Nonsporulating Firmicutes
Key Genera include Staphylococcus, Micrococcus, Streptococcus, Lactobacillus, Sarcina.
Staphylococcus and Micrococcus
Staphylococcus and Micrococcus are both aerobic organisms with a typical respiratory metabolism. They are catalase-positive, and this permits their distinction from Streptococcus and some other genera of gram-positive cocci.
Gram-positive cocci are relatively resistant to reduced water potential and tolerate drying and high salt (NaCl) fairly well. Their ability to grow in media containing salt provides a selective means for isolation. For example, if an appropriate inoculum such as a skin swab, dry soil, or room dust is spread on a rich-medium agar plate containing 7.5% NaCl and the plate is incubated aerobically, gram-positive cocci often form the predominant colonies. Many species are pigmented, and this provides an additional aid in selecting gram-positive cocci.
Micrococcus is an obligate aerobe and produces acid from glucose only under aerobic conditions, whereas Staphylococcus is a facultative aerobe and produces acid from glucose both aerobically and anaerobically. Staphylococcus also typically forms cell clusters, whereas Micrococcus does not.