In this article we have complied various notes on bacteria. After reading this article you will learn about:- 1. Meaning of Bacteria 2. General Characteristics of Bacteria 3. Economic Importance 4. Distribution 5. General Morphology 6. Size 7. Structure 8. Cell Division 9. Staining 10. Nutrition 11. Respiration 12. Reproduction 13. Genetic Recombination 14. Host-Bacterial Interactions 15. Bacterial Culture and Others.
Contents:
Notes on the Meaning of Bacteria
Notes on the General Characteristics of Bacteria
Notes on the Economic Importance of Bacteria
Notes on the Distribution of Bacteria
Notes on the General Morphology of Bacteria
Notes on the Size of Bacteria
Notes on the Structure of Bacteria
Notes on the Cell Division of Bacteria
Notes on the Staining of Bacteria
Notes on the Nutrition of Bacteria
Notes on the Respiration of Bacteria
Notes on the Reproduction of Bacteria
Notes on the Genetic Recombination of Bacteria
Notes on Host-Bacterial Interactions
Notes on Bacterial Culture
Notes on the Growth of Bacteria
Notes on the Variability in Bacteria
Notes on Bacteriophage
Notes on the Relationships of Bacteria
Notes on the Classification of Bacteria
Notes on Bacterial Nomenclature
Note # 1. Meaning of Bacteria:
Bacteria are microscopic organisms, which are often known as ‘germs’ and ‘microbes’. They are among the simplest forms of life known, and, hence show the characteristics of both plants and animals. Their relationship to other living organism is very obscure.
Though they are placed in the plant kingdom, this does not mean that they are more closely related to plants than to animals. Their inclusion under the plant kingdom is entirely for the sake of convenience.
Since the discovery of bacteria in 1676 by Anton van Leeuwenhoek, a Dutchman, they were objects of curiosity until middle of the nineteenth century, when Louis Pasteur and Robert Kock demonstrated that these organisms are responsible for some of the most important diseases of human beings and animals.
Their brilliant work revolutionized medicine and pioneered the field of antiseptic surgery paving the way for the development of the study of bacteria into independent science of bacteriology. It was Pasteur who disproved the theory of spontaneous generation by furnishing irrefutable evidence that microorganisms arise from pre-existing living entities.
He also demonstrated that fermentation is a biological phenomenon rather than a purely chemical one.
Koch’s experimental methods made possible the formulation of rules or postulates, which are followed whenever possible, before a suspected organism could be finally accepted as the cause of a specific disease. These are known as Koch’s Postulates or Rules of Proof.
The Koch’s postulates are as follows:
1. The organism must be consistently associated with the disease in question.
2. The organism must be isolated from the diseased plant or animal and grown in pure culture and accurately described.
3. The organism of the pure culture, when inoculated back into healthy plant or animal, must be capable of reproducing the characteristic symptoms of the disease.
4. The organism must be re-isolated from the diseased plant or animal tissue, grown in pure culture and must be identical with the original inoculated organism.
All these postulates have been accepted as working guides by workers of diseases of human beings and other animals, and also of plants. But they are not applicable in case of viruses, insects, and fungi which cannot be grown on artificial media.
Note # 2. General Characteristics of Bacteria:
Bacteria are among the smallest of living organisms. They are unicellular (some nulticellular) or thalloid living organisms, sometimes with some differentiation of cells, but without differentiated tissues. Bacterial cells are procaryotic possessing datively simple nucleus without any nucleous and nuclear membrane.
Some species of bacteria are parasites. They attack the living cells of other plants or of animals and secure their food from that source. But most bacteria grow as saprophytes on dead remains or the products of plant and animal life without a direct relationship with living cells.
Parasitic bacteria are responsible for some of the diseases of plants and animals, whereas, the saprophytic kinds may be beneficial in one way or the other.
While most of the bacteria are heterotrophic, a few are photosynthetic and chemosynthetic autotrophs and are capable of synthesizing carbohydrates out of carbon dioxide and water. Photosynthetic autotrophs contain photosynthetic pigment known as bacteriochlorophyll and can carry on a type of photosynthesis — photoautotrophic.
While others, lack such pigment, but can manufacture carbohydrates by chemosynthesis— chemoautotrophic.
More than 3,000 species of bacteria have been described in Bergey’s Manual. It is estimated that each person carries some 1014 bacteria, and that the total human population excretes from its collective gut 1022— 1023 bacteria per day.
Bacterial physical environments range from hot springs at 80°G to refrigerated foods, and from distilled water with trace contaminants to the Dead Sea.
Bacteria reproduce by binary fission. Some forms produce spores. Sexuality in bacteria is a very intriguing problem which is being thoroughly investigated. Transfer of genetic material from one cell to the other has, however, been reported in some bacteria. It has also been reported that bacteria maintain certain specific, inheritable attributes through countless series of cell generations.
They also possess some nuclear mechanism of inheritance. Both DNA and RNA are present in bacterial cells.
Note # 3. Economic Importance of Bacteria:
Bacteria are of major importance to main kind for they include some of his most destructive enemies and also some of his greater benefactors. Injurious species include those responsible for disease of human beings and other animals and those that destroy food and reduce crop production by inducing various diseases of crop plants.
Beneficial species liberate fertilizer elements for growing crops, destroy sewage and other wastes. Activities of many bacteria have been harnessed for various industrial purposes to produce valuable chemicals, medicines and various other products essential for human society.
I. Beneficial Activities.
Some of the beneficial activities of bacteria are:
1. Bacteria and soil fertility:
Of all the living organisms found in soils, bacteria are among the most active. They are especially abundant in the ‘surface layers of soil, decreasing in numbers with depth of soil. These bacteria, along with other soil organisms, play a dominant role in soil fertility.
In general, they succeed in converting insoluble or unavailable materials into forms that can be used by higher plants. Of the essential elements secured by the green plants from the soil, nitrogen is, next to hydrogen and oxygen, the one which is needed in the largest quantities. This element makes up a large part of the molecule of the different proteins of which protoplasm largely consists.
Large quantities of the nitrogen from the soil which has been built into plant proteins of crop plants are removed from the field in the grain or other useful parts.
Although the waste products from domestic animals containing much of the nitrogen of the food which they have eaten and utilized in their metabolism, are in part returned to the soil as fertilizer, much of the nitrogen of the crops removed from the soil is never returned to it. Maintenance of the supply of soil nitrogen is thus one of the principal problems of soil fertility.
