sarA-Mediated Biofilm Formation in Staphylococcus aureus: Investigating Exceptions to the Rule

Staphylococcus aureus is an important human pathogen that causes a variety of infections, including serious conditions such as osteomyelitis, pneumonia, and necrotizing fasciitis. We have focused on musculoskeletal infections associated with a biofilm, such as osteomyelitis and infections of implanted medical devices. These biofilm-associated infections are difficult to treat because the biofilm itself confers intrinsic resistance to antibiotics. Treatment of biofilm-associated infections requires extensive debridement of the tissues and/or removal of the device, followed by six weeks of intravenous antibiotic therapy. We are interested in developing means of treating these infections without the requirement for debridement and/or device removal. One possible drug target is sarA, mutation of which results in a limited ability to form a biofilm and an increased sensitivity to current antibiotics. The phenotypic effects after mutation of sarA are consistent among diverse clinical isolates of S. aureus with two exceptions: the strains RN6390 and Newman. In both of these strains, mutation of sarA has little effect on biofilm formation. RN6390 expresses agr at high levels, and it is thought that this accounts for its inability to form a biofilm. Recently, serious, life-threatening S. aureus infections have occurred in otherwise healthy individuals in the community (CA-MRSA). These CA-MRSA infections also express high levels of agr. However, despite similar levels of agr to RN6390, we demonstrate CA-MRSA isolates are capable of forming a robust biofilm. Mutation of sarA in Newman has little effect on biofilm formation. We show this is due to a point mutation in saeS (saeSL18P) that results in constitutive activation of the saeRS two-component system. We created a CA-MRSA strain containing the saeSL18P mutation and found this mutation is does compromise sarA-defined in vivo phenotypes in our bacteremia model. However, the saeS L18P mutation had little effect on sarA-defined phenotypes in our in vitro and in vivo biofilm models. To the extent that osteomyelitis and device-related infection are biofilm-associated, this suggests that increased saeRS activity would not compromise the efficacy of a sarA inhibitor in CA-MRSA biofilm-associated musculoskeletal infections. We conclude that an inhibitor of sarA could be a valuable therapeutic tool in treating CA-MRSA musculoskeletal infections.