The bacteria are the smallest and the most primitive unicellular organisms. They are mostly unicellular but few form colonies. They are of various shapes and the average size of the organizer is between 0.5 or 2m or even longer.
The form of the bacterium cell is generally constant and typical for each species. Morphologically, the bacteria are of the following types:
Every species of bacteria has a definite form and shape and on this basis of their structure, they are classified as under:
1. Cocci: They are oval or spherical in shape. If they occur singly, they are called as micrococcus; if in pairs - Diplococcus; if in a linear chain streptococcus; and if in clusters like grapes— Staphylococcus.
2. Bacilli: They are usually rod-like, may or may not have flagella. The flagella may be present at one or both the ends or around the entire cell surface. These bacteria may occur singly— bacillus; or may form filaments— Streptobacillus.
3. Vibrio: These bacteria are small and often curved like commas. They are usually motile and have a fiagellum at one end.
4. Spirillum: They are twisted like a screw and have flagella at one end or both the ends.
Bactria
Fungi algae
Under this system, organisms are classified into three domains and six kingdoms. The domains are Archaea, Bacteria, and Eukarya. The kingdoms are Archaebacteria (ancient bacteria), Eubacteria (true bacteria), Protista, Fungi, Plantae, and Animalia.
Organisms are classified into three Domains and into one of six Kingdoms of life. These Kingdoms are Archaebacteria, Eubacteria, Protista, Fungi, Plantae, and Animalia. Organisms are placed into these categories based on similarities or common characteristics
There are three basic bacterial shapes, according to "Mims Medical Microbiology." Round bacteria are referred to as cocci (singular: coccus); cylindrical, capsule-shaped bacteria as bacilli (singular: bacillus); and spiral bacteria are aptly called spirilla (singular: spirillum).
Comma-shaped bacteria - e.g. Vibrio Cholera.
Spherical-shaped bacteria (Cocci) - e.g. Staphylococcus and Streptococcus.
Rod-shaped bacteria (Bacilli) - e.g. E.Coli and Salmonella.
Spiral-shaped bacteria (Spirilla) - e.g. Treponema and Borellia.
Flagellated bacteria - e.g. Tetanus Bacteria.
It became very difficult to group some living things into one or the other, so early in the past century the two kingdoms were expanded into five kingdoms: Protista (the single-celled eukaryotes); Fungi (fungus and related organisms); Plantae (the plants); Animalia (the animals); Monera (the prokaryotes).
Microscopic organisms are tiny life forms, often consisting of a single cell, and very sensitive to change. They are vitally important in the food chain and to the health of our planet. They are the base of the marine food web and, directly or indirectly, are food for everything else in the open sea
On the basis of distribution of flagella, bacteria are classified as follows:
1. Atrichous: Flagella are absent.
2. Monotrichous: There is a single flagellum only.
3. Amphitrichous: There are two flagella, one at both the ends of bacterium.
4. Cephalotrichous: There are many flagella that occur at one end only.
5. Lophotrichous: There are many flagella that occur at both the ends of the bacterium.
6. Peritrichous: There are many flagella that occur equally distributed all over the surface of bacterium.
Bacteria are unicellular and ultra-microscopic organisms, yet they play an important role in nature. They are of tremendous importance to man. They play an important role in agriculture and medicine and are the basis of many industries. Some are beneficial to man directly or indirectly, others are very harmful as they cause various plant and animal diseases. Bacteria are considered as our friends and foes due to their beneficial and harmful activities. Their economic importance can be studied under two aspects:
(A) Beneficial aspects.
(B) Harmful aspects.
(A) Beneficial Aspects:
The beneficial activities of the bacteria can be classified as follows:
(1) Role in agriculture.
(2) Role in industry and medicine.
(1) Role in Agriculture:
The activities of bacteria are very important in agriculture in the following aspects:
(a) Decaying of organic substance:
Most of the bacteria are very useful in bringing about decomposition of dead organic matter of plants and animals by the secretion of enzymes. The enzymes convert the fats, carbohydrates and nitrogenous compounds into simpler forms, such as, CO2 water, ammonia, hydrogen sulphide, phosphates, nitrates etc that are used as raw material by the green plants. Thus, these bacteria not only decompose the organic compounds but also remove the harmful waste from the earth and thus function as nature's scavengers.
