D-2215. Emergence of Staphylococcus aureus Strains Associated with Glycopeptide Failures: Biological Features and Detection Issues
Session: Poster Session: Staphylococcus aureus Antimicrobial Susceptibility Testing
Monday, October 27, 2008: 12:00 AM
Room: Hall C
Background: There is a concern with glycopeptide treatment failures for S. aureus infections, especially when caused by strains showing elevated MIC values. Here we report on a case series of S. aureus infections that failed to respond to glycopeptide therapy. Methods and Results: During a three-month period (January to April, 2008), an unusual number (n=9) of S. aureus infections which did not respond to glycopeptide treatment were observed at the Cisanello Hospital (Pisa, Italy). Infections included 3 SSTIs, 2 BSIs, 2 cIAIs, one VAP and one osteomyelitis, and occurred in 6 different wards. One MRSA isolate exhibited vancomycin and teicoplanin MICs lower than the breakpoints (0.5 and 0.12 mg/L, respectively), while eight isolates (of which 7 were MRSA) exhibited vancomycin and/or teicoplanin MICs at the breakpoint using the CLSI broth (BMD) and agar dilution methods. Using Etest on MH agar, the 8 isolates with MICs at the breakpoint exhibited vancomycin and teicoplanin MICs of 2.5-3 and 3-8 mg/L, respectively, and population analysis profiling revealed a behavior typical of h-VISA or VISA strains. PFGE analysis and SCCmec typing revealed clonal and SCCmec cassette diversity (SCCmec types I, II and IV). Most infections were successfully treated with linezolid or tigecycline. Linezolid plus ertapenem combination showed fully synergistic activity against 5 strains, partially synergistic activity against 4 strains. Conclusions: An increasing trend was observed for glycopeptide failures in treating S. aureus infections at our Hospital. Most of these infections were caused by strains showing glycopeptide MICs at the breakpoint using BMD, but vancomycin MIC higher than the breakpoint using Etest. A dual susceptibility testing approach (BMD plus vancomycin Etest on MH agar) would seem useful for detection of similar strains, and alternative treatments should be considered upon their detection.
Alessandro Leonildi1, Carlo Tascini2, E Mantengoli3, Francesco Menichetti4, Gian Maria Rossolini, MD5, Giulia Gemignani, MD1 and  C. Tascini, None., (1)Azienda Ospedaliera Universitaria Pisana, (2)Azienda Ospedaliera Universitaria Pisana, Pisa, Italy, (3)Microbiology Institute, Siena University, (4)Cisanello Hospital, (5)Univ. of Siena Med. Sch., Siena, Italy