Susceptibility of Staphylococcus aureus isolated from skin and wound infections in the United States 2005-07: laboratory-based surveillance study

Abstract

Objectives: The aim of this study was to describe the rates of antimicrobial susceptibility of Staphylococcus aureus from skin and wound infections reported from nine regions of the USA during 2005-07 and to identify the regional variation in patterns of resistance.

Methods: The Surveillance Network (TSN) comprises 296 laboratories across the nine census regions of the USA. TSN laboratories reported the susceptibility data for six antimicrobials by isolate with source and other relevant data. Antimicrobial susceptibility data were analysed by individual drug resistance, multidrug resistance and geographical distribution of resistance phenotypes.

Results: There were over 380 000 isolates of S. aureus tested and reported for the period 2005-07. Methicillin resistance was observed in 57.8% in 2007, with little change from 2005. There was little difference in rates of methicillin resistance between community and hospital strains, although strains from intensive care units (ICUs) tended to be slightly more resistant overall. Resistance to other antimicrobials was also reported. A regional variation in resistance rates was noted with the highest rates in the Central states and lowest in the New England and Mid-Atlantic regions. There was high activity observed with trimethoprim/sulfamethoxazole and gentamicin. Linezolid resistance was rare. Oxacillin resistance was similar among paediatric and elderly cohorts, whereas ciprofloxacin and clindamycin resistance was significantly (P < 0.01) more common in elderly patients when compared with both paediatric and adult populations. Less than a third of all isolates showed no resistance mechanism, 30.3%. Three distinct resistance phenotypes accounted for 46% of all resistant strains. Overall, there were more highly drug-resistant isolates from the ICU with four, five or six drug-resistant phenotypes accounting for over a third of all strains.

Conclusions: S. aureus has become methicillin-resistant in both the community and hospital settings; however, little change has been seen in the past 3 years. Multiresistant strains now are seen in all settings, but due to regional variation, empirical therapy should be guided by local susceptibility patterns. Currently, among the agents studied, only trimethoprim/sulfamethoxazole, gentamicin and linezolid exhibit susceptibility rates of >95%.