{b) Fertility of the soil:
Some bacteria maintain and others increase the fertility of the soil. They bring about physical and chemical changes in the soil by converting insoluble materials into soluble ones. These bacteria are the ammonifying, nitrifying and the nitrogen fixing Bacteria.
(i) Ammonifying Bacteria:
The decay bacteria decompose the proieinous compounds into amino acids, which are reduced to ammonia by ammonifying bacteria. The free ammonia combines in the soil to form ammonium salts. This conversion is known as ammonification. Examples are Bacillus ramosus, B. vulgaris etc.
(ii) Nitrifying Bacteria:
These bacteria convert ammonium salts into nitrates, which are absorbed by the plants. The nitrifying bacteria are the Nitrobacter and Nitrosomonas. The Nitrosomonas oxidize the ammonium salts into nitrous acid, which forms nitrites in the soil. The Nitrobacter then converts the nitrites into nitrates. This conversion of ammonium salts into available nitrates is called nitrification.
(iii) Nitrogen Fixing Bacteria:
These bacteria take up nitrogen from the atmosphere and convert it into organic nitrogen compounds. It is known as nitrogen fixation. The nitrogen-fixing bacteria are of two types. One type includes Azotobacter and Clostridium, which live freely in the soil and fix nitrogen of the air in their bodies in the form of nitrogenous organic compounds. The other types of bacteria are the nodule bacteria, the Bacillus radicicola. Rhizobium lives as symbiont in the roots of leguminous plants and forms nodules. These bacteria absorb free nitrogen from the bacterial cell. The leguminous plants thus enrich the fertility of the soil. They are grown for green manuring and rotation of crops.
(2) Role in Industry and Medicine:
Bacteria play a very important role in various industries. The products obtained as a result of bacterial activities cannot be chemically prepared. Their activities are involved in the following industries:
(a) Preparation of Alcohols:
Ethyl alcohol and butyl alcohol are manufactured by the bacterial acivities in the sugar solution, e.g., Clostridium acetobutylicum.
(b) Preparation of Vinegar:
Vinegar is prepared by the activities of Acetobacter aceti in the sugarcane juice.
(c) Preparation of Butter, Cheese etc.:
The preparation of butter, cheese etc. is done by bacteria. The Lactobacillus lactis is responsible for souring of milk resulting in curd (dahi) preparation. Bacterial activities also impart the typical flavours.
(d) Preparation of Tea, Coffee etc.:
Bacteria are very useful in preparation and flavouring of tea, coffee, cocoa etc. e.g., Micrococcus.
(e) Preparation of Tobacco:
Tobacco leaves are cured and flavoured by the bacteria. Typical types of bacteria are cultured for this purpose, e.g., Micrococcus.
(f) Preparation of Hemp fibres:
Fibres from the hemp are isolated after rotting the stems by activity of bacteria (e.g., Clostridium butyricum). The bacteria eat up the protoplasmic tissues but leave the sclerenchyma fibres.
(g) Preparation of Leather and Tanning:
The hairs and fats are removed from the skin by the action of bacteria in the leather industry.
(h) Preparation of Antibiotics:
The bacteria are also used in the preparation of antibiotics. According to Sir Alexander Fleming, the growth of harmful Staphylococcits is checked by Penicillium natatum. With this discovery, large number of antibiotics has been prepared which are of great importance in the medical world. Tyrothricin, Subtillin, Polyximin-B, Bacitracin, Streptomycin, Aureomycin, Terramycin are some well-known antibiotics.
(B) Harmful Aspects:
Bacteria are also harmful to man directly or indirectly. They cause various diseases in plants, human beings or domestic animals. The harmful bacteria are of the following types ;
(i) Animal pathogenic Bacteria.
(ii) Plant pathogenic Bacteria.
(iii) Food destroying Bacteria.
