Mupirocin and Chlorhexidine Genotypic Resistance Found in Staphylococcus aureus Isolated From Young Infants Below 90 Days Old: A Genetic Basis for Eradication Failure

Objectives: To investigate the genetic characteristics associated with eradication failure of Staphylococcus aureus in infants below 90 days old. Methods: S. aureus isolated from clinical specimen cultures (blood, surgical tissue, or drainage, pus, etc.) and routine screening cultures in the neonatal intensive care unit (nasal and axillary skin swab) from patients below 90 days old were collected prospectively for 1 year, from August 2017 to July 2018. The isolates underwent typing and screening for genes associated with chlorhexidine (qacA/B), quaternary ammonium (smr), and mupirocin resistance (iles mutation, mupA, mupB), as well as Panton-Valentine leukocidin (PVL) toxin. Results: During the study period, 40 nonduplicate isolates were included for analyses, of which 70.0% were methicillin-resistant S. aureus (MRSA). Mupirocin resistance was found in 25% of the total isolates; 17.4% of the colonizers; and 35.3% of the pathogens (P = 0.196). Chlorhexidine resistance gene was found in 3 MRSA isolates colonized in the nares of preterm infants. All isolates harbored the disinfectant quaternary ammonium compound (QAC) resistance gene. PVL toxin gene was found in 57.5%, and the presence of PVL gene among colonizers and pathogens was similar (69.6% vs. 41.2%, P = 0.072). Conclusions: Mupirocin, chlorhexidine, and QAC-resistant MRSAs harboring the PVL toxin gene were found in the nasal carriages of preterm infants. In this highly vulnerable patient population, one-fourth of the isolates harbored mupirocin-resistant genes, and all were resistant to QAC disinfectants. These strains are associated with persistence in both carriage and environmental reservoirs within the hospitals.