LitR Is a Repressor of syp Genes and Has a Temperature-Sensitive Regulatory Effect on Biofilm Formation and Colony Morphology in Vibrio (Aliivibrio) salmonicida

Vibrio (Aliivibrio )salmonicida is the etiological agent of cold water vibriosis, a disease in farmed Atlantic salmon (Salmo salar ) that is kept under control due to an effective vaccine. A seawater temperature below 12°C is normally required for disease development. Quorum sensing (QS) is a cell density-regulated communication system that bacteria use to coordinate activities involved in colonization and pathogenesis, and we have previously shown that inactivation of the QS master regulator LitR attenuates theV. salmonicida strain LFI1238 in a fish model. We show here that strain LFI1238 and a panel of naturally occurringV. salmonicida strains are poor biofilm producers. Inactivation oflitR in the LFI1238 strain enhances medium- and temperature-dependent adhesion, rugose colony morphology, and biofilm formation. Chemical treatment and electron microscopy of the biofilm identified an extracellular matrix consisting mainly of a fibrous network, proteins, and polysaccharides. Further, by microarray analysis of planktonic and biofilm cells, we identified a number of genes regulated by LitR and, among these, were homologues of theVibrio fischeri symbiosis polysaccharide (syp ) genes. Thesyp genes were regulated by LitR in both planktonic and biofilm lifestyle analyses. Disruption ofsyp genes in theV. salmonicida ΔlitR mutant alleviated adhesion, rugose colony morphology, and biofilm formation. Hence, LitR is a repressor ofsyp transcription that is necessary for expression of the phenotypes examined. The regulatory effect of LitR on colony morphology and biofilm formation is temperature sensitive and weak or absent at temperatures above the bacterium's upper threshold for pathogenicity.