With 130 species in 20 genera this is the third largest phylum of bacteria. It contains a variety of physiological types including both obligate aerobes and obligate anaerobes. The genus bacteroides mostly consists of commensals living in the guts of animals and as such are obligate anaerobes. They are probably the most common bacteria in the human large intestine where 1010 to 1011 cells can occur per gram of faeces. Some species can also be pathogenic causing gastrointestinal disorders.
The genus flavobacterium contains primarily aquatic species though they are also found in food processing plants. F. meningosepticum is believed to be a cause of infant meningitis.
The genera Cytophaga and Sporocytophaga are long gliding rods in form. They are widespread soil species with the habit of attaching themselves to cellulose strands before digesting them. Both genera are obligate aerobes and are ecologically important as cellulose degraders in aerobic environments. Cytophaga spp. can also be pathogenic (cause disease) in fish, ie C. columnaris causes Columnaris disease and C. psychrophila causes cold-water disease.
This is a small phylum of 17 species sometimes referred to as Green Sulphur Bacteria. They are all obligately anaerobic phototrophic species. Most species contain bacteriochlorophyll a and either c, d or e. Light energy trapped by bacteriochlorophylls c, d and e is channelled to the bacteriochlorophyll a which is where the photosynthesis takes place. Chlorobium tepidum is a thermophilic species which forms dense microbial mats in hot springs with a high sulphide content. Some species in this group have the interesting habit of 'consorting' with other larger non-phototrophic bacteria. Normally between 12 and 20 Chlorobium spp. will be in consort with a single non-chlorobium cell. These aggregations are called 'consortia'. They are more than just casual groupings because both the Chlorobium and the non-chlorobium cells divide at the same time. Scientists do not yet know the ecological significance of these 'consortia'.
Spirochetes are a distinct group of bacteria, they are gram negative and most of them are tightly-coiled, long and slender in shape. They have one or more flagella (up to 100) at each end of their cells which when rotated allow them to move. There is a great diversity of size within the spirochetes and the larger species can be 0.25 of a millimetre long while being only 0.00075 millimetres in diameter, ie Spirochaeta flicatilis. Members of the genus Spirochaeta are common in ponds and lakes. Some species are endosymbionts of animals such as termites. The genus Cristispira contains only one species which lives generally within the 'style' of molluscs such as clams and oysters. The genus Treponema contains one of the few spirochetes to be pathogenic to humans, T. pallidum which causes syphilis in humans. T. denticola, T. macrodentium and T. oralis all live in the human mouth where teeth and gums meet. Other species of Treponema such as T. saccharophilum are commensals in the rumen of cattle.
Leptospira and Leptonema are two aerobic genera of spirochetes. Leptospira interrogans is an internal parasite of mammals. It is normally found in rodents but can infect both humans and dogs. In humans it most commonly causes leptospirosis, a disease of the kidneys which can be fatal.
The genus Borrelia contains the most pathogenic of the spirochetes and most species in this genus are parasites in animals. Borrelia recurrentis causes relapsing fever in humans, it is commonly transmitted via the body louse. Borrelia burgdorferi causes Lyme disease which is transmitted by ticks. B. burgdorferi is of interest scientifically because it is one of the very few exceptions to the rule that 'bacteria have circular DNA'. In other words its DNA is linear, a characteristic normally only seen in Eukaryotes.
This is a small phylum of chemotrophic and autotrophic bacteria. The two best studied genera are Thermomicrobium containing two gram negative chemotrophic species and Chloroflexus a thermophilic phototroph. Thermomicrobium is unique in that its lipids form neither ester linkages like other bacteria, nor either linkages like the Archaea and the Eukaryotes. This is because the lipids are made up of 1,2-dialcohols instead of glycerol.
Chloroflexus is an inhabitant of neutral to alkaline hot springs where it often forms thick mats. Photosynthesis is carried out via bacteriochlorophyll a. Interestingly, Chloroflexus spp. can also grow in the dark by means of chemo-organotrophic respiration. Chloroflexus is an ancient genera with a photosynthetic metabolism that is part way between that of the Purple Bacteria and the Green Sulphur Bacteria. Some scientists think Chloroflexus may be one of the earliest photosynthetic bacteria.
The Fusobacteria is a small phylum of bacteria most of which occur in the genus Fusobacteria. Fusobacteria are filamentous bacteria which secondary colonists on the dental plaque on your teeth, after it has been formed by species of Streptomyces, thus thickening and reinforcing the bacterial attack on your teeth.