chapter  11
16 Pages

Gram-Positive Resistance—Clinical Implications /

ByJohn C. Rotschafer, Mary A. Ullman, Isaac Mitropoulos

Throughout the modern antibiotic era, Staphylococcus aureus has

remained a formidable and adaptable pathogen. Through the production

of penicillinase, S. aureus has averted the effect of penicillin G and

congeners (1). Through a penicillin-binding protein 2 alteration (PBP-2a),

the pathogen has overcome semisynthetic penicillins and first-generation

cephalosporins (2). Infections caused by this altered form of S. aureus

have been shown to cause increased morbidity and mortality and are more

expensive to care for than methicillin-susceptible S. aureus (MSSA)

strains (3). While often referred to as methicillin-resistant S. aureus

(MRSA), this term does not reflect contemporary susceptibility testing

procedures that are now performed with oxacillin not methicillin, so the

more suitable term would be oxacillin-resistant S. aureus (ORSA) (4).

Some investigators have promoted cefoxitin as the best b-lactam substrate for identifying S. aureus with a PBP-2a and a cefoxitin disk along

with an oxacillin minimum inhibitory concentration (MIC) is recommend

by the CLSI as the standard testing procedure for MRSA/ORSA (4).

Additionally, the presence of MRSA can now be confirmed using various

gene probe tests that determine the presence of mecA, the gene respon-

sible for the alteration of PBP-2. Borderline-resistant S. aureus (BORSA)

are hyper-b-lactamase-producing strains that are mecA negative and can be misinterpreted to be MRSA in antibiotic susceptibility testing (2).