Inducibility and potential role of MecA-gene-positive oxacillin-susceptible Staphylococcus aureus from colonized healthcare workers as a source for nosocomial infections

To determine the carrier rate of methicillin-susceptible mecA-positive Staphylococcus aureus (dormant MRSA) among healthcare workers (HCWs), 447 nurses and physicians from 13 general wards and intensive care units were investigated for nasal or oropharyngeal S. aureus carriage during one year whenever an MRSA patient was treated. Induction of phenotypic resistance in all mecA-positive oxacillin-susceptible aureus was attempted by 24 h exposure to oxacillin and cefotaxime. Organisms from the broth tube with the highest antibiotic concentration and visible growth after incubation were re-exposed for a total of seven repetitive exposures. Two mecA-negative oxacillin-susceptible S. aureus served as negative control. A population analysis before and after antibiotic exposure was performed. A third of the HCWs were found to be S. aureus carriers. Only three nurses were MRSA positive (0.7%). Seven isolates of dormant MRSA were isolated in six nurses and one doctor (1.6%). After four days of repetitive antibiotic exposure six of seven dormant MRSA were highly resistant to oxacillin. Resistance of the two control S. aureus without the mecA gene was not changed by repetitive antibiotic exposure. Two of the seven dormant MRSA were clonally related as shown by pulsed-field gel electrophoresis (PFGE). The PFGE pattern of one dormant MRSA (HCW) was identical to an MRSA (HCW). The pattern of another dormant MRSA was indistinguishable from an MRSA isolated from a patient who was treated at the same time on the same ward suggesting transmission from the HCW to the patient. Dormant MRSA may be isolated twice as often as MRSA from HCWs. Transmission to patients is possible, which may lead to clinical infections. It might be useful to screen methicillin-susceptible S. aureus isolates from HCWs for the mecA gene when recurrent infections with MRSA occur on a ward and a source cannot be found.