Comparative antibacterial activity of retapamulin, cephalothin, gentamicin and erythromycin against Staphylococcus aureus, including molecularly characterised isolates of MRSA recovered from uncomplicated skin infections

Abstract number: O96

Scangarella N., Shawar R., Li G., Twynholm M., Dalessandro M., Breton J., West J., Miller L., O'Hara P., Becker J., Madsen H., Goering R., Payne D.

Objectives: In order to better recognise differences in susceptibility profiles, the in vitro activity of retapamulin, a new topical antibacterial agent, and comparators was determined against Staphylococcus aureus, including molecularly characterised isolates of MRSA from uncomplicated skin infections.

Methods: Bacterial isolates were obtained from the skin specimens of patients enrolled in five global phase III clinical trials conducted to evaluate the safety and efficacy of retapamulin. Susceptibility testing was conducted according to current CLSI standards. Susceptibility to retapamulin, cephalothin, gentamicin and erythromycin was determined on all S. aureus isolates using broth microdilution panels and susceptibility to meticillin was determined by disk diffusion using cefoxitin and/or oxacillin disks. All meticillin-resistant Staphylococcus aureus (MRSA) isolates were further analysed by pulsed-field gel electrophoresis (PFGE), multilocus sequence typing, SCCmec typing, and testing for the Panton-Valentine leukocidin genes.

Results: The overall rate of meticillin resistance among S. aureus isolates was 7% (105/1,442). Of the 105 MRSA isolates, 50% (53) were determined to be USA300. MIC90s (mg/mL) against S. aureus isolates obtained at the baseline visit are shown in the table.

Retapamulin demonstrated excellent in vitro activity against all S. aureus with a MIC range of 0.008–0.5 mg/mL and MIC90s of 0.12 mg/mL. Erythromycin demonstrated poor in vitro activity against all S. aureus with MIC90s of geqslant R: gt-or-equal, slanted32 mg/mL. Based on MIC90 values, gentamicin was >128-fold more active and cephalothin was 32-fold more active against USA300 MRSA isolates than against non-USA300 MRSA isolates.

Conclusions: Retapamulin was highly active in vitro against all S. aureus tested, irrespective of meticillin resistance or PFGE type. However, the MIC results for cephalothin and gentamicin suggest a possible difference between in vitro activity against USA300 and non-USA300 MRSA types. It should be noted that in vitro activity does not always correlate with clinical efficacy.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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