The element nitrogen passes repeatedly through a cycle which is known as nitrogen cycle, in which it is first converted from simple to complex compounds and then returned to the simple form in a cyclic order by the activity of bacteria and fungi in collaboration with higher plants and animals. Most plants absorb nitrogen from the soil principally in the form of nitrate ions, although they can also absorb ammonium ions.
Since nitrate ions are continually being removed from the soil by green plants the supply of nitrates would eventually be exhausted if there were no way of replenishing it. Neither the elaborated nitrogenous wastes excreted by animals are directly available for reuse by most green plants.
The conversion of nitrogen from proteins and other nitrogenous compounds into the form of nitrates involves several steps and is due in large part to the activities of bacteria, although some of the earlier stages are also carried on by fungi.
Certain groups of bacteria are involved in the decomposition of nitrogenous organic compounds of both plant and animal bodies and the nitrogenous waste excreted by animals and nitrogen transformation in the soil and thereby play an important role in the maintenance of soil fertility. They are ammonifying bacteria, nitrifying bacteria, and nitrogen fixing bacteria.
The bacterial decomposition of nitrogenous organic compounds in the absence of abundant oxygen usually results in the formation of materials of offensive odour chiefly of sulphur compounds.
Such anaerobic decomposition is termed putrefaction. The decomposition of organic compounds in the presence of oxygen and without the development of odoriferous substance is called decay. Actually there is no sharp line of differentiation between putrefaction and decay.
Both the processes are carried on by bacteria in the soil. The decomposition of organic compounds is important to human society from at least two standpoints. It prevents the accumulation of organic matter, both plant and animal, on the earth, and it results in the formation of simple compounds or set free elements that are returned to the soil to be used again by plants.
The ammonifying bacteria transform various proteinaceous substances into ammonia in the soil. The process is known as ammonification. In the first step in the process, the proteins of the soil organic matter are broken down to amino acids. This occurs during ordinary decay of organic matter and is brought about chiefly by bacteria and fungi.
In the next step, the ammonifying bacteria, together with certain fungi, convert the amino-nitrogen to ammonia. Some of the ammonia escape into the air and is lost.
But most of it usually reacts with water to form ammonium hydroxide without the intervention of bacteria. Ammonium ions can be absorbed directly by most plants and used as a source of nitrogen. The ammonia of the ammonium salts is oxidized to stable nitrates by the nitrifying bacteria and the process is known as nitrification.
In the first step, the ammonia is oxidized to nitrite by bacteria known as the Nitrosomonas group. Finally, the oxidation of nitrite to nitrate is due chiefly to Nitrobacter. Nitrate is most readily utilized by green plants.
Ammonifying and nitrifying bacteria are directly concerned in the transformation of protein compounds of plant and animal dead bodies and the animal wastes into nitrates, and thus are essential in maintaining a supply of nitrates in the soil.
Nitrogen-fixing bacteria assimilate atmospheric nitrogen (which green plants cannot utilize) by converting into organic nitrogen compounds, which are subsequently decomposed by other bacteria and transformed through, a series of stages into nitrates. The process by which they assimilate free nitrogen is called nitrogen-fixation.
There are two types of nitrogen-fixing bacteria:
(i) Non-symbiotic nitrogen-fixing bacteria live independently in the soil. Two prominent genera of this group are Azotobacter and Clostridium. These free-living bacteria fix independently atmospheric nitrogen in their body and convert to organic nitrogen compounds. When they die, the organic nitrogen compounds of their body are made available to higher plants through the activities of other bacteria.
(ii) Symbiotic nitrogen-fixing bacteria, consisting of species of Rhizobium, live symbiotically in small, swellings or nodules on the roots of various seed plants, chiefly leguminous plants. These bacteria fix atmospheric nitrogen to synthesize organic nitrogen compounds. When the leguminous plants are harvested, the roots with nodules containing bacteria are left in the soil.
When they decay, the organic nitrogen compounds made by bacteria become available as nitrates through the process of nitrification. A leguminous crop in a rotation thus possesses an advantage besides that of the crop itself, it increases the amount of nitrogen in the soil.
Each type of leguminous plant requires a particular strain of nodule-forming bacteria in its roots. If this strain is not present in the soil, it can be added artificially by inoculating the seeds before they are sown. The relationship between nodule- forming bacteria and their leguminous plants is one of symbiosis.
The bacteria secure food from the tissues of the leguminous plant, and the leguminous plant obtains nitrogen which is fixed by the bacteria.
The nodule-forming bacteria are thus also known as symbiotic nitrogen-fixing bacteria. These bacteria are widely distributed in soils, so that even in the absence of seed inoculation with these bacteria, leguminous seedling roots sooner or later become infected with symbiotic nitrogen- fixing bacteria.
2. Bacterial Metabolism—its Commercial Importance:
The metabolic processes and their products of bacteria have been utilized in many industries. In their metabolic activities bacteria excrete waste products some of which have exceedingly important commercial uses. The large-scale production of valuable substances from bacterial metabolism is a relatively new field in the economic utilization of bacteria, and, as such, is one in which new discoveries are frequent and often very startling.
(i) Source of Antibiotics:
Soil is perhaps the most important source of microorganisms which produce antibiotic substances. These include filamentous bacteria (actinomycetes). Some of the antibiotic substances are secreted outside the cells and into the environment; others are retained largely within cells and must be separated by extraction.
The modern period of antibiotics began in 1939 with the finding of tyrothricin, produced by a soil bacterium.
Prominent among the antibiotics from actinomycetes are:
streptomycin, discovered in 1944 and obtained from Streptomyces griseus, strtptothricin from S. lavendulae; Chloromycetin, in 1947 obtained from S. venezuelae; aureomycin, in 1948 obtained from S. aureofaciens; neomycin, in 1949 obtained from S. fradiae and terramycin, in 1950 obtained from S. rimow.
Besides these, bacitracin and polymyxin are antibiotics produced by Bacillus subtilis and Bacillus sp. respectively were reported long ago.
(ii) Fermentation—its Industrial Application:
The process of bacterial fermentation and its products have been utilized in various industries. Some of them are: Clostridium acetobutylicum ferments carbohydrates producing acetone, methyl alcohol, and n-butyl alcohol, which have important industrial uses. Very recently, vitamin B2, a commercially important product has been discovered as a product of fermentation of carbohydrates by this species of Clostridium. This vitamin has been one of the more expensive vitamins to obtain in quantity, is valuable as a preventive of certain nervous disorders and other diseases.
