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Current Category » Introductory Plant Pathology

Classification of Bacteria or Prokaryotes – Volume III

The classification of bacteria or prokaryotes (of significance in agriculture and allied fields) given in “Bergey’s Manual of Systematic Bacteriology” is listed as follows.

Volume: III

Section 18: Anoxygenic Phototrophic Bacteria:

These bacteria are Gram-negative, photolithographic or photoautotrophic containing bacteriochlorophyll and various water soluble carotenoid pigments. These bacteria grow phototrophically only under anaerobic conditions and are incapable of forming O2 (hence called Anoxygenic). Anoxygenic phototrophic bacteria are divided into two major groups, on the basis of their pigmentation i.e a) Purple bacteria and b) Green bacteria.

a) Purple Bactria:

Have two important families are i) Chromatiaceae (Purple sulphur bacteria) and ii) Rhodospirillaceae (Purple non-sulphur bacteria)

i) Family, Chromatiaceae:

In this have four important genera viz. Chromatium (cells ovoid to rod shaped), Thiocystis (coccoid), Thiospirillum (helical), Thiosarcina. All genera are photolithographic H2S or elemental sulphur as the electron donor for CO2 fixation. Most species are anaerobic and cannot grow in the dark even under microaerophilic conditions.

ii) Family, Rhodospirillaceae:

It contains purple, non-sulphur bacteria with variable shapes: helical (Rhodospirillum), ovoid or spherical (Rhodopseudomonas) or spherical (Rhodopseudomonas) or ovoid (Rhodomicrobium). They are chemoorganotrophs for which organic substances serve both as source of carbon and electrons for the reduction of CO2. Photosynthesis occurs only under anaerobic conditions in the presence of light.

b) Green Bacteria: Two families and Important Genera are

i) Family: Chlorobiaceae, genera: Chlorobium (Green sulphur bacteria) Prosthecochloris.

ii) Family: Chloroflexaceae, genus: Chloroflexus (Green non-sulphur bacteria)           

Section 19: Oxygenic Phototrophic Bacteria:

The typical bacteria included in this section are Cyanobacteria ( Blue green algae). These are widespread in soil, freshwater, and marine habitats. Cyanobacteria are useful in N2 fixation, adding organic matter in soil and preventing incipient erosion. Some Cyanobacteria grow in symbiosis with other organisms. E.g. Lichen. They are also associated with certain protozoa, called as “Cyanellae”. Cyanobacteria contain chlorophyll ‘a’ rather than bacteriochlorophyll and other pigments such as water-soluble cartoenoids and phycobillins. BGA plays important role in N2 fixation in paddy crop. E.g. Anabaena, Nostoc, Chlorela, etc.     

Section 20: Aerobic, Chemolithotrophic bacteria and associated organisms:

This section includes chemolithotrops (E.g. nitrifying bacteria) , colourless sulphur bacteria ( Thiobacillus, Thiospira), obligately chemolithotrophic hydrogen bacteria ( Hyderogenobacter),iron and manganese oxidizing and or depositing bacteria siderocapsa and magnetotatic bacteria Aquaspirillum.  The families and important genera under this section are as follows.

Family:

Nitrobacteriaceae, Nitrifying bacteria are morphologically either, rods, cocci or helical. They are aerobic autotrops, incapable of chemolithotrophic growth with the exception of one species, Nitrobacter wingoradskyi. They nitrifying bacteria comprises two distinct metabolic groups viz. i) ammonia oxidizers ( oxidize ammonia to nitrate) E.g. Nitrosomonas, Nitrosolobus, Nitrosococus and Nitrosospira, and ii) Nitrate oxidiers ( oxidize nitrate to nitrate) e.g. Nitrobacter, Nitrococcus, Nitrospina. Nitrifying bacteria are commonly present in soil, where they play in important role in the nitrogen cycle, N2 fixation and in maintaining the fertility of soil.

Sulphur Bacteria:

The cultivable genera of sulphur metabolizing bacteria are: Thiobacillus ( Gram –negative , short rods) and Thiomicrospira ( helical). Both genera are widely distributed in soil, fresh water, coal-mines, and marine environments. They derive energy from oxidation of reduced sulphur compounds (E.g. Sulfides, elemental sulphur, thiosulfate, sulfite etc.) and the final oxidation product is sulphate. Some species are obligate autotrophs (E.g. Thiooxidans, T. thioparatus and Thiomicrospira pelophila) and others are facultative autotrophs (E.g. Thiobacillus novellas).

Section 25: Archaeobacteria or Archaebbacteria:

The Archaebacteria differs from eubacteria in at least two bio-chemical traits. i.e. i) they lack peptidoglycan and nuranic acid in their cell walls and ii) Membrane lipids or fatty acids are either – linked to glycerol in Archaebacteria and esterlinked in eubacteria. At present, at least three main categories of Archaebacteria are recognized. a) Methane producers, b) the red extreme halophiles and c) the thermoacidophiles.

a) Methenogenic Bacteria:

These are stringent anaerobics that obtain energy through oxidation of H2 or format and reduce CO2 with the formation of methane gas (CH4). The important genera of methane –producing or Methenogenic bacteria are: Methanobcterium ( Gram-negative , short rods), Methanosarcina ( Gram –positive Cocci) , Methanococcus ( Gram-positive , pleomorphic cooci), methanomicrobrium ( Gram-negative , short rods) , Methanogenium ( Gram-negative , pleomorphic cocci), Methanomicrobium ( Gram-negative, short rods), Methanogenium ( Gram – negative , pleomorphic cocci) and Methanospirillum ( Gram-negative , curved rods or filaments).

b) Extreme Halophiles:

These are chemoorganotrophic, Gram – negative, aerobic rods (E.g. Holobacterium) or cocci (Helococcus).

c) Thermoacidophiles:

These are aerobic, Gram-negative Archaebacteria with ability to grow under highly acidic conditions at high temperatures. Two important genera are: Thermoplasmas and Sulfolobus.

Current Category » Introductory Plant Pathology