Quorum sensing regulation of virulence gene expression in Vibrio harveyi in vitro and in vivo during infection of gnotobiotic brine shrimp larvae

In this study, we investigated quorum sensing regulation of virulence genes in Vibrio harveyi by determining their expression levels, both in vitro and in vivo during infection of gnotobiotic brine shrimp. The quorum sensing master regulator luxR and the vhp metalloprotease showed around threefold and fivefold higher expression levels in a luxO mutant with maximum quorum sensing activity than in a luxO mutant with minimum quorum sensing activity. There was no difference in expression of the vhh haemolysin gene between the two mutants. There was however more than 2.5-fold lower expression in an AI-2-negative mutant, suggesting that this gene is specifically regulated by AI-2 quorum sensing through a yet unknown signal transduction cascade. The in vivo expression data showed a peak in expression of the quorum sensing master regulator luxR and the vhp metalloprotease after 24 h of incubation in wild-type V. harveyi and the luxO mutant mimicking a maximally activated quorum sensing system, whereas the expression remained low in the luxO mutant mimicking a completely inactivated quorum sensing system. The vhh haemolysin gene showed a peak in expression after 24 h in the wild type and a constantly low expression in an AI-2-negative mutant