A potential role of transposon IS431 in the loss of mecA gene

Acquisition of a vancomycin-resistance-determinant may trigger deletion of the mecA gene. However, the molecular mechanisms involved remain largely unknown. In this study, we successfully produced vancomycin-intermediate-resistant Staphylococcus aureus (VISA) from Methicillin-resistant-S. aureus (MRSA) through serial passages with vancomycin. Five MRSA isolates achieved a vancomycin MIC of >8 mg/ml after 45-day serial exposure to vancomycin. After 20-day passages in media without antibiotics, three of the isolates were restored to pre-induction levels, whilst the remaining 2 (3503-1 and 4126-1) retained a vancomycin MIC >6 mg/ml. The oxacillin MICs for strain 3503-1 and its induced equivalents 3503VR6 and 3503VR10, were 512 μg/ml, <2 μg/ml, and <2 μg/ml, respectively. Oxacillin MICs for 4126-1 and its induced strain 4126VR10 were 512 μg/ml and 128 μg/ml, respectively. Strains 3503-1 and 3503VR6 were sensitive to gentamicin while 4126-1 and 4126VR10 were resistant. PFGE analysis demonstrated that comparing to the parental strain 3503VR6 and 3503VR10 lacked a DNA fragment of 40-kb and 80-kb, respectively. Both deleted regions localized around the transposon IS431. The deletion region of 3503VR10 was further investigated by whole-genome sequencing. We conclude that transition from MRSA to VISA may cause deletion of the mobile genetic element staphylococcal cassette chromosome mec (SCCmec), and possibly be mediated by IS431, resulting in increased susceptibility to oxacillin.