Important producers of atmospheric oxygen, also fix nitrogen 2. Gram negative, coccoid bacteria that are obligate intracellular parasites. Many are normal flora in the human mouth, others such as Treponema pallidum , Borrelia burgdorferi and Leptospira are pathogens. Most abundant organism in the human intestines. Frequent cause of peritonitis.
Clostridia are obligate anaerobic rods that form endospores and produce powerful exotoxins.
Members of the genus Bacillus are aerobic spore-formers. Lactic acid producing bacteria. Aerotolerant, but grow slowly in air. Lactobacillus species are found in the vagina, intestines and mouth. Other species are used in various food production industries.