Clinical and Molecular Characteristics of qacA/B-Positive Methicillin-resistant Staphylococcus aureus Causing Bloodstream Infections [Epidemiology and Surveillance]

The increasing use of chlorhexidine for methicillin-resistant Staphylococcus aureus (MRSA) decolonization has raised concerns about the emergence of resistance to these agents. However, the clinical significance of MRSA positive for the qacA/B-chlorhexidine tolerance genes has not been established. We investigated the clinical features and predictive factors of MRSA bloodstream infection (BSI) isolates, caused by qacA/B-positive MRSA, from 2010 to 2016 at a tertiary hospital in South Korea. A total of 246 MRSA BSI isolates were included; 71 (28.9%) isolates carried qacA/B. The annual frequency of qacA/B-positive MRSA bacteremia did not change significantly over the study period. Patients infected with qacA/B-positive MRSA had common risk factors for healthcare-associated infections, including prior antibiotic use, central venous catheterization in situ, intensive-care-unit-acquired bacteremia, and nosocomial infection. The qacA/B-positive isolates were also associated with an increasing chlorhexidine MIC and resistance to non-β-lactam antibiotics. The qacA/B-positive isolates were more likely to belong to sequence type 5 (ST5), which is a common healthcare-associated MRSA strain in South Korea. In multivariable analyses, qacA/B-positive MRSA isolates were found to be associated with agr dysfunction (aOR, 6.45; 95% CI, 2.59–16.10), ST5 MRSA strain (aOR 4.96; 95% CI, 1.85–13.26), nosocomial infection (aOR, 4.88; 95% CI, 2.20–10.83), and antibiotic use within the previous 3 months (aOR, 2.59; 95% CI, 1.20–5.59). These findings suggest that the microbiological features of qacA/B carriage may provide a selective advantage for specific MRSA strains in hospital environments.

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