Staphylococci

Staphylococci are spherical gram-positive bacteria that divide in multiple planes, producing the characteristic irregular clusters seen on Gram stain. They are catalase-positive, which distinguishes them from the catalase-negative streptococci — the first branch point in the standard gram-positive cocci identification algorithm. They are facultative anaerobes and tolerate high salt concentrations, a property exploited by mannitol salt agar.

The next branch point is the coagulase test. Staphylococcus aureus is coagulase-positive and beta-hemolytic, ferments mannitol, and produces a large arsenal of virulence factors including protein A, coagulase, hemolysins, and toxins responsible for toxic shock syndrome, scalded skin syndrome, and preformed-toxin food poisoning. Clinically it causes impetigo, cellulitis, abscesses, osteomyelitis, septic arthritis, acute endocarditis, and pneumonia. The coagulase-negative species include S. epidermidis, which forms biofilms on prosthetic joints, catheters, and heart valves, and S. saprophyticus, a common cause of urinary tract infection in young sexually active women and the species distinguished by novobiocin resistance.

Treatment turns on resistance. Most staphylococci produce beta-lactamase, so penicillin fails and anti-staphylococcal penicillins such as nafcillin and oxacillin are used instead. Methicillin-resistant S. aureus (MRSA) carries the mecA gene encoding an altered penicillin-binding protein, defeating nearly all beta-lactams — the anti-MRSA cephalosporin ceftaroline is the exception — and requiring vancomycin or other agents active against resistant gram-positive organisms.

Staphylococci are among the highest-yield organisms on USMLE Step 1 microbiology, tested through the catalase and coagulase algorithm, virulence factors, and drug resistance mechanisms. They also appear in CCMA content on common infections — staphylococcal species cause styes and bacterial conjunctivitis — and in PTCE antimicrobial material, where the relevant point is which antibiotic classes retain activity against them.