The manufacture of vinegar is one of the oldest processes in human history which involves bacterial metabolism. Vinegar production begins with the fermentation of sugars in apple juice to alcohol by yeasts. In the presence of oxygen, the vinegar bacterium, Acetobacter aceli, oxidizes alcohol to acetic acid, which is responsible for the characteristic odour and flavour of vinegar.
Lactic acid as one of the products resulting from the souring of milk, has many uses in the processing of foods, in pharmaceuticals and in the chemical industry. It is named after the milk constituent lactose, or milk sugar. It is probably the oldest known acid, having been discovered by Scheele in 1780.
A group of bacteria designated as ‘lactic acid bacteria’, ferment lactose of milk to lactic acid. All these bacteria are classified in the family Lactobacteriaceae which includes the genera Lactobacillus, Leuconostoc, and Streptococcus.
The dairy industry finds bacteria an essential aid in a number of processes. Butter is sometimes made from cream which has been allowed to undergo ‘ripening’—that is, a lactic acid fermentation, causing it to become sour. The cream is pasteurized, incubated and then churned. This fermentation eventually results in the formation of substances responsible for the characteristic odour of butter.
The process of pasteurization was named after Louis Pasteur, the French scientist who developed it to prevent the spoilage of wines. It is chiefly associated with the treatment of milk to eliminate pathogenic bacteria. Two methods are used to pasteurize milk. In the older of these, the holding process; the milk is heated to 140°F. and held at that temperature for 30 minutes.
Another short time, high temperature method, sometimes termed flash pasteurization, the milk, in thin layers is exposed to a temperature of 160°F. for a minimum of 15 seconds. The milk is then cooled as rapidly as possible to a temperature which retards bacterial growth. Pasteurization destroys pathogenic bacteria without appreciably affecting tie physical and chemical properties of the milk.
The pasteurization of milk is widely recognized as one of the most important public health measures. It is not a process of sterilization. Spores of bacteria are not killed, and since the temperature falls far short of boiling, neither are all the vegetative cells. This is notably true of the lactic acid bacteria although the number of these is greatly reduced.
Pasteurized milk will, therefore, be normally sour, as will raw milk, although more slowly. Besides all these processes, a number of complex processes involving lactic acid or other kinds of bacteria occur during the ripening of cheese.
(iii) Retting of fibres:
Bacteria play an important role in the retting of jute, flax, and hemp fibres. They hydrolyze the pectic substances which act as cement like materials that bind the fibres together. Retting is carried out by immersing the stalks of jute, flax or hemp in water and weighing them down. Water is absorbed by the tissues, causing swelling and the extraction of water soluble substances.
The water becomes highly coloured containing substances that have been extracted from the submerged stalks. This highly coloured water now becomes a good culture medium for the growth of many kinds of organisms. The aerobic organisms reduce the concentration of dissolved oxygen and create an environment suitable for the growth of anaerobes.
The pectic substances are slowly fermented and dissolved by the anaerobes, leaving the fibres intact. During fermentation, various organic acids and gases are produced. The submerged stalks should be removed from water when the reaction has gone to completion; otherwise overrating will result.
They are then thoroughly washed to remove the organic acids, odours, and other undesirable substances and the fibres removed from them are then spread out in the sun or air to dry. The dried material is then ready for dressing. Bacteria responsible for retting of fibres are: Bacillus subtilis, B. polymyxa, Clostridium tertium, and C. felsimium.
(iv) Other fermentation processes:
Certain streptococci and Iactobacilli are used for the preparation of silage for the consumption of cows.
Tobaco is cured and fermented to improve its colour, texture and aroma. Bacteria of the Bacillus megatherium group are used for the curing process. A satisfactory fermentation is associated with a rapid increase in numbers of the Micrococcus candicans and Bacillus subtilis types.
3. Bacteria as human symbionts:
Bacteria promote digestive and possibly other physiological processes in the intestinal tracts of animals. The digestion of cellulose by such herbivorous animals as horses and cattle results in part from cellulose- digesting enzymes excreted by bacteria inhabiting the intestines of these animals.
Bacteria also dwell in large numbers in the human intestinal tract, particularly in the lower part of the small intestine and in the large intestine.
Escherichia coli, formerly called Bacterium coli and Bacillus coli, is a regular inhabitant of the lower intestinal tract of man and other vertebrates. About 40 per cent, of human feces consists bacteria mainly E. coli in milk, water, or food is used as an index of fecal contamination.
E. coli has common been considered chiefly a commensal, a partner of reciprocal parasitism (commensalism). It is also occasionally known as a pathogen. It is now known, however, that E. coli and other intestinal bacteria synthesize considerable quantities of some of the B vitamins and release these into the intestine.
These bacteria may therefore be properly regarded as normally being symbionts rather than mere commensals. The destruction of the bacterial flora of the colon by vigorous dosage with certain antibiotic drugs sometimes causes functional disturbances which suggest that the usefulness of these organisms in human physiology is not limited to the production of known vitamins.
The activity of these bacteria in the human gut and the significance of this activity are not known positively; some physiologists believe that these bacteria carry on certain digestive activities of value of the human body, while others believe that lactic acid and other metabolic products of these normally occurring bacteria inhibit the growth of putrefactive and possibly certain pathogenic bacteria.
Whatever may be their specific physiological significance, it is certain that the maintenance of a normal bacterial flora in the human intestinal tract is essential to the health of the human organism and that any major disturbance in this intestinal bacterial flora results in derangements of health.
II. Harmful Activities by Bacteria:
Among the harmful or wasteful results of bacterial activity arc these:
1. Reduce soil fertility:
Certain soil bacteria reduce soil fertility by depleting the nitrogen content of soil. They are known as denitrifying bacteria. These bacteria are especially abundant and active under anaerobic conditions in wet soil and soil with high organic matter content.
They break down nitrates through intermediate compounds to free nitrogen gas, which escapes into the air and thereby the soil fertility is reduced to a great extent. This process of breaking up nitrates to free nitrogen is designated, as denitrification. Denitrification is favoured by poor aeration of the soil, and nitrates may eventually disappear from water-logged soils.
