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More than 95% of clinical isolates of Staphylococcus aureus obtained worldwide are resistant to penicillin, ampicillin, and the anti-pseudomonal penicillins, and remain sensitive to all other tested antimicrobials. Methicillin-resistant S. aureus (MRSA) represents the most common resistance threat currently posed by this organism. In contrast to methicillin-sensitive strains, isolates of MRSA are commonly resistant to multiple classes of antimicrobial agents. The mecA gene produces the penicillin binding protein (PBP) 2a, which has decreased affinity for ß-lactams, yielding resistant to all ß-lactams, penicillins, cephalosporins, carbapenems, and other penems. Resistance to other classes of antibiotics is due to transposition and site-specific integration. A major newly emerging threat is S. aureus with reduced susceptibility to vancomycin (formerly known as vancomycin-intermediate S. aureus, or VISA; however, although an MIC of 4 µg/mL is still interpreted as being susceptible, it should be viewed as having reduced susceptibilty to vancomycin.) Any isolate of S. aureus having a vancomycin minimum inhibitory concentration (MIC) of > 4 µg/mL should be considered to be a possible VISA. A major threat which has not yet been seen clinically is S. aureus with true resistance to vancomycin (VRSA). VRSA has been created in the laboratory using vancomycin-resistance conferring genes obtained from vancomycin-resistant Enterococcus (VRE). | |
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Coagulase-negative Staphylococcus spp., such as S. epidermidis, are usually encountered as skin contaminants of rather low virulence, but they can cause disease; e.g., endocarditis, or cystitis caused by S. saprophyticus. Coagulase-negative Staphylococcus spp. are usually resistant to multiple classes of antimicrobial agents, including methicillin/oxacillin. Many produce PBP 2a. Vancomycin-resistant coagulase-negative Staphylococcus spp. have rarely been encountered clinically. “Coagulase-negative staphylococci are felt to be a reservoir of resistance genes amplified through antibiotic selection that occurs when antibiotics administered to patients achieve low concentrations in the skin. The resistance gene can be transferred to Staphylococcus aureus, making it resistant to multiple agents.” | |
