Occurrence of vancomycin-tolerant and heterogeneous vancomycin-intermediate strains (hVISA) among Staphylococcus aureus causing bloodstream infections in nine USA hospitals.
BACKGROUND
The bactericidal activities of vancomycin and daptomycin were evaluated in a large collection of methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia strains from nine major USA medical centres.


OBJECTIVE
To evaluate the occurrence of heterogeneous vancomycin-intermediate S. aureus (hVISA) among MRSA strains tolerant to vancomycin and/or with increased vancomycin or daptomycin MIC values. The accuracy of the macro Etest method (MET) compared with population analysis profiling (PAP) for the detection of hVISA was also assessed.


METHODS
A total of 1800 MRSA strains were collected from bloodstream infections at the nine sites (40 strains per year, per medical centre during the 2002-06 study period). Vancomycin and daptomycin MIC testing was performed by reference broth microdilution (all strains) and MBC tests on 50% of strains (randomly selected). A subset of isolates (n = 268) having an increased vancomycin MBC (> or =16 mg/L), an increased vancomycin MIC (> or =1 mg/L) and/or an increased daptomycin MIC (>0.5 mg/L) were tested for susceptibility to vancomycin and teicoplanin by MET.


RESULTS
Overall, 181 of 900 (20.1%) MRSA tested exhibited vancomycin tolerance, varying from 10% to 43% among the medical centres evaluated, and from 11.7% in 2004 to 27.8% in 2005. No resistance trend was observed in any medical centre or in the overall study data. Daptomycin showed bactericidal activity against all strains tested. The accuracy of MET for identifying hVISA strains varied significantly with the criteria applied for positivity.


CONCLUSIONS
The most frequently used criteria to define hVISA, i.e. MET reading values > or =8 mg/L for both vancomycin and teicoplanin or > or =12 mg/L for teicoplanin only, detected 20 of 36 PAP-positive strains (55.6% sensitivity), indicating that the prevalence of hVISA could be higher than currently appreciated. Daptomycin was bactericidal against hVISA strains.
Sader
Helio S
HS
JMI Laboratories, North Liberty, IA, USA. helio-sader@jmilabs.com
Jones
Ronald N
RN
Rossi
Kerri L
KL
Rybak
Michael J
MJ
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D016470:Bacteremia / Q000382:microbiology*; D017576:Daptomycin / Q000494:pharmacology; D004361:Drug Tolerance; D006761:Hospitals; D006801:Humans; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests; D050296:Microbial Viability / Q000187:drug effects; D013203:Staphylococcal Infections / Q000382:microbiology*; D017334:Teicoplanin / Q000494:pharmacology; D014481:United States; D014640:Vancomycin / Q000494:pharmacology*; D020713:Vancomycin Resistance
D000900:Anti-Bacterial Agents; D017334:Teicoplanin; D014640:Vancomycin; D017576:Daptomycin