2. Spoilage of foodstuffs:
Of all agents involved in the spoilage of foodstuffs, the activities of living organisms are undoubtedly the most important. Spoilage is caused principally by bacteria, yeasts and molds. The organisms may be pathogenic or non-pathogenic. Different kinds of organisms produce different types of changes in food. Bacteria are more exacting in their requirements than either the yeasts or the molds.
They require relatively large amounts of moisture, hydrogen-ion concentration usually near the neutral point, and relatively low osmotic pressures. Bacteria cause spoilage of human food in the form of rotting meat and fish, spoiling butter, all kinds of vegetables and fruits. Certain bacteria cause severe types of food poisoning in persons who eat bacteria-contaminated food.
The bacteria found on meat surfaces are usually of Achromobacter and Pseudomonas. These organisms, growing on meat, are probably not danger us to health.
Some of the bacteria concerned with putrefaction of meat are: Bacillus subtilis, B. cereus, Escherichia coli, Proteus vulgaris, and Aerobacter cloacae.
Spoilage of fish is caused largely by bacteria. They belong to the genera Achromobacter, Flavobacterium, Micrococcus, Micro- bacterium and Pseudomonas. Bacteria chiefly responsible for food poisoning are: Micrococcus pyogenes var. aureus, Clostridium botulinum, Salmonella enteritidis and Solmonella typhimurium.
The most common kind of food poisoning is Staphylococcus food poisoning which follows the consumption of food contaminated by staphylococci. A second and less common food-borne disease is caused by bacteria of the genus Salmonella, and is commonly known as Salmonella food infection. Although infection has been traced to several kinds of foods, including milk, meat is usually involved.
Salmonella also infects domestic animals and infections in human beings may result from the consumption of such meat when it is insufficiently cooked. Another type of food poisoning is, Botulism food poisoning which follows the consumption of bacteria contaminated food. It is caused by Clostridium botulinum.
This organism thrives on meat and other high-protein foods like peas and bean in the absence of air, and the spores often survive the temperatures used in home canning. C. botulinum multiplies in food liberating a potent toxin which causes botulism.
3. Cause animal diseases:
That bacteria cause diseases of animals was detected as early as 1850, but the final proof of the bacterial cause of animal disease was obtained in 1876 by Robert Koch.
Bacteria cause tuberculosis of cattle, anthrax of sheep, chicken cholera, pneumonia, glanders in horses, sheep and goats. One of the oldest diseases of animals is anthrax which is caused by Bacillus anthracis.
Species of the genus Rickettsia cause diseases like Rocky Mountain spotted fever, classical typhus fever of man and other animals. They are transmitted to humans via arthropod vector. Strains of Chlamydia trachomatis cause keratoconjunctivitis, trachoma that often results in blindness.
A number of microorganisms isolated from animal mucous membrane and sewage were termed pleuropneumonia-like organisms (PPLO). They were subsequently designated as mycoplasmas. Mycoplasma pneumoniae is the causative agent of primary atypical pneumonia.
4. Cause human diseases:
Many of the serious human diseases are caused by bacteria. Some of them are: tuberculosis (caused by Mycobacterium tuberculosis var. hominis), diphtheria (caused by Corynebacterium diphtheriae), leprosy (caused by Mycobacterium leprae) and tetanus (caused by Clostridium tetani).
5. Cause plant diseases:
Many of the plant diseases are induced by bacteria. Bacteria are usually selective in attacking only particular hosts. It has been seen that bacteria that attack plants do not attack animals.
Comparatively bacterial diseases are more common in animals than plants. Plant pathogenic bacteria are distributed among 8 genera: Pseudomonas, Xanthomonas, Rhizobium (under certain environmental conditions), Agrobacterium, Birwinia, Corynebacterium, Streptomyces, and Mycoplasma.
Most bacteria enter the host through wounds, stomata, hydathodes, lenticels, and nectaries.
Species of Streptomyces are known to penetrate the cuticle directly and the legume nodule bacteria (Rhizobium) penetrate the non-cuticularized root hairs. Since the discovery of Professor T. J. Burrill of the University of Illinois, U.S.A. in 1879, that bacteria are capable of causing plant diseases, many bacterial diseases of plants have been discovered. Bacteria cause various plant diseases.
Some of them are:
(i) Leaf spots:
Invasion and multiplication within the substomatal chamber and the intercellular spaces lead to necrosis of the invaded plant tissue causing spots. The organisms responsible for leaf spot disease produce a vigorous attack on the plant, with the result that the cells become heavily infected and strongly discoloured. The discoloured areas dry up and frequency fall out, leaving holes in the leaves.
Some of the organisms causing leaf spot disease are Pseudomonas angulata, the causal agent of angular leaf spot of tobacco; P. maculicola, causes cauliflower spot; P. meliea, the etiological agent of leaf spot of tobacco; Xanthomonas cucurbitae, causes leaf spot of squash; X. malvacearum, the agent of angular leaf spot of cotton; X. ricinicola, the causal agent of leaf spot of castor bean.
(ii) Extensive blights:
Here the progress of the bacteria is more rapid than leaf spots causing a more extensive and rapid necrosis. Organisms producing blight diseases are capable of penetrating considerable distances between cells, leaving the neighbouring tissue intact. The bacteria grow in the plant juices without producing any digestion of the tissues.
Usually a discolouration of the leaves and branches is produced. Death is due probably to an interference with the flow of the plant sap. Some of the organisms producing blights include Erwinia amylovora, the agent of fire blight or pear blight; E. lathyri, the cause of the streak disease of sweet peas and clover.
(iii) Soft rots:
The major effect is a slimy softening of the tissue by the secretion of an enzyme which diffuses in advance and dissolves the middle lamella of the cells, plasmolysis and death of the cells follow, and the bacteria grow upon the dead plant tissue rather than upon the living cells.
Organisms responsible for soft rots reduce the plant tissue to a soft, very moist, pulpy mass. The organisms producing soft rots differ from the other forms found in the soil in that they have the power to attack healthy plant tissue by the secretion of an extracellular enzyme.
The enzyme dissolves the pectin or cement-like material that binds the plant cells. The action is probably hydrolytic, resulting in the liberation of soluble sugars, which are utilized by the bacteria for food. The result is that the plant tissue is reduced to a mass of separate cells, which become converted later into a slimy, pulpy material.
