PURPOSE.To determine the prevalence, genetic diversity, and clinical relevance of the lukSF-PV gene, encoding the bacterial toxin Panton-Valentine leukocidin, in Staphylococcus aureus isolates from cases of bacterial keratitis in the United Kingdom.METHODS. Multiplex PCRs investigating carriage of lukSF-PV and mecA were performed on S. aureus isolates from patients. The lukSF-PV operon was sequenced to investigate its diversity, and multilocus sequence typing to test for a clonal relationship between lukSF-PV isolates. Antimicrobial minimum inhibitory concentrations (MICs) and clinical outcome data were compared for isolates characterized as lukSF-PVþve, mecAþve, and lukSF-PV/mecA-ve.
RESULTS.Of 95 isolates, 9 (9.5%) were lukSF-PVþve, 9 (9.5%) mecAþve, and 1 was positive for both. Five single nucleotide polymorphisms were found in lukSF-PV genes of seven strains. There was no significant difference between the MICs of lukSF-PV/mecA-ve and lukSF-PVþve isolates to the antimicrobials tested, except for tigecycline (P < 0.05). The mecAþve isolates had significantly higher mean MICs to meropenem and fluoroquinolones (P < 0.05). There were nonsignificant trends for healing and treatment times, ulcer and scar size, and overall clinical score to be greater in the lukSF-PVþve group (P < 0.05). The proportion of patients, however, who required surgery was significantly greater among patients with lukSF-PVþve isolates with an odds ratio of 7.8 (95% CI 1-42, P ¼ 0.018) for patients requiring surgery.CONCLUSIONS. lukSF-PVþve isolates were associated with a trend to worse clinical outcome and more surgical interventions, with an effect unrelated to MICs. This suggests that lukSF-PV may be an important virulence factor in S. aureus-associated keratitis.Keywords: keratitis, Panton-Valentine leukocidin, Staphylococcus aureus keratitis T he pathogenicity of Staphylococcus aureus is mediated by a multitude of secreted virulence factors, such as bacterial surface adhesins, immune evasion proteins, and toxins. PantonValentine leukocidin (PVL) is a pore-forming toxin produced by S. aureus.1 It is a bicomponent toxin that consists of the polypeptides LukS-PV and LukF-PV. 2 The cognate LukS-PV and LukF-PV bind to neutrophils, monocytes, and macrophages, but not to lymphocytes.3 Following the binding of monomers of LukS-PV and LukF-PV, further monomers attach to the cell wall forming a heptameric structure that forms a pore in the host cell surface. 2,4 This pore formation results in leukocyte cell death and the release of inflammatory cytokines. 3 The involvement, however, of PVL in the virulence of S. aureus is equivocal and its link with clinical outcome remains uncertain. In vivo studies have produced conflicting data. Murine models of S. aureus infection have shown that the absence of PVL results in an increase in virulence, 5-8 whereas studies in rabbits indicate that the presence of PVL increases the virulence of S. aureus.9-11 These discrepancies could, however, be attributed to differences in the immunology of the models. 11 Mo...