(iv) Soil fertility destroying Bacteria.
(i) Animal Pathogenic Bacteria:
There are a large number of parasitic bacteria, which cause various serious diseases in man and domestic animals, sometimes in epidemic form. They arc invisible enemies. Some of the common human diseases producing bacteria are Mycobacterium tuberculosis causing tuberculosis, Eberthela typhosa causing typhoid, Clostridium tetani causing tetanus, Shigella dysenteriae causing dysentery, Hemophilous influenzea causing influenza, Corynebacteriaum diphtlteriea causing diphtheria, Diplococats pneumonias causing pneumonia, Vibrio cholerae causing cholera, Streptococcus causing blood poisoning, Treponema pallidium causing syphilis, Gonococcus causing gonorrhoea, Bacillus pestis causing plague etc. In domestic animals various diseases are caused by bacteria, e.g., Anthrax, Pneumonia, Tuberculosis, Cholera, Glanders etc.
(ii) Plant Pathogenic Bacteria:
Many parasitic bacteria cause serious diseases in cultivated plants, which cause great harm to the crops. Important diseases are Citrus canker, fire blight of pear, cotton root rot, walnut blight, potato rot, pineapple rot etc. The Canker of Citrus (orange and lemon) is caused by Xanthonionas citri. The rot diseases cause black spots on potato, tomato, cabbage, carrot etc.
(iii) Food destroying Bacteria:
Some saprophytic bacteria are responsible for the decay of human foodstuffs including meat, milk, vegetables, fruits etc. These bacteria spoil foodstufs and make them unpalatable and poisonous, e.g., souring of milk, rotting of meat, vegetables, fruits etc. Staphylococcus and Clostridium botulinum cause food poisoning when rotten food is eaten,
(iv) Soil Fertility destroying Bacteria:
These are denitrifying bacteria in the soil, which reduce the nitrates, and the ammonium salts to free nitrogen, which escapes into the atmosphere. This process is known as denitrification, which decreases the fertility of the soil, e.g., Bacillus denitrificans.
These bacteria are often abunbant in the poorly drained and heavily manured soil. So the denitrifying bacteria are the natural enemies of the farmers.
Thus with the study of economic importance of bacteria we conclude that bacteria are our friends due to their beneficial aspects and enemies due to their harmful aspects. But in the bacteria beneficial aspects overweigh their harmful aspects. We can control the harmful activities but their beneficial activities cannot be replaced by artificial processes. So the bacteria are our friends more and enemies less.
Archaebacteria are the most primitive bacteria, probably by the first form of life, which evolved in a different line from that of true bacteria. These are adapted to extreme conditions like anoxygeny, pH, salinity and temperature. They differ from true bacteria by cell wall lacks murein, cell membrane with lipids (glycerol isopronyl ether) of branched alipathic chains, smaller size of DNA and base constituents in the RNA. These are of three types- Methanogens (methane producing, cause explosion in coal mines), Halophiles (live in extreme saline conditions, contain purple pigment bacteriorhodopsin for ATP synthesis from sunlight) and Thermophiles or Thermoacidophiles (withstand high temperature and acidity, mostly grow in hot sulphur springs).
Cyanobacteria (or blue-green algae) are photosynthetic organisms occur in terrestrial and aquatic habitats, sometimes forming planktons on the surface of freshwater and as endophytes. They may be unicellular, colonial or filamentous (trichome). Cyanobacterial cell is typically prokaryotic with a mucilage sheath (or slime layer or capsule) surrounding the cell wall which is made of peptidoglycan.
Cytoplasm is differentiated into outer coloured chromoplasm and central colourless centroplasm. Chromoplasm contains pigments, oil droplets, lipid, globules, cyanophycean starch and protein; carboxysomes rich in Rubisco enzyme for photosynthesis, 70S ribosomes and contractile vacuoles. Pigments are located on thylakoid membranes present in chromoplasm. Centroplasm contains the naked DNA (nucleoid). Some members possess special elongated cells called heterocysts economic importance. Spirulina is an important source of protein, Oscillatoria is used for sewage disposal by oxidizing organic wastes and Lyngbia is a source antibiotics.
