Longitudinal analysis of chlorhexidine susceptibilities of nosocomial methicillin-resistant Staphylococcus aureus isolates at a teaching hospital in Taiwan

Background: Chlorhexidine has been widely used for hand hygiene to prevent transmission of nosocomial pathogens, including methicillin-resistant Staphylococcus aureus (MRSA). However, data on longitudinal surveillance of the susceptibility of MRSA isolates to chlorhexidine are limited. Methods: A total of 240 nosocomial MRSA isolates obtained in 1990, 1995, 2000 and 2005 at National Taiwan University Hospital (NTUH), a hospital where chlorhexidine gluconate was used for hand hygiene for more than 20 years, were included in the study. Chlorhexidine susceptibility, molecular typing using multilocus sequence typing and distribution of the qacA/B gene of these MRSA isolates were studied. Results: The proportion of tested MRSA with a high MIC of chlorhexidine (>= 4 mg/L) was 1.7% in 1990, 50% in 1995, 40% in 2000 and 46.7% in 2005. Among these 83 isolates with high chlorhexidine MICs, 55.4% carried the qacA/B gene. MRSA isolates carrying the qacA/B gene were first detected in 1995 and belonged to a single clone at that time. However, the qacA/B gene was detected in MRSA isolates belonging to seven different clones in 2005. Conclusions: The proportion of tested MRSA isolates with high chlorhexidine MICs at NTUH increased from 1990 to 1995 and remained steady thereafter. The presence of the qacA/B gene may contribute to the spread of specific MRSA clones.