The important species causing soft rots include the following:
Erwinia aroideae, the cause of soft rot of potato; eggplant, cauliflower, radish, cucumber, cabbage, parsnip, turnip, and tomato; E. atroseptica, responsible for black rot of stem and tuber of potato and other vegetables; E. betivora, the etiological agent of sugar beet rot; E. carotovora, the cause of soft rot in carrot and cabbage.
(iv) Vascular diseases:
In some cases the bacterial invasion in the vascular system becomes systemic, in others, bacteria multiply in the vascular system resulting acute wilt. The bacterial wilts constitute a group of very important and destructive plant diseases. The infecting organisms multiply and accumulate in large numbers in the vascular system, causing an interruption in the flow of sap in the plant.
A complete interruption in the flow of sap results in a rapid wilting of the plant. Some important organisms causing wilt diseases are Bacterium stewartii, the cause of wilt disease of maize; Corynebacterium insidiosum, the agent of vascular disease of alfalfa; Erwinia tracheiphila, the etiological agent of wilt of cucumber, pumpkin, and squash.
(v) Bacterial galls:
Here the primary effect of the bacteria on the host is the stimulation of the cell division hyperplasia leading ultimately to hypertrophy, the combined effect of which results in the gall formation. These abnormal growths are produced by the action of organisms on the meristematic tissue of the plants. In some infections the galls remain small; in others they may assume large proportions. The important organisms producing intumescence diseases include: Agrobacterium rhizogenes the agent of hairy root of apple; Bacterium pseudotsugae, the agent of galls of Douglas fir.
(vi) Canker:
During early part of disease development bacteria are contained primarily in the phloem, rather than the xylem and are more active in the living host tissue at the outer edge of the canker. Example, Cornebacterium michiganense, the cause of canker of tomato.
(vii) Mycoplasmal disorders:
Species of the genus Mycoplasma induce various plant diseases. Some of them are: Little leaf of Brinjal, Greening disease of Citrus, and Grassy shoot disease of sugarcane (GSD).
Note # 4. Distribution of Bacteria:
Bacteria are widely distributed in nature under varied conditions. They vary in numbers from one locality to another, depending upon the environmental conditions. Bacteria are present on and beneath the surface of the earth, in fresh-water and in the sea, on and in other organisms, and on the dust particles which float in the air.
They do not generally occur inside normal, healthy cells of other organisms, but otherwise they are usually found wherever food is available to them.
Some bacteria grow belt at 0°C; others require temperatures above 45°C and may even grow at 80°C. Some require atmospheric oxygen; others are indifferent to or inhibited by it.
Almost all naturally occurring organic compounds can be used as food by one or another kind of bacteria. On or in a substratum suitable for their growth and development they may become extremely numerous. Decaying vegetable and animal materials and solutions rich in organic matter are usually excellent places for the growth of saprophytic species.
Bacteria are useful criteria of water and air pollution and cause many serious diseases of human beings, domesticated animals, and cultivated plants.
These are known as Pathogenic Bacteria. Bacteria constitute a normal part of:
(1) The soil flora,
(2) The intestines of animals promoting digestive and possibly physiological process and are particularly abundant in the tropics. Some can withstand extreme heat (40-75°C) —thermophilic and others extreme cold (0-30°C) psychrophilic. Whereas, the majority of bacteria are intermediate in their temperature requirements (20-46°C)—mesophilic.
Most bacteria are not only harmless but absolutely necessary for the existence of living things—non-pathogenic bacteria. Life could not exist in the complete absence of bacteria. Plants and animals owe their existence to the fertility of the soil, and this in turn depends upon the activity of microorganisms which inhabit the soil.
Bacteria are, involved in nitrogen transformations in the soil and help maintenance of soil fertility.
Note # 5. General Morphology of Bacteria:
Most bacteria are unicellular; there are some mycelioid forms too. Cross-walls are absent except in the mycelioid ones (actinomycetes), where septation does take place previous to sporulation and as a prerequisite to it. Bacterial cells may be motile bearing flagella or non-motile without any flagella.
Shape and Arrangement (Fig. 330):
According to the shape of cells, bacteria may be of four types:
(i) Coccus (pl. Cocci)—cells spherical, (ii) Bacillus (pl. Bacilli)—cells rod-shaped, (iii) Spirillum (pl. Spirilla)—cells cork-screw-like, and (iv) Vibrio or Comma (pl. Commas)—cells ‘comma’-like.
Cocci may be of different forms:
(a) Micrococcus, cells are small and occur singly;
(b) Diplococci, cells divide in one plane and remain attached predominantly in pairs;
(c) Streptococci, cells divide in one plane and remain attached to form chains;
(d) Tetracocci, cells divide in two planes and characteristically form groups of four cells;
(e) Staphylococci, cells divide in three planes in an irregular pattern producing bunches of cocci;
(f) Sarcinae, cells divide in three planes in a regular pattern producing a cuboidal arrangement of cells.
Bacilli are not arranged in patterns as complex as those of cocci, and most occur singly or in pairs—diplobacilli. Some species form chains—streptobacilli, others form trichomes which are similar to chains but have a much larger area of contact between the adjacent cells, and again in others the cells are lined side by side like matchsticks— palisade arrangement.
There are other kinds of bacteria that exhibit a considerably different morphology.
For example, some species of bacteria possess appendages, bacteria of the genus Saprosphira form helical filaments, individual cells of the genus Caulobacter are rod- shaped or fusiform with a stalk sometimes protruding from one pole, species of Streptomyces produce a well-developed branched mold-like mycelium through much narrower than fungal mycelium.
Although most bacterial species have cells that are of a fairly constant characteristic shape, some have cells that are pleomorphic, i.e., that can exhibit a variety of shapes. A bacterial species is generally associated with a definite cell form when grown on standard media under certain specified conditions, such as temperature of incubation of the medium.
Bacteria usually exhibit their characteristic morphology in young cultures and on media possessing favourable conditions of growth.
They are of three forms:
(1) The embryonic,
(2) The mature, and
(3) The senescent.
The embryonic forms correspond to the growth phase and are of long slender uniform cells. The mature form corresponds to the resting phase and is characterized by short cells of small size but more variable in form. The senescent form corresponds to the death phase and show’s great variation in both form and size.
Some morphologists have considered them as definite stages in an orderly life cycle of an organism. Young cells are, in general, larger than old ones of the same species. The changes in age are only temporary; the original forms appear when the old cells are transferred to fresh medium.