Anything that divides a species into subgroups, which do not freely interbreed, is said to be the isolated members of the species. Dobzhanky (1937) used the term isolating mechanisms in his book “Genetics of the origin species”.
Isolating Mechanism:
This is broadly classified into two groups:
1. Geographic Isolation
2. Reproductive Isolation
Geographic Isolation:
When populations are separated by geographic barrier, it is called geographic isolation. Wagner (1868) was the first who stated that geographic isolation is a factor for formation of new species. The populations of two different localities are separated by geographic barrier like mountain, landmass, water body, forest desert etc. the high mountain range, deserts, dense forests and extremes of the temperature also acts as effective barrier. e.g. finches of Darwin are geographically isolated in different areas of the Galapagos islands. Such isolated populations are completely cu off another genetically. Thus, genetic divergence develops in the isolated population.
3. Reproductive Isolation:
An isolating mechanism that prevents the gene exchange between Mendelian populations is called reproductive isolation. It is sub-divided into two groups: (i) Pre-zygotic isolation (ii) post-zygotic isolation.
(i) Pre-zygotic Isolation:
This mechanism prevents inters specific crosses (i.e. fertilization and zygote formation).
It is of following types:
(a) Ecological or Habitat Isolation:
It occurs when species occupy different habitats in the same territory.
(b) Seasonal or Temporal Isolation:
It is found between populations in which the members reach sexual maturity or flowering at different times of the year.
(c) Ethological Isolation:
In some difference in the sexual behaviuor (mating, dance, song, light, scent) prevents interbreeding.
(d) Mechanical Isolation:
In animals the structural difference in the external genetalia prevents interbreeding. In flowers, structural difference also prevents pollen transfer.
(e) Physiological Isolation:
In this mechanism, two different species mate but fertilization is prevented by Physiological differences between them.
(ii) Post-zygotic Isolation
(a) Gametic Mortality:
When there is mating between two different species, to the gametes are killed because of antigenic reaction in the genital tracts.
(b) Zygitic Mortality:
When fertilization is successful by the interbreeding of two species the zygote dies.
(c) Hybrid Sterility:
During inter-specific interbreeding viable hybrids are produced but they are sterile.
Role of Isolation in Speciation:
Geographic isolation is essential factor for development of genetic diversity and subsequent origin of species. The allopatric speciation in undisputed as speciation start at the complete geographic isolation. Sympatric speciation (speciation occurring in single population) can only occur under restrictive conditions and it has been controversial. (Bush 1994)
Colony Morphology of Bacteria; How to describe Bacterial Colonies?
Bacteria grow on solid media as colonies. A colony is defined as a visible mass of microorganisms all originating from a single mother cell. Key features of these bacterial colonies serve as an important criteria for their identification.
Characteristics of Bacterial colonies
Colony morphology can sometimes be useful in bacterial identification. Colonies are described as to such properties as size, shape, texture, elevation, pigmentation, effect on growth medium. In this blog post you will find common criteria that are used to characterize the bacterial growth.
Colony Shape: It includes form, elevation and margin of the bacterial colony.
Form of the bacterial colony: – The form refers to the shape of the colony. These forms represent the most common colony shapes you are likely to encounter. e.g. Circular, Irregular, Filamentous, Rhizoid etc.
Elevation of bacterial colony: This describes the “side view” of a colony. These are the most common. e.g. Flat, raised, umbonate (having a knobby protuberance), Crateriform, Convex, Pulvinate (Cushion-shaped)
Margin of bacterial colony: The margin or edge of a colony may be an important characteristic in identifying an organisms. Common examples are Entire (smooth), irregular, Undulate (wavy), Lobate, Curled, Filiform etc. Colonies that are irregular in shape and/or have irregular margins are likely to be motile organisms. Highly motile organism swarmed over the culture media. Such as Proteus spp.
