Subinhibitory Concentrations of Mupirocin Stimulate Staphylococcus aureus Biofilm Formation by Upregulating <i>cidA</i>

Abstract

Previous studies have shown that the administration of antibiotics at subinhibitory concentrations stimulates biofilm formation by the majority of multidrug-resistant <i>Staphylococcus aureus</i> (MRSA) strains. Here, we investigated the effect of subinhibitory concentrations of mupirocin on biofilm formation by the community-associated (CA) mupirocin-sensitive MRSA strain USA300 and the highly mupirocin-resistant clinical <i>S. aureus</i> SA01 to SA05 isolates. We found that mupirocin increased the ability of MRSA cells to attach to surfaces and form biofilms. Confocal laser scanning microscopy (CLSM) demonstrated that mupirocin treatment promoted thicker biofilm formation, which also correlated with the production of extracellular DNA (eDNA). Furthermore, quantitative real-time PCR (RT-qPCR) results revealed that this effect was largely due to the involvement of holin-like and antiholin-like proteins (encoded by the <i>cidA</i> gene), which are responsible for modulating cell death and lysis during biofilm development. We found that <i>cidA</i> expression levels significantly increased by 6.05- to 35.52-fold (<i>P</i> < 0.01) after mupirocin administration. We generated a <i>cidA</i>-deficient mutant of the USA300 <i>S. aureus</i> strain. Exposure of the <i>ΔcidA</i> mutant to mupirocin did not result in thicker biofilm formation than that in the parent strain. We therefore hypothesize that the mupirocin-induced stimulation of <i>S. aureus</i> biofilm formation may involve the upregulation of <i>cidA</i>.