Expression and activity of a Xenorhabdus nematophila haemolysin required for full virulence towards Manduca sexta insects

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Abstract
As an insect pathogen, the gamma-proteobacterium Xenorhabdus nematophila likely possesses an arsenal of virulence factors, one of which is described in this work. We present evidence that the X . nematophilahaemolysin XhlA is required for full virulence towards Manduca sexta larvae. Lrp (leucine-responsive regulatory protein), FlhDC (regulator of flagella synthesis), and iron (II) limitation positively influenced xhlA transcript levels, suggesting XhlA expression is linked with nutrient acquisition and motility regulons. To help understand the role of XhlA in virulence, we examined its cellular targets and found that XhlA was a cell-surface associated haemolysin that lysed the two most prevalent types of insect immune cells (granulocytes and plasmatocytes) as well as rabbit and horse erythrocytes. Taken together, the need for xhlA for full virulence and XhlA activity towards insect immune cells suggest this haemolysin functions in X. nematophila immune evasion during infection. Analysis of a gene located immediately upstream of the xhlA locus, hcp (haemolysin co-regulated protein) revealed that its transcript levels were elevated during iron (III) limitation and its expression was Lrp-dependent. Further characterization of xhlA, hcp, and lrp will clarify their regulatory and functional relationships and their individual roles during the infectious process.