1013. Description of oxacillin-susceptible mecA positive Staphylococcus aureus without inducible oxacillin resistance
Session: Poster Abstract Session: Detecting, Identifying, and Typing Bacteria
Saturday, October 22, 2011
Room: Poster Hall B1
Background: Methicillin-resistant Staphylococcus aureus (MRSA) isolates are detected by the presence of the mecA gene or an oxacillin minimum inhibitory concentration (MIC) ≥ 4 ug/ml. Recently, unusual stains have been described as mecA positive but oxacillin–susceptible. After exposure to oxacillin, these MRSA strains can develop resistance to oxacillin and may be reported as oxacillin-susceptible. This study describes two patients with mecA positive methicillin-susceptible S. aureus strains (MSSA) without inducible resistance.

Methods: From October to December 2010, we identified two strains of oxacillin susceptible, mecA positive S. aureus strains from a nasal swab for MRSA screening and from a blood culture. These strains were detected by growth on chromogenic MRSA agar with cefoxitin but were oxacillin-susceptible. We also collected risk factors for MRSA and outcomes of these patients. Induction of resistance was performed using two protocols: incubation of strains on Mueller-Hinton agar containing oxacillin (6ug/ml) for 72 hours and using incoula grown in subinbitory concentrations of oxacillin and then incubation in two-fold dilutions of oxacillin. The presence of the mecA gene was detected using previously published primers. Finally, we sequenced mecA gene for detection of specific sequence.

Results: Both patients had important risk factors for MRSA acquisition with previous colonization or recent hospitalisation. One patient remained colonized in the nares but the other patient was bacteremic with underlying pneumonia and responded to vancomycin. These strains had respectively an MIC of 0.5 and less 0.25 ug/ml by Vitek and 2,0 and 1,5 ug/ml by Etest. These stains remained susceptible to oxacillin despite prolonged incubation at 72 hr; resistance could not be induced with subinhibitory concentrations of oxacillin. They were mecA positive by PCR.

 Conclusion: Our study confirmed that these unusual strains could be missed by conventional detection of resistance by oxacillin disk diffusion test or MIC testing. For blood cultures or sterile body fluids that are MSSA positive, the presence of the mecA gene should be performed to rule out these unusual strains.  Further genetic analyses of the mecA gene of these two strains will be performed.

Subject Category: D. Diagnostic microbiology

Andreanne Jean, MD, Infectious Disease and Medical Microbiology, McGill University Health Centre, Montreal, QC, Canada, Vivian Loo, MD, MSc, Microbiology, Royal Victoria Hospital, Montreal, QC, Canada; McGill University Health Centre, Montreal,, QC, Canada, Makeda Semret, MD, McGill University, Montreal, QC, Canada, Charles Frenette, MD, Infectious Diseases and Medical Microbiology, McGill University Health Centre, Montreal, QC, Canada and Susan Fenn, Infectious Disease, McGill University Health Centre, Montreal, QC, Canada


A. Jean, None

V. Loo, None

M. Semret, None

C. Frenette, None

S. Fenn, None

Findings in the abstracts are embargoed until 12:01 a.m. EST Thursday, Oct. 20 with the exception of research findings presented at IDSA press conferences.