Correlation of mupirocin resistance with biofilm production in methicillin-resistant Staphylococcus aureus from surgical site infections in a tertiary centre, Egypt

• 82.2% of MRSA isolates were mupirocin-susceptible. 17.8% were mupirocin-resistant.
• Of the resistant-isolates, 38.5% showed low-level resistance. 61.5% were high-level mupirocin-resistant.
• The mupA gene was detected in 75.0% of high-level mupirocin-resistant strains. No low-level mupirocin-resistant strains.
• 5.0% of mupirocin-sensitive isolates were non-biofilm-producers and 95.0% were biofilm-producers. Both low- and high-level mupirocin-resistant isolates were biofilm-producers.
• 15.3% of high-level mupirocin-resistant strains were mupA-negative. However, they showed evidence of biofilm formation.

The aim of this study was to detect mupirocin-resistant isolates from pus/wound swabs taken postoperatively in a tertiary centre in Egypt and to determine their ability to form biofilm in order to establish its correlation with mupirocin resistance. This was a prospective study including 513 pus/wound swabs from patients suffering from postoperative surgical site infections over the period July 2013–January 2015. Samples were cultured and isolates were identified by coagulase activity, DNase test, mannitol fermentation by mannitol salt agar followed by API Staph 32. Oxacillin agar screen test, agar dilution test for mupirocin, and mupA gene detection by PCR were performed for all methicillin-resistant Staphylococcus aureus (MRSA) isolates. Biofilm detection was carried out by the microtitre plate and Congo red agar methods. Of the 161 S. aureus isolates identified, 73 (45.3%) were MRSA, among which 82.2% were mupirocin-susceptible and 17.8% were mupirocin-resistant. Among the resistant isolates, 38.5% showed low-level resistance and 61.5% were high-level mupirocin-resistant. The mupA gene was detected in 75.0% of high-level mupirocin-resistant strains and in none of the low-level mupirocin-resistant strains. Among the mupirocin-susceptible isolates, 95.0% were biofilm-producers and 5.0% did not produce biofilm. All mupirocin-resistant isolates produced biofilm. Moreover, 15.3% of high-level mupirocin-resistant strains were negative for the mupA gene but showed evidence of biofilm formation. In conclusion, biofilm formation may be suggested to play a role in mupirocin resistance besides the presence of a genetic element encoding abnormal isoleucyl-tRNA synthetase, however further studies are needed to confirm these findings.