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Coagulase-negative staphylococci (CoNS) are part of the normal flora of human skin. These organisms have relatively low virulence but are increasingly recognized as agents of clinically significant infection of the bloodstream and other sites. Risk factors for CoNS infection include indwelling foreign devices and immune compromise.
Resistance to methicillin and semisynthetic penicillins has been observed in more than 80 percent of coagulase-negative staphylococcal isolates. The genes responsible for resistance are often found on plasmids, facilitating the horizontal exchange of resistance genes among strains. The methicillin minimum inhibitory concentration (MIC) breakpoints for CoNS (except Staphylococcus lugdunensis) are ≤0.25 mg/L for susceptibility to oxacillin and ≥0.5 mg/L for resistance to oxacillin according to Clinical and Laboratory Standards Institute guidelines.
There have been case reports of CoNS with reduced susceptibility to vancomycin. CoNS glycopeptide resistance appears to occur in the form of heteroresistant subpopulations of organisms. Therefore, accurate detection in the laboratory can be difficult. The vancomycin MIC breakpoints for CoNS are susceptible if ≤4 mg/L, intermediate if 8 to 16 mg/L, and resistant if ≥32 mg/L.
The agent of choice for empiric treatment of infections due to CoNS is vancomycin; it is generally the mainstay of treatment for methicillin-resistant CoNS infections. In the setting of infection due to CoNS that is known to be methicillin susceptible, the preferred agent is nafcillin or oxacillin. Additional agents with potential activity against CoNS include daptomycin, linezolid, telavancin, ceftaroline, and quinupristin-dalfopristin.
The duration of treatment of CoNS infection depends on the clinical site of infection. Bacteremia in the absence of involvement of other sites may be treated for 7 to 14 days.