Molecular and cellular characterisation of Staphylococcus aureus in chronic wounds

• Main Author: Emanuel, Charlotte
• Published: Cardiff University 2011
• Subjects: 616.9
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.585157

Chronic skin wounds (CW) represent a significant world health problem including reservoirs of multi-drug resistant bacterial biofilms. The precise role of bacteria in the aetiology of chronic inflammation and non-healing remains unclear. This study characterised MRSA from human CW and investigated how they inhibit wound healing, modulate immunological responses and resist treatment in comparison to control MRSA from asymptomatic nasal carriers (NC). Routine cultural analysis of 150 chronic wounds revealed 50% were colonised with <italic>S. aureus</italic>, of which 22.6% were MRSA. Multi-locus sequence typing identified two new sequence types and demonstrated that wound MRSA represented two clonal complexes (22 and 30) with almost 90% identified as hospital-acquired EMRSA-15. MRSA isolated from CW and NC were characterised for virulence factor (VF) expression and modulation of the innate immune system. Presence and expression of MRSA VFs indicated an association with sequence type. Greater expression of colonisation- (cna) and degradation- associated (hysA) VFs was evident in the wound MRSA, suggesting that modulation of virulence is important for non-healing. The ability of conventional biocides (iodine, silver, and potassium permanganate) to treat chronic wound bacteria, using the carrier-test method, revealed iodine as the only effective biocide. In vitro stimulation of the ability of the MRSA to induce innate immunity showed that CW-MRSA exhibited decreased TLR, cytokine and complement responses compared with NC-MRSA (IL-8, TNFa, complement activation PO.05). Moreover, biofilm- induced reductions in immunogenicity were observed compared with planktonic growth in monocyte and complement assays (PO.05). Scratch wound assays indicated that MRSA failed to inhibit keratinocyte migration (P>0.05), although bacterial growth conditions (biofilm vs. planktonic) significantly affected the observed cellular migration (PO.05). Virulence factor production and ability to modulate/evade the host innate immune response are important potential mechanisms by which MRSA are able to colonise chronic wounds. These studies provide important new insights into the role of MRSA in delayed dermal healing.