Size of the bacterial colony: The size of the colony can be a useful characteristic for identification. The diameter of a representative colony may be measured in millimeters or described in relative terms such as pin point, small, medium, large. Colonies larger than about 5 mm are likely to be motile organisms.
Appearance of the colony surface: Bacterial colonies are frequently shiny and smooth in appearance. Other surface descriptions might be: dull (opposite of glistening), veined, rough, wrinkled (or shriveled), glistening.
Mixed growth of mucoid Lactose fermenting colonies and NLF colonies in MacConkey Agar
Consistency/Texture : Several terms that may be appropriate for describing the texture or consistency of bacterial growth are: dry, moist, viscid (sticks to loop, hard to get off), brittle/friable (dry, breaks apart), mucoid (sticky, mucus-like)
Color of the colonies (pigmentation): Some bacteria produce pigment when they grow in the medium e.g., green pigment produces by Pseudomonas aeruginosa, buff colored colonies of Mycobacterium tuberculosis in L.J medium, red colored colonies of Serratia marcescens.
Opacity of the bacterial colony: Is the colony transparent (clear), opaque (not transparent or clear), translucent (almost clear, but distorted vision–like looking through frosted glass), iridescent (changing colors in reflected light).
Some important terminologies
Draughtsman colonies
Draughtmans colonies of S. pneumoniae
Young colonies of Streptococcus pneumoniae(pneumococci) have raised center but as the culture ages, they become flattened, with a depressed central part and raised edges giving them a ringed appearance also known as ‘draughtsman colonies’.
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Pathogenic Microbes with characteristics smell (good and bad)
Microorganisms can produce different types of volatile compounds which may give characteristics smell, pleasant scent or pungent odor. Production of these volatile chemicals depends on the metabolic characteristics of that particular organism. Sometimes scent or odor produced by bacteria can give important clue in the identification of microorganisms, but it is not a reliable rule.
You may have seen Microbiologist/technician trying to “smell the difference” between bacterial culture. Though, some laboratory personnel regard sniffing as a useful tool other regards it as a biohazard (1). I suggest you to be cautious while sniffing culture plates. Sniffing culture plates is not a good idea and you may contract a disease (2).
Pungent/Unpleasant smellAnaerobes are particularly pungent due to their reliance on sulfhydryl compounds to maintain redox balance. When an anaerobic infection is suspected, specimen is often foul-smelling. Gram negative anaerobes are often responsible for ‘morning breath’.
Bacterioides fragilis: Pus containing Bacteroides species has a very unpleasant smell.
Under anaerobic conditions, Clostridium perfringens multiplies and produces alpha toxin and other toxins which result in therapid destruction of tissue carbohydrate with the production of gas in decaying tissues, particularly muscle. Affected tissue is foul-smelling.
Yeast like smell: The colonies of Candida albicans have a distinctive yeast smell.
Smell like a soil Certain cyanobacteria and actinomycetes synthesize geosmin, a volatile chemical that is reminiscent of potting soil.
Rotten cooked fishy odor: Proteus mirabilis produces a very distinct fishy odour. On Salmonella-Shigella (SS) agar, Proteus usually smells like “rotten cooked fish”.
Sweet grape-like scent Pseudomonas aeruginosa produces a sweet grape-like scent, so wound dressings and agar plates are often sniffed for organism identification. Pseudomonas aeruginosa can famously generate a “grape juice” smell in infected burn patients (3). Cultures of pseudomonas have a distinctive smell due to the production of 2-aminoacetophenone.
Ammoniacal smell
Woman infected with Gardnerella vaginalis complaints of grey, offensive, fishy ammoniacal smell. Fishy ammoniacal smell becomes more intense after adding a few drops of 10% potassium hydroxide.
pseudomallei cultures give off an ammoniacal smell.
Bleach-like odor: Eikenella corrodens, a gram negative rod, responsible for wound infections gives a bleach-like odor when grown in Blood Agar or Chocolate Agar.
Caramel odor: Streptococcus milleri produces diacetyl (caramel odor). The detection of diacetyl (caramel odor) can be used in presumptive identification of the “Streptococcus milleri” group (4).
