Staphylococcus aureus

Gram Stain

• Gram positive cocci in clusters (aerobic)

Clinical Significance

This organism may colonize the nose and/or skin of humans. It is associated with skin/soft tissue, bone/joint, wound and medical device-related infections, bacteremia, septicemia, endocarditis, and pneumonia.

Note: With exception of indwelling long term urinary catheter as risk factor, the isolation of this organism in pure growth from urine should warrant investigation for infection at a distal site.

Usual Susceptibility Pattern

Most strains are resistant to penicillin.

Cloxacillin susceptible strains are susceptible to cefazolin, beta-lactamase inhibitor combinations, cefepime, ceftobiprole, and carbapenems.

Cefuroxime, cefotaxime, and ceftriaxone have some activity against MSSA but do not bind as avidly as cloxacillin or cefazolin. Once S aureus is confirmed, these antibiotics are not adequate for serious S aureus infections.

Cloxacillin resistance (MRSA) is mediated by an altered penicillin binding protein that results in resistance to most beta-lactams. MRSA incidence is geographic location and facility dependent.

Certain strains exhibit borderline (cl)oxacillin resistance (BORSA) as a result of hyperproduction of beta-lactamase. For these isolates, beta-lactamase inhibitor combinations are not recommended. Cefazolin and carbapenems may also exhibit diminished activity and should be used with caution.

Rare strains of S. aureus can produce a penicillinase able to hydrolyze cefazolin at high inocula (in infections such as osteomyelitis, endocarditis, medical device related infections). This may lead to clinical failure of cefazolin (but not cloxacillin) in MSSA.

Clindamycin resistance is significant.

Most isolates remain susceptible to vancomycin, doxycycline, and TMP/SMX. Vancomycin MICs can be elevated (mutation in cell wall thickness) and necessitate higher vancomycin doses or an alternate agent such as linezolid or daptomycin. True vancomycin resistance is rare.

Note: chronic infections with small colony variants of Staph aureus are difficult to treat with beta-lactams but may respond to moxifloxacin + rifampin - if these test susceptible