Effect of Co-inhabiting Coagulase Negative Staphylococci on S. aureus agr Quorum Sensing, Host Factor Binding, and Biofilm Formation

Abstract

<i>Staphylococcus aureus</i> is a commensal colonizer of both humans and animals, but also an opportunistic pathogen responsible for a multitude of diseases. In recent years, colonization of pigs by methicillin resistant <i>S. aureus</i> has become a problem with increasing numbers of humans being infected by livestock strains. In <i>S. aureus</i> colonization and virulence factor expression is controlled by the <i>agr</i> quorum sensing system, which responds to and is activated by self-generated, autoinducing peptides (AIPs). AIPs are also produced by coagulase negative staphylococci (CoNS) commonly found as commensals in both humans and animals, and interestingly, some of these inhibit <i>S. aureus agr</i> activity. Here, we have addressed if cross-communication occurs between <i>S. aureus</i> and CoNS strains isolated from pig nares, and if so, how properties such as host factor binding and biofilm formation are affected. From 25 pig nasal swabs we obtained 54 staphylococcal CoNS isolates belonging to 8 different species. Of these, none were able to induce <i>S. aureus agr</i> as monitored by reporter gene fusions to <i>agr</i> regulated genes but a number of <i>agr</i>-inhibiting species were identified including <i>Staphylococcus hyicus</i>, <i>Staphylococcus simulans</i>, <i>Staphylococcus arlettae</i>, <i>Staphylococcus lentus</i>, and <i>Staphylococcus chromogenes</i>. After establishing that the inhibitory activity was mediated via AgrC, the receptor of AIPs, we synthesized selective AIPs to explore their effect on adhesion of <i>S. aureus</i> to fibronectin, a host factor involved in <i>S. aureus</i> colonization. Here, we found that the CoNS AIPs did not affect adhesion of <i>S. aureus</i> except for strain 8325-4. When individual CoNS strains were co-cultured together with <i>S. aureus</i> we observed variable degrees of biofilm formation which did not correlate with <i>agr</i> interactions. Our results show that multiple CoNS species can be isolated from pig nares and that the majority of these produce AIPs that inhibit <i>S. aureus agr</i>. Further they show that the consequences of the interactions between CoNS and <i>S. aureus</i> are complex and highly strain dependent.