Properties of a high rate of MRSA colonization in the nasal cavity of intensive care unit doctors

Background: Methicillin-resistant Staphylococcus aureus (MRSA) is a major causative agent of healthcare-associated infections. Aims: To survey S. aureus/MRSA colonization in the nasopharyngeal cavities of intensive care unit (ICU) doctors at a university hospital. Methods: Surveys on nasopharyngeal S. aureus/MRSA colonization in 29 ICU doctors at a university hospital were conducted during July 2011 and January 2012. Polymerase chain reaction (PCR) analysis revealed mecA-positive strains as MRSA. The antimicrobial susceptibilities and toxin gene profiles of the isolates were additionally examined. Results: A total of 52% of the doctors examined during the first survey and 64% during the second survey showed S. aureus colonization, and 81% of the isolates were confirmed to be MRSA. Most of the MRSA strains had partially mutated mecA, as determined by PCR. The MRSA isolates, except for three, were susceptible to oxacillin, suggesting that these isolates could be misidentified as methicillin-sensitive S. aureus (MSSA) in hospital laboratories, whereas several resistant colonies appeared after an additional 3 days of incubation in the presence of oxacillin. Among the MRSA isolates, only four were tst-positive, and none were eta/etb-positive. Conclusion: A high rate of MRSA colonization in the nasal cavity of ICU doctors at a university hospital was observed. Most MRSA isolates, as determined through mecA detection, were susceptible to oxacillin, but produced resistant mutants in the presence of oxacillin. Therefore, we strongly suggest monitoring and/or eradication of colonized MRSA in the nasal cavity of ICU doctors.