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Bacteria
Bacteria belong to the group of prokaryotes (with no defined nucleus) whose structures vary from those of eukaryotes (those possessing defined nucleus) in many ways.
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Differences between eukaryotes and prokaryotes in cell structure
Structural Features |
Eukaryotes (Fungi, Protozoa, Algae) |
Prokaryotes (bacteria including blue greens) |
1. Nuclear structure |
Nucleus with chromosomes associated with histone Proteins |
Single naked circular chromosome |
2. Nuclear membrane |
Present |
Absent |
3. Endoplasmic reticulum |
Present |
Absent |
4. Mitochondria |
Present |
Absent |
5. Ribosomes |
80S type |
70s type |
6. Lysosomes |
Present |
Absent |
7. Plasma membrane |
Present, contains sterols |
Present, no sterols except in mycoplasma |
8. Cell wall |
Absent or composed of cellulose or chitin |
Complex structure with peptidoglycan layer, protein and lipids |
9. Capsule |
Absent |
Frequently present |
They are the most dominant group of microorganisms in soil and probably equal one half of the microbial biomass in soil. They are present in all types of soil but their population decreases as the depth of soil increases. In general, horizon A of a soil profile consists of more microorganisms than B and C horizons. Under anaerobic conditions (in the absence of oxygen), bacteria dominate the scene and carryon microbiological activities in soil since fungi and actinomycetes do not grow well in the absence of oxygen.
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Bacteria live in soil as cocci (spheres, 0.5 µ), bacilli (rods, 0.5 to 3.0 µ) or spirilli (spirals). The bacilli are common in soil whereas spirilli are very rare in natural environments. In 1925, Winogradasky classified soil microorganisms in general and bacteria in particular into two broad categories―the autochthonous and the zymogenous organisms. The autochthonous or indigenous population is always uniform and constant in soil since their nutrition is derived from native soil organic matter (example, Arthrobacter and Nocardia).
On the other hand, zymogenous or fermentative organisms require an external source of energy and their normal population in soil is low (examples, Pseudomonas and Bacillus). When specific substrates are added to soil the number of zymogenous bacteria increases and gradually declines when the added substrate is exhausted.
To this category belong the cellulose decomposers, nitrogen utilizing bacteria and those splitting ammonium into nitrate.
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Growth in the presence or absence of oxygen is taken as the criterion to distinguish bacteria into anaerobic, aerobic and facultative anaerobic, that is, those capable of developing under oxygenated as well as non-oxygenated conditions.
Under the Bergy's system of Determinative Bacteriology, bacteria are classified into taxonomic groups, orders, families, genera and species based on the classical Linnaen concept of binomial nomenclature. Ten orders are included in the class Schizomycetes. Of these, three orders―Pseudomonadales, Eubacteriales and Actinomycetales contain the species of bacteria which are predominantly encountered in soil.
The most common soil bacteria come under the genera Pseudomonas, Arthrobacter, Clostridium, Achromobacter, Bacillus, Micrococcus, flavobacterium, Corynebacterium, Sarcina and Mycobacterium. Escherichia is encountered rarely in soils except as a contaminant from sewage whereas Aerobacter is frequently encountered and is probably a normal inhabitant of certain soils.
Another group of bacteria common in soils is the myxobacteria belonging to the genera Myxococcus, Chondrococcus, Archangium, Polyangium, Cytophaga and Sporocytophaga. The latter two genera are cellulolytic and hence are dominant in cellulose-rich environments. Myxobacteria feed on other Gram-negative bacteria through lysis.
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It is not easy to determine the total population of bacteria in any soil accurately. Apart from the inherent limitations of the soil dilution and plate methods, their numbers vary with the texture, water content and many other parameters especially the availability of organic substrates in soil.
Bacteria can withstand extremes of climate although temperature and moisture influence their population. In Arctic zones where temperature is below the freezing point, bacteria can thrive as luxuriantly as they do in arid desertic soils where temperatures are very high. The inherent faculty of many bacteria to form spores possessing tough outer covering facilitates.
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| A.Diplococci |
B.Streptococci |
C.Staphylococci |
D.Bacilli |
E.Coccobacilli |
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| F.FusiformBacilli |
G.Filamentous Bacillary Forms |
H.Virbios |
I.Spirilla |
J.Sarcinae |
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| A.Monotrichous |
B. Amphitrichous |
C. Lophotrichous |
D. Peritrichous |
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Bacterial Forms and Structure
| 1. Robosomes |
2. Cytoplasm |
| 3. Cytoplasmic Membrane |
4. Nucleoid |
| 5. Mesosome |
6. Pili |
| 7. Capsule |
8. Cell Wall |
| 9. Flagellum |
10. Granular Inclusion |
| A. Bacillus Megaterium |
B. Azotobacter Chroococcum |
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| C. Desulfovibrio |
D. Desulfovibrio Sp |
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The survival of bacteria in all adverse environments. Survival by spore formation under extreme conditions ought to be differentiated from tolerance to different temperature ranges, which is one of the factors determining the population of bacteria in soil.
Based on this criterion, bacteria are grouped as mesophiles (15 to 45°C), psychrophiles (below 20°C) and thermophiles (45 to 65°C). The mesophilic bacteria, however, constitute the bulk of soil bacteria. Other factors affecting bacterial population in soil are pH, farm practices, fertilizer and pesticide applications and organic matter amendments.
Bacteria are also classified on the basis of their nutritional requirements into those requiring amino acids, B-vitamins, amino acids + B-vitamins, unidentified growth factors in yeast extract or soil extract and soil extract + yeast extract. The source of B-vitamins and other growth factors in soils is difficult to explain and the occurrence of fastidious species in soil requiring growth substances can only be explained on the basis of mutual dependence of different bacterial strains on extracellular products.
Autotrophic as well as heterotrophic bacteria are present in soil. Autotrophs synthesize their own food whereas heterotrophs depend on preformed food for nutrition. Photoautotrophs are those whose food energy is derived through mediation of sunlight, as in the instance of photosynthetic bacteria as opposed to chemoautotrophs which oxidize inorganic materials to derive energy and at the same time utilize the carbon from CO2 for growth. In the latter category, a group of bacteria known as obligate chemoautotrophs, are included which prefer specific substrates.
Examples of this kind are Nitrobacter which utilizes nitrite, Nitrosomonas which utilizes ammonium, Thiobacillus which converts inorganic sulphur compounds to sulphate and Ferrobacillus capable of converting ferrous iron to ferric iron. Several of the reactions involved in nitrogen transformations in soil depend on the chemoautotrophic Nitrobacter and Nitrosomonas and hence chemoautotrophy of bacteria in soil is intimately related to crop production.
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