Staphylococcus aureus carrier of oxacillin-sensitive mecA gene (OS-MRSA):

another challenge for microbiology laboratories

Abstract

Staphylococcus aureusis a common human pathogen that is able to acquire resistance mechanisms for different antibiotics. Oxaxicillin resistance mainly results from mecA or mecC genes which determine an oxacillin minimum inhibitory concentration (MIC) ³4 mg/L. Expression of these genes is variable and the isolation of mecA gen with oxacillin MIC ³2 mg/L (phenotypically susceptible) have been described carriers has been described, having been named OS-MRSA. S. aureus bacteria was isolated from the synovial fluid and blood culture in a girl with a joint infection. Isolation of the blood culture was classified as methicillin resistant (MRSA) while thet of the synovial fluid was methicillin sensitive (MSSA), by using the automatized Vitek 2 system. Both of them present oxacillin inhibition halos > 15 mm, and inhibition halos of 19 and 17 mm for cefoxitin, respectively. Both isolations proved mecA, sea, seb and type V SCCmec genes were present. Genetic comparison by SmaI-PFGE showed identical profiles for both cultures, suggesting it was the same strain. These report informs the identification of an OS-MRSA isolation in Uruguay and points out the limitation of a few laboratory procedures to correctly identify resistant phenotypes associated to mecA gene in clinical isolations of S. aureus.