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Reduction in vancomycin susceptibility may occur in MSSA
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Reduced vancomycin susceptibility can occur in Staphylococcus aureus regardless of background methicillin susceptibility, according to findings published recently.
Satish K. Pillai, of Beth Israel Deaconess Medical Center in Boston and other researchers involved in the study wrote that the development of intermediate vancomycin susceptibility in methicillin-susceptible S. aureus (MSSA) may lead to increased tolerance to several classes of antibiotics used to combat staphylococcal infections.
The researchers examined differences in vancomycin population analysis profiles, chemical autolysis, bactericidal activities and vancomycin, oxacillin and daptomycin minimum inhibitory concentrations.
The study aim was to determine whether the evolution of reduced vancomycin susceptibility in MSSA is similar to that seen in methicillin-resistant S. aureus (MRSA). The studied isogenic series permitted that determination to be made, according to the researchers.
Results demonstrated that progressive vancomycin resistance correlated with increasing daptomycin susceptibility.
“Chemical autolysis and the bactericidal activity of vancomycin, oxacillin and daptomycin were reduced in the final, vancomycin-intermediate S. aureus isolate, compared with the vancomycin-susceptible MSSA progenitor,” the researchers wrote.
Pillai SK et al. Clin Infect Dis. 2009;49:1169-1174.
This carefully done study of genetically-related MSSA isolates (from one patient with osteomyelitis treated with vancomycin over several months) is potentially very important. Vancomycin MIC "creep" was confirmed and associated with clinical failure. Also disturbing was the associated rise in MIC to daptomycin despite the lack of daptomycin exposure. Vancomycin should be avoided when treating MSSA osteomyelitis.
- George Pankey, MD
Infectious Disease News Editorial Board member