Note # 6. Size of Bacteria:
Bacteria vary greatly in size according to the species. Regardless of their size, Hone can be clearly seen without the aid of a microscope. The bacteria most frequently studied in the laboratory measure approximately 0.5 to 10 µm in diameter.
Staphylococci and streptococci may have diameters ranging from 0.75 to 1.25µm. Rod (bacillus) forms, such as typhoid and dysentery bacteria often have a width between 0.5 and 1 µm and a length of 2 to 3 µm. Some filamentous forms may exceed 100 µm in length and diameter between 0 5 and 1.0 µm.
Note # 7. Structure of Bacteria:
A bacterial cell has certain definite structures inside (internal) and outside (external) the cell wall. Some cellular parts, such as the cell wall and cytoplasm are common to almost all cells, again some structures are present in only certain species; still others are more characteristic of certain species than of others.
a. Structures Internal to the Cell Wall:
Immediately beneath the cell wall is a thin membrane or covering called the cytoplasmic membrane (Fig.331 A), also called bacterial cell membrane or simply the plasma membrane. It is semipermeable, selective membrane that controls the passage of nutrients and waste products into and out of the cell.
The bacterial cell membrane is an important centre of metabolic activity where all enzymes for the replication of DNA and other metabolic reactions are located.
It also contains many different kinds of proteins, each of which probably has a specific catalytic function responsible for the transport of many organic and inorganic nutrients into the cell and biosynthesis of membrane lipids and various classes of macromolecules that compose the bacterial cell wall (peptidoglycans, teichoic acid, lipopolysaccharideg and simple polysaccharides).
It often contains important components of the machinery of ATP generation. In some bacteria, the membrane appears to have a simple contour, which closely follows that of the enclosing cell wall.
In others, it is infolded, at one or more points into cytoplasmic region resulting in the increase of surface area of the membrane. The infoldings of the bacterial cell membrane are known as mesosomes. The mesosome performs the functions of DNA replication and septum formation in a bacterial cell. It is a link between cytoplasmic membrane and nuclear material.
Mesosome may be lamellae type, as in the species of Lactobacillus or vesicular type structure found in the species of Bacillus. Bacterial cells are characterized by the absence of mitochondria and Golgi bodies. Hence a large number of enzymes involved in the generation of energy in a bacterial cell are attached to the cytoplasmic membrane (membrane-bounded enzymes).
Mesosomes, like mitochondria, also control the Respiratory activities of the bacterial cells, they are also known as chondrioids.
Cell material, contained within the cytoplasmic membrane may be divided into the cytoplasmic area and chromatinic or nuclear area (nuclear material) (Fig. 331A) which is less dense than the surrounding cytoplasm. The cytoplasmic area is granular and is rich in RNA.
The RNA in combination with protein forms macro- molecular bodies. These RNA-protein particles are called ribosomes which contain enzymes useful in protein biosynthesis.
Ribosomes are small granular structures ranging from 10/µm to 13/µm in size and are about 10,000 to 15,000 in number in a cell. Electron microscope study reveals that each ribosome is composed of two unequal and roughly spherical halves having nearly equal amount of RNA and protein.
The greater the rate of protein synthesis greater is the number of ribosomes. Ribosomes active in protein synthesis occurring groups are the polysomes (polyribosomes). They are held together by messenger RNA. All rapidly growing bacteria contain numerous ribosomes.
The cytoplasm of a bacterial cell also contains volutin granules, also known as metachromatic granules, various kinds of polysaccharides as food reserves; and one or more vacuoles.
Globules of fat also accumulate in bacterial cell cytoplasm. In anoxygenic photosynthetic bacteria there occur extensive intracullar membrane systems which are the site of the photosynthetic apparatus, the infoldings of which accommodate bacteriochlorophyll.
The chromatinic or nuclear area is rich in DNA which can be made visible under the light microscope by Feulgen staining. Bacterial cell does not contain nucleus characteristic as in eucaryotic cell. There is no evidence of nuclear membrane separating the nuclear area from the cytoplasm.
Since the DNA of the bacterial cell does not form a discrete nucleus and is not capable of mitotic and meiotic divisions, it has been suggested that this structure be designated as chromatin body, nucleoid, nuclear equivalent, and even bacterial chromosome.
Besides giving rise to enzymes and other proteins of specific nature in biological systems, DNA molecules are responsible for passing on the necessary information to make a new Cell from one generation to the next.
The basic proteins (histones) are not associated with bacterial DNA.
Electron microscopy reveals that fibrils of extremely long circular molecule of DNA are highly folded to form a compact mass known as chromosome, though it differs from the chromosome of eucaryotic cell as because the DNA molecule does not undergo shortening or thickening. It may assume various shapes, from a sphere to an elongated or dumbell or otherwise.
The bacterial chromosome is about one millimetre long and has a molecular weight of about 2×109. It is covalently closed and occupies about 10 per cent of the volume of the cell. The ends of the polynucleotide threads are covalently joined to make a continuous circular molecule.
DNA in such a configuration is said to be in the covalently closed, circular (CCC) form (Fig. 332a). The chromosome is double stranded having succession of genes (about 10,000 in all) arranged either singly or in groups (operons). The individual genes and operons are controlled by regulatory mechanisms of variable complexity.
Replication of the circular DNA of the bacterial chromosome is unidirectional and autonomous. When replication begins the chromosome is attached to a specific point on the bacterial cell membrane, this is known as replicator site with which the enzyme machinery responsible for DNA replication is associated.
The replication fork is located at the replicator site, and its passage along the chromosome involves movement of the chromosome past the attachment site. Prior to the initiation of replication, a new replicator site adjacent to the old one is formed on the membrane, and the free end of the broken DNA strand is attached to it.
Separation of daughter chromosome is effected by the localized synthesis of the membrane in a region situated between the old and new attachment sites. Membrane growth consequently spreads the attachment sites farther and farther apart (Fig. 332).
Most bacteria carry pieces of DNA, either as extra-chromosomal elements called plasmids or in the form of bacterial viruses (bacteriophages) carried in a quiescent form by the cells. Plasmids are closed circular autonomously replicating double-stranded DNA molecules like the chromosome having molecular weight ranging from 106 and 108 and are 1 per cent of the chromosome in size.
They usually contain three to four genes. Plasmids are attached somewhere on the inner face of their host’s cell membrane.
