Bacterial Classification

The taxonomy of bacteria is not very definitively worked out yet, especially the higher levels of classification. Some authorities believe that the degree of variance between different bacterial groups is sufficient to give them each 'Kingdom Status' of their own. Thus in the 9th edition of "Brock: Biology of Microorganism" you will find reference to 13 Kingdoms of Bacteria. From the point of view of these pages it is not really important whether you think of the different categories as Phyla or Kingdoms as long as you are aware that the bacteria are an incredible diverse group of organisms. Here I have followed the classification scheme laid out in the 2nd edition of "Bergey's Manual of Systematic Biology".


Gram Staining

You will find bacteria referred to as 'Gram +' or 'Gram Positive' and 'Gram -' or 'Gram Negative' this is a reference to how the bacteria responds to the Gram staining method. Staining methods are designed to make a staining agent bind to the cell wall of the bacteria. The Gram staining method is named after Christian Gram who invented the method in 1884.

In testing for gram stain response, microbiologists first spread some bacteria on a slide, then fire it by passing the slide through a flame briefly. The next step is to flood the slide with crystal violet solution for 1 minute. Then they add iodine solution for 3 minutes - at this stage all cells are purple. Adding alcohol for 20 seconds results in Gram negative cells becoming clear again, ie they lose their purple staining. Lastly, the cells are restained with safranin. This results in gram positive cells remaining purple and gram negative ones being red or pink. Gram staining is nearly always the first step in identifying a new sample or species of bacteria. Nowadays, gram staining can be done in one step using a fluorescent dye and a fluorescence microscope.

The Phyla of Bacteria (as used on this site and taken from Bergey's Manual of Systematic Bacteriology 1st Ed.)
Name of Phylum Number of Species Number of Genera
Aquificae 27 12
Xenobacteria 29 11
Chrysogenetes 1 1
Thermomicrobia 13 6
Cyanobacteria 78 62
Chlorobia 17 6
Proteobacteria 1644 366
Firmicutes 2474 255
Planctomycetes etc. 13 5
Spirochaetes 92 13
Fibrobacter 5 3
Bacteroids 130 20
Flavobacteria 72 15
Sphingobacteria 76 22
Fusobacteria 29 6
Verrucomicrobia 5 2

Phylum 1 - Aquificae

This is a small group of thermophilic to hyperthermophilic chemolithotrophic bacteria, meaning that they derive their energy from inorganic molecules and they live in hot environments. Members of the genus Aquifex can live at temperatures as high as 95 degrees C and they have an optimum growth temperature of 85 degrees C. Aquifex is an aerobic bacterium but it can only tolerate low quantities of oxygen. Most of the rest of the phylum are anaerobic and cannot tolerate the presence of oxygen at all. Aquifex utilizes H2S or S2032- as its energy source.

Thermodesulfobacterium is another member of this group. As its name suggests, it is a sulphur reducing bacterium, reducing SO42- to H2S. Thermodesulfobacterium likes it a little cooler than Aquifex having an optimum growth temperature of 70 degrees C, and it is anaerobic.

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Phylum 2 - Xenobacteria

This group comprises a number of aerobic chemoorganotrophic bacteria. The two best studied genera are Thermus and Deinococcus. Thermus is a thermophilic bacterium. The enzyme Taq DNA Polymerase comes from Thermus aquaticus. This is the major enzyme used in Polymerase Chain Reaction (PCR) techniques for amplyfying DNA. The best known Deinococcus species is D. radiodurans, so named because of its incredible ability to survive high does of radiation. D. radiodurans can survive 30,000 Gy of ionizing radiation (1 Gy =100 Rad). A human being can be killed by less than 5 Gy, making D. radiodurans 6000 times better at surviving radiation than us.

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Phylum 9 - Planctomyces and Allies


Chlamydiaceae is the smallest of the the two families in this phyla , containing only 3 species all in the genus Chlamydia. All three are obligate parasites of warm-blooded animals; C. trachomatis and C. pneumoniae of humans and C. psittaci of birds and occasionally mammals, including humans. All are pathogenic. C. trachomatis causes trachoma the leading cause of blindness in humans, as well as otitis, a non-gonococcal urethritis, urethral inflammation, Lymphogranuloma venereum and cervicitis. C. pneumoniae causes a variety of respiratory problems similar to pneumonia. C. psittaci causes epidemic Psittacosis in birds, particularly parrots, as well as pneumonia, arthritis and conjunctivitis in young mammals such as kittens, calves, foals and piglets.



This is another small but distinct group of bacteria. They are unique because of the stalk that they produce, which unlike that in Caulobacter is made of protein. They are also interesting because they are budding bacteria. The stalk is believed to be used as a means of attachment to substrates. They are primarily aquatic aerobic chemoorganotrophs. There are 4 genera and 10 species; Pirellula, Planctomyces, Gemmata and Isophaera a gliding filamentous form. Little is known about the ecology of any of these genera.






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