The Staphylococcus genus is classified into two major groups: aureus and non-aureus. The coagulase positive S. aureus subspecies aureus (S. aureus) and S. intermedius are important pathogens of domestic animals, where their coagulase production correlates with their pathogenicity. Coagulase negative staphylococci are usually low virulence and but occasionally cause disease in animals and man.
Staphylococci are Gram positive cocci, approx 1um in diameter, and tend to occur in clusters resembling bunches of grapes. They grow in colonies which are usually white, opaque and approximately 4mm in diameter. Some bovine and human strains of S. aureus are golden in colour. Most are facultative anaerobes, which are catalase positive, non-motile, oxidase negative and do not form spores. Four types of haemolysins are recognised in staphylococci, these are alpha, beta, gamma and delta and when present they cause haemolysis in sheep or blood agar. Strains of staphylococci vary in their haemolysin producing ability but animal strains of both S. aureus and S. intermedius usually produce both alpha and beta haemolysins and on ruminant agar a double zone of haemolysis is often seen.
Staphyloccocal species occur as commensals on the skin of man and animals, they are also found in the upper respiratory tract, mucous membranes, in the digestive tract and lower urogenital tract. Staphylococci are pus generating bacteria and as such are often associated with suppurative lesions.