Some of the plasmids are non-transferable and are lost, others have the ability to transfer themselves from one cell to another. Plasmids transfer genetic material between bacteria to spread resistance to antibiotics and genes for the degeneration of complex organic compounds in geochemical cycle. Like bacteriophages, plasmids are employed as cloning vehicles or vectors.
Both chromosome and plasmids carry genetic information to daughter cells. But plasmids are independently replicating DNA pieces which may sometimes fail to replicate or be distributed to daughter cells leading to loss from the bacterial cell.
Besides plasmids, bacterial cells also have attached to the cell membrane pieces of DNA called episomes which are one-hundredth times the size of the bacterial chromosome. Episomes sometimes replicate autonomously in the cytoplasm and at other times become integrated into the chromosome DNA and replicate with it.
This behaviour distinguishes an episome from a plasmid because the latter does not integrate into the chromosome.
b. Structure of Cell Wall:
External to the cytoplasmic membrane and immediate contact with the cytoplasm is the cell wall which provides a rigid framework to the bacterial cell and also supports and protects the more labile protoplasmic entities from osmotic damage. Thickness of cell wall ranges from 10 to 25 µm. Bacterial cell wall is essential for bacterial cell growth and division.
It is a mucocomplex structure and is composed mainly of diaminopimelic acid (DPA), muramic add, and teichoic acid, in addition to which are present amino acids, aminosugars, carbohydrates, and lipids which form a polymeric substance known as peptidoglycan (mucopeptide murein).
The mucopeptides form the rigid part of the bacterial cell wall. Two different groups of bacteria are commonly differentiated on the basis of their response to a differential stain (Gram stain) named after Danish Bacteriologist Christian Gram.
Those species of bacteria which retain the crystal violet stain are designated as Gram-positive, while others which do not are Gram-negative. The cell wall of Gram-positive bacteria is thicker, more rigid and chemically less complex than that of Gram-negative ones. It is composed of homogeneous layer of 10-50 nm wide having chemical composition—peptidoglycan, teichoic acids, and polysaccharides.
Whereas in Gram-negative bacteria the cell wall is more complex than that of Gram-positive ones. It is thin being composed of inner layer 2-3 nm wide and outer layer 7-8 nm wide with chemical composition—peptidoglycan, phospholipids, proteins and lipopolysaccharides. The lipopolysaccharide which is also known, as endotoxin material determines antigenicity, toxigenicity, and sensitivity to phase infection.
c. Structures External to the Cell Wall:
Structures that may be present external to the bacterial cell wall are: flagella, pili, prosthecae, and capsule. Flagella are extremely thin hair-like helical appendages that protrude through the cell wall and are responsible for swimming motility. They are 0.01 to 0.02 µm in diameter and upto 70 µm in length.
Each flagellum has three parts:
a basal body associated with the cytoplasmic membrane and cell wall, a short hook, and a helical filament visually several times as long as the cell.
A bacterial flagellum usually consists of a single proteinaceous fibril —monofibrillar, which in some species has been d mionstrated to be composed of a sheath and a core. Flagella are composed of protein subunits called fiagellin which has the properties of fibrous proteins as kerasin -and myosin. The number and arrangement of flagella vary with different bacteria but they are generally constant for each species.
Motile bacteria exhibit their motion with the help of flagella which commonly have a rotary motion. Certain helical bacteria exhibit swimming motility, particularly in highly viscous media, yet they lack flagella. They have flagella-like structures located within the cell, just beneath the outer cell envelope.
These are called periplasmic flagella or axial flagella or endoflagella. Some other bacteria are motile only when they are in contact with a solid surface. As they glide they exhibit a sinuous, flexing motion, known as gliding motility.
Bacteria without flagella are known as atrichous (as in the genus Streptococcus) and those with a single polar flagellum—monotrichous (Fig. 333A) (as in the genera Vibrio, Spirillum), with a tuft of flagella at one pole—lophotrichous (Fig. 333 B & E) (as in the genus Pseudomonas), a tuft of flagella at each pole—amphitrichous (Fig. 333C) (as in Alkaligenes faecalis), or several flagella originating at various points over the surface of cell—peritrichous (Fig. 333D) (as in Escherichia coli and genus Salmonella).
Majority of the spherical bacteria are non-motile flagella being rare- among bacilli, there are many motile as well as non-motile species; and most spiral forms are motile.
Bacteria may possess very fine, hollow, nonhelical, filamentous appendages much smaller, shorter and more numerous than flagella which do not form regular waves as flagella do. These appendages are called pili (sing, pilus) or fimbriae (Fig. 331B). They measure less than 10nm in diameter and usually less than one pm long and can be seen only by electron microscopy.
Pili have no Junction for motility. There are many morphological types of pili, at least ten have been recognized.
They are named according to their function. Pili have several functions. One kind of pili known as F-pilus (or sex pilus) serves as the port of entry of genetic material (DNA) between donor and recipient cells during bacterial mating. The sex pili are little hollow protein tubules which serve as conjugation tubes in which there is just enough room for the DNA to transfer itself.
Some pili are concerned with attaching two bacterial cells together prior to the transfer of DNA from one cell to the other. Other types of pili cause bacteria to adhere to one another and to foreign cells, such as red blood cells, epithelial cells, etc. Whereas others serve as attachment sites for bacterial viruses- and pathogenic bacteria.
Again others may also serve to keep bacteria near the surface of liquid or where oxygen is most available.
Some bacteria produce semirigid extensions of the cell wall and cytoplasmic membrane known as prosthecae (sing, prostheca). Prosthecae increase surface area of cells for nutrient absorption and may also serve as aids in attachment to surfaces when they have adhesive substance at the end. Certain non-living ribbon-like or tubular appendage excreted by the cell is termed stalk.
Many bacteria produce a nonliving secretion of viscid material around the external surface of their cell wall.
According to the amount of material produced and the degree of its association with the cell, one of the three terms may be applied: capsules, slime layers and sheaths.
If the material is closely adherent to the cell and detectable only with difficulty it is known as a microcapsule; if equally sharply defined but extensive and readily visible it is called a macrocapsule; if copious in quantity and only relatively loosely associated with the cell and parts freely from it, it is slime layer or free slime or gum and it is designated as sheath ‘ when the amount of material is very insignificant.
