Regulators H-NS and LeuO inversely control swarming motility and biofilm formation in Vibrio parahaemolyticus

Vibrio parahaemolyticus is a halophile present in marine environments worldwide and is a leading cause of bacterial seafood-borne gastroenteritis. Free living Vibrio parahaemolyticus planktonic cells can either attach to surfaces to form swarming cells or develop into a sessile three-dimensional biofilm structure. Swarming motility requires lateral flagella (laf operon) and the expression of the surface sensing operon scrABC to produce a spreading cauliflower colony morphology. Biofilms are formed from capsule polysaccharide (CPS) encoded by the cpsA-K operon that is positively regulated by CpsR and CpsQ. In enteric bacteria, H-NS is a global gene silencer and LeuO is an antagonist of H-NS. In this work, we examined the role of these regulators in the decision between swarming and biofilm behaviors using deletion mutants of leuO, hns, and a double deletion leuO/hns. The wild type and {Delta}leuO strains produced swarming colonies whereas {Delta}hns produced a hyper swarming whereas in {Delta}leuO/{Delta}hns, the phenotype reverted to wild type. Transcriptional reporter assays of PlafB-gfp and PscrA-gfp showed significantly increased fluorescence in {Delta}hns compared to wild type. In the {Delta}leuO/{Delta}hns mutant, PlafB-gfp fluorescence reverted to wild type levels and PscrA-gfp showed increased fluorescence compared to wild type. In CPS assays, {Delta}leuO had a less dense rugose morphology compared to wild type and {Delta}hns produced a smooth colony, which also produced significantly less biofilm. {Delta}leuO/{Delta}hns had an opaque morphology and produced significantly more biofilm. Reporter expression assays of PcpsA-gfp and PcpsR-gfp confirmed the roles of both H-NS and LeuO in CPS and biofilm formation. ImportanceThis study determined the role of LeuO and H-NS in controlling the decision between two surface based behaviors, swarming motility and sessile biofilm formation in V. parahaemolyticus. The effects of deletions of leuO, hns, and a double {Delta}leuO/{Delta}hns mutant showed that H-NS was a negative regulator of swarming, but a positive regulator of biofilm formation. The mechanism of this control was in part due to H-NS inhibition of LeuO at loci required for swarming and biofilm formation.