mstaph.htm version 990301

SAARS: Staphylococcus spp. (microbiology)

The staphylococci are facultative anaerobes which produce a large flat gray or white to yellow colony after 24 hour incubation at 35°C on sheep blood agar. Unlike the streptococci and enterococci, staphylococcal colonies grown under anaerobic conditions are smaller than those grown aerobically. Also, the staphylococci are catalase-positive, while the streptococci and the enterococci are not (see, however, the discussion concerning weak positive catalase reactions exhibited by enterococci). Microscopically, Gram-positive cocci occurring in typical grape-like clusters with individual cells being spherical can be presumed to be staphylococci.

In terms of clinical relevance, the staphylococci are conveniently divided into two groups: those which produce a positive coagulase test within 24 hours (coagulase-positive staphylococci, commonly identified and reported as Staphylococcus aureus) and those which do not (coagulase-negative staphylococci, commonly identified and reported as coagulase-negative Staphylococcus sp., or CNS). It is possible to perform a rapid slide clumping test which has a good correlation with the coagulation test, but the slide test detects the presence of an enzyme distinct from that detected in the coagulase test. Some laboratories may be content to classify staphylococci isolated from low-qualtiy specimens based only on a slide clumping test result, but those isolates obtained from higher-quality specimens (e.g., cerebrospinal fluid, other high-quality fluids, blood, and tissues) should always have a coagulase test performed for purposes of identification.

Many laboratories now routinely speciate coagulase-negative staphylococci, although the clinical usefulness of species identification of CNS and reliability of automated identification systems are generally dubious. It may be valuable to identify S. saprophyticus from urine cultures by testing for novobiocin resistance since this species is associated with cystitis in sexually-active females. Some staphylococci other than S. aureus are coagulase-positive, but these are not recovered from human infections. Similarly, on rare occasion an isolate of S. aureus may give a false negative coagulase test, but unless the organism has been recovered from a high-quality specimen it may be unnecessary to perform further identification testing, and the organism can be reported as CNS, with or without susceptibility testing, depending upon the circumstances of its isolation.