Differential susceptibility of airway and ocular surface cell lines to FlhDC-mediated virulence factors PhlA and ShlA from .

is a bacterial pathogen that causes ventilator-associated pneumonia and ocular infections. The FlhD and FlhC proteins complex to form a heteromeric transcription factor whose regulon, in , regulates genes for the production of flagellum, phospholipase A and the cytolysin ShlA. The previously identified mutation, , resulted in highly elevated expression of the operon. The mutant was observed to be more cytotoxic to human airway and ocular surface epithelial cells than the wild-type bacteria and the present study sought to identify the mechanism underlying the increased cytotoxicity phenotype. Although FlhC and FlhD have been implicated as virulence determinants, the mechanisms by which these proteins regulate bacterial cytotoxicity to different cell types remains unclear. This study aimed to evaluate the mechanisms of FlhDC-mediated cytotoxicity to human epithelial cells by . Wild-type and mutant bacteria and bacterial secretomes were used to challenge airway and ocular surface cell lines as evaluated by resazurin and calcein AM staining. Pathogenesis was further tested using a infection model. The increased cytotoxicity of bacteria and secretomes to both cell lines was eliminated by mutation of and . Mutation of the flagellin gene had no impact on cytotoxicity under any tested condition. Elimination of the phospholipase gene, , had no effect on bacteria-induced cytotoxicity to either cell line, but reduced cytotoxicity caused by secretomes to airway epithelial cells. Mutation of and , but not , reduced bacterial killing of larvae. This study indicates that the FlhDC-regulated secreted proteins PhlA and ShlA, but not flagellin, are cytotoxic to airway and ocular surface cells and demonstrates differences in human epithelial cell susceptibility to PhlA.