There are following groups of bacteria
- Archeobacteria: They are most primitive organism on the earth. The lack peptidoglycan in their cell wall. The live in the deep sediments of oceans. Example: methanogenic bacteria
- Actinomycetes: They form colonies of branching hyphae tubular filaments resembling those of fungi). Reproduction takes place by fragmentation of the end of hyphae into spores. Most actinomycetes live in the organic litter of soil. Example: Mycobacterium.
They have flagellated rods. They produce endospores. They are both obligate anaerobes and aerobes. Example: Bacillus, Clostridium
- Enteric bacteria: These are gram-negative, facultative anaerobes. They undergo anaerobic respiration using NO3 as the electron donor. They inhabit the intestinal tracts of animals. Example: Escherichia cob
- Mycoplasma: They are the smallest bacteria. They lack cell wails. They grow extracellularly. They are saprobes and animal pathogens. Example: Mycophanta
S. Myxobacteria: They are soil-dwelling chemoheterotrophs. Individual cells move by gliding. The cells form fruiting body during dry season. Spores are released and grow into new colonies. Example: Myxococcus
- Nitrogen fixing aerobic bacteria: They are common free-living and inutualistic species. The genus Rhizobium lives in root nodules on leguminous plants. It fixes the atmospheric nitrogen. Example: Azotobacter, Rhizobium.
- Phototropic anaerobic bacteria: They are photoautotroph. They have different photosynthetic equipment than plants. They use H2S as electron source. They are anaerobes. They are found in pond, lake, and ocean sediments. They have different groups like bacteria and the green sulfur bacteria. Example: Chromatium
- Pseudomonads: They are present in nearly all aquatic and soil habitats. Their cells are rod-shaped with gram-negative cell wall. They are flagellated at one end. They are chemoheterotrophs. Example: Pseudomonas
- Rickettsias and Chiamydias: They are obligate intracellular parasites of animals. They have reduced gram-negative cell wall. Rickettsias alternate between arthropod and mammal hosts. chey cause different diseases. Example: Rickettsia Chlamydia
- Spirochetes: They have helical cells. They sometimes become very long. They show corks crew like movement. They are free-living saprobes and parasites. Example: Borrelia