The above viscous substance is composed of polysaccharides of several types—dextan, levan, and cellulose. It is believed that capsule and free slime are morphologically and biochemically distinct— the capsule is a part of the cell, whereas the free slime is a secretion.
Capsule formation depends upon the composition of the culture medium, but slime layer may be modified layer of the cell wall formed as an extracellular material of slimy or gelatinous nature.
Capsules are not essential structures because:
(i) They are not synthesized under all environmental conditions,
(ii) mutants exist which have lost the ability to produce them
(iii) cells from which they have been removed by enzymic digestion remain viable.
They may be 10µ thick and are stainable by copper salt. The presence of a capsule is often associated with the virulence of pathogenic bacteria as they are able to resist phagocytosis by the white cells of the blood. Capsules may act as ion exchangers.
Gapsule formation may be responsible for considerable economic loss in dairy and other food industries. Encapsuled bacteria are better protected against unfavourable conditions. They are pathogenic to humans. A number of bacteria often adhere together with their gelatinous material forming the zoogloea stage. The capsule may often function in the storage of food substances or waste materials.
In some aerobic bacteria the capsules form a raft in which are found the actively growing cells of other organisms. Bacterial capsules are species-specific and can therefore be used for immunological distinction of closely related species, for example, Acetobacter xylinum and Pseudomonas aeruginosa.
Note # 8. Cell Division of Bacteria:
Bacterial cells are haploid. In rapidly growing bacterial cells nuclear division proceeds ahead of cell division. The bacterial cell division is completed by the doubling of all the cell constituents followed by partitioning of the cell to produce two daughter cells.
The sequence of events is:
(i) Duplication of DNA,
(ii) Separation of two strands of DNA, and
(iii) Cross-wall formation leading to separation of two daughter cells.
The duplicated DNA separates into two strands and replicates individually. Each of the DNA strands are distributed in two daughter cells. During cross-wall formation, transverse plasma membrane is laid down which is followed by the centripetal growth of cell wall and the membrane is splitted into two halves. Sometimes chain of cells are formed by continued division.
Note # 9. Staining of Bacteria:
Due to smallness, bacteria are invisible to the naked eye. This necessiciates one to study the size, shape and structural characteristics of bacteria with the help of micros- scope which permits a wide range of magnifications. Many techniques have been developed by which specimens of bacteria can be prepared for examination in detail under the microscope.
Depending upon the principle of magnification, microscopes are of two categories:
light (or optical) and electron. Optical microscopy includes: bright-field, dark-field, ultraviolet, fluorescence and phase-contrast. Bright-field microscope is the most widely used instrument to study bacteria.
For this, two general techniques are employed to make microscopic preparations:
(i) Hanging-drop of suspension of organisms in a liquid; and
(ii) Dried, fixed, and stained films or smears.
Hanging-drop preparations are made by placing a drop of the bacterial suspension on a cover slip and inverting it over the concave area of a hollow-ground slide.
Fixed, stained preparations are more widely used for the observation of the morphological characteristics of bacteria. The essential steps in the preparation of a fixed, stained smear are: preparation of the film or smear, fixation and application of one or more staining solutions.
A large number of coloured organic compounds (dyes) are available for staining bacteria. These dyes may be acid, basic, or neutral. Acid dyes generally stain basic cell components, and basic dyes generally stain acidic cell components. A neutral dye is a complex salt of a dye acid with a dye base.
The staining process is dependent upon the nature of the bacterium. Some forms strain easily, others do not. Spores and flagella are difficult to stain.
Simple Staining Method:
A dried fixed smear of bacteria is flooded with a dye solution (say, methylene blue) for a specified period of time, after which this solution is washed off with water and the slide blotted dry. The cells of bacteria usually stain uniformly. This may not be true with some bacteria where the interior part of the cell may be deeply stained than the rest of the cell.
Other Staining Methods:
Several bacterial staining methods are used for studying bacteria.
Some of the commonly used staining methods are given below in brief:
(i) Gram staining:
In this process the fixed bacterial smear is subjected to the solutions of: Crystal Violet, Iodine solution, 95 per cent alcohol (decolourizing agent), and Safranin or some other suitable counter stain.
Bacteria stained by the Gram staining method fall into two groups: Gram-positive bacteria retain crystal violet and appear deep violet; and Gram-negative bacteria lose the crystal violet but appear red as they are stained by the Safranin. The Gram stain has its greatest use in characterizing bacteria.
Earlier workers used Gram-staining technique to classify bacteria into two groups: Gram-positive and Gram-negative. Even now Gram staining is widely used in bacteriological study. In addition to Gram-positive and Gram-negative bacteria, some bacteria are Gram-variable being Gram-positive at sometimes, and Gram-negative at others.
(ii) Acid-fast staining:
In this technique the fixed bacterial smear is subjected to the solutions—Garbolfuchsin (heated), acid-alcohol, and methylene blue in regular sequence. Bacteria retaining the Garbolfuchsin appear red are classified as acid-fast bacteria, and those decolourized by the acid-alcohol and counterstained by the methylene blue are designated as non-acid-fast bacteria.
(iii) Ziehl’s Carbol-Fuchsin:
This is a good spore stain. It is prepared by adding 10 c.c. saturated alcoholic solution of basic fuchsin to 100 c.c. of 5 per cent aqueous carbolic acid.
Flame fixed bacterial smear on a coverslip is to be treated for 5 to 10 seconds with 5 per cent aqueous chromic acid, washed in water, then stained in Ziehl’s Garbol-Fuchsin for one minute heating the solution until it steams, destained in 5 per cent aqueous sulphuric acid, washed in water, counterstained with methylene blue, to be washed dried in the air, and finally mounted in Canada balsam.
Staining combination of 5 per cent aqueous malachite green counterstained with 0 5 per cent aqueous safranin also gives good results.
Ziehl’s Garbol-Fuchsin stain with some procedural modifications is also used for staining bacterial flagella.
To demonstrate bacteria in the host tissue, stain combinations which give excellent result are: Garbol-Fuchsin and light green, Giemsa and safranin.
(iv) Endospore staining:
The endospore smear is allowed to remain in contact with crystal violet or methylene blue stain for a long time by gentle heating and then steaming for 30 to 60 seconds. Heating results in loosening of the vegetative cells from the smear and ultimately only the endospores remain attached to the slide.
Endospores are-coloured green, after washing they may be counterstained with safranin. After rinsing and drying the preparation is ready