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Current Category » Introductory Plant Pathology

Classification of Bacteria or Prokaryotes – Volume I

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.


The spirochetes, the two important families are i.e. i. Leptospiracear, important genus : Leptospira and ii. Spirochaetaceae, important genus is Spirochaets.
Section -2:

Aerobic or microacerophilic, motile, helical or viborid gram negative bacteria. The important genera (of academic interest) in this section are: Azospirillium, Campylobacter and Badellovibrio. Azospirillium cells are vibrioid with single polar flagellum, present or found within the roots of grasses, wheat, corn and other plants or as free-living soil organisms. They are either aerobic or microerophilic and fix N2 within plant roots. Important species are A.limpoferum and A.brasilense.

Non – motile, Gram negative curved bacteria: Family- Spirosomaceae, Genera: Spirosoma, Runella and Flectobacillus.


Gram –negative, aerobic rods and cocci: It is on the largest section containing most diverse group of bacteria important in agriculture, such as:

i) Family: Pseudomonadaceae, Three Important genera are………

a. Pseudomonas:

Several species are pathogenic to humans, animals and plants; cause spoilage of meat and other foods, species like P. syringae, cause diseases like leaf spots, leaf stripe, wilt and necrosis, P. fluorescens, is common soil saprophyte that produce a fluorescent pigment.

b. Xanthomonas:

Produce characteristics yellow pigment Xanthomonadin, all species are pathogenic to plants causing diseases such as spots, streaks, cankers, wilts and rots. Xanthomonads, produce exocellular polysaccharides i.e. xantham gums useful in industrial application.

c. Zoogloea:

Cells are embedded in gelatinous matrix to from slimy masses with finger like morphology. Species are saprophytic, commonly found on tricking filter beds, in sewage treatment plants, where they oxidize the organic matter of the sewage.

2. Family: Azoobacteriacea

Two important genera are Azotobacter and Azomonas. The species are saprophytes found in soil, water and plant rhizosphere, fix N2 under aerobic conditions and forms desiccation resistant spores called “Cysts”.
3. Family: Rhizobiaceae:

Three important genera are Rhizobium, Brandyrhizobium and Agrobacterium. Bacteria of the genus Rhizobium and Brandyrhizobium fix atmospheric. N2 symbiotically in legumes by including root nodules. Whereas, species of the genus Agrobacterium, do not fix N2 but they are plant pathogenic inducing tumors in crown, roots and stems of dicotyledons.  
4. Family: Methylococcaceae:

 Two important genera are Methyloccus and Methylomonas. Bacteria are obligate methane- Oxides, use methane gas as a sole carbon and energy source under aerobic and microacerophilic conditions.

5. Family: Acetobacteriaceae:

Two important genera are Acetobacter and Glucanobacter Member of these two genera are saprophytes, found in sugar or alcohol, enriched acidic environments such as flower, fruits , bear , wine , vinegar, honey etc. They are industrially important. Acetobacter peritrichous are used to make vinegar and Glucanobacters (Polar Flagella) are involved in the manufacture of chemical like dihydroxyacetone, sorbose etc.

Section -5:       

Facultatively anaerobic Gram-negative rods-

Two important families and important genera in respective families are:

1. Family: Enterobacteriaceae, Important genera are…………

a. Escherichia:

E.g. E.coli, inhabitant of lower portion of the intestine of human and warm-blooded animals, caused gastrogenteritis and urinary tract infections.

b. Shigella:

Species are pathogenic, causing bacillary dysenter in human called Shigellosis.
c. Salmonella:

All species are pathogenic in humans causing enteric fevers typhoid and paratyphoid fevers gastroenteritis and septicaemia.

d. Enterobacter:

Species occur in water, sewage, soil, meat plants and vegetables. Some species are opportunistic human pathogens.

e. Erwinia:

Species mainly associated with plants, causing disease such as blights, cankers, die-back, leaf spot, wilts, discolouration of plant tissues and soft rots.

f. Yersinia:

These are parasites of animals but can also cause infections in humans such as plague. (Y.pestis).



Important Genera are: Vobrio, Aeromonas

Most Vibrio species are harmless saprophytes, but some species are pathogenic in human. E.g. V. cholerae, causing cholera in humans.

Section-7: Dissimilatory Sulfate or Sulphur reducing Bacteria:

The bacteria or organism belonging to this section are obligate anaerobes using sulfate, sulphur or other oxidized sulphur compounds as election acceptors and reducing them to H2S. These are gram-negative found in mud and marine environments and in intestinal tract of humans and animals. The important genera are Desulfovibrio (Vibrioid and helical cells), Desulfococcus (Spherical cells) and Desulfosarcina.

Section-8: The Rickettsias and Chlamydias:

These are tiny, non motile, Gram- negative bacteria. They are obligate parasites, able to grow only within host cells. The two important genera are Rickettisia and Coxiella. The species of Rickettisai caused diseases like Rocky Mountain spotted fever, classical typhus fever, scrub typhus and the single of species of genus Coxiella causes Q fever a type of pneumaonia (Coxiella burnetii).

Section-9: The Mycoplasma:

These are very small organisms devoid of cell wall. Because of lack of cell wall, mycoplasmas are not inhibited by penicillin antibiotic; however they can be inhibited by antibiotics that affect protein synthesis (E.g. tetracycline or Chloramphenicol). They can be cultivated invitro (In laboratory) on synthetic media as facultative anaerobes or obligate anaerobes. Mycoplasmas are placed in the Division –Teericutes , class –Mollicutes and Order- Mycoplasmatales, containing three families, viz.

i) Mycoplasmataceae, genera Mycoplasma and Ureplasma, 
ii) Acholeplasmatoceae, genus Acholeplasma
iii) Spiroplasmataceae, genera, Spiroplasma, Anaeroplasma and Thermoplasma

Species of the genus, Mycoplasma are pathogenic to human and animals, E.g. M. pneumoniae, causing primary atypical pneumonia in humans, members of the genus Ureplasma cause arthritis in humans, pneumonia and urogential disease in cattle. Citrus stubborn is one of the important disease caused by Spiroplasma (S. citri) in citrus.

Current Category » Introductory Plant Pathology