Identification and Characterization of OscR, a Transcriptional Regulator Involved in Osmolarity Adaptation in Vibrio cholerae

Abstract

Vibrio cholerae is a facultative human pathogen. In its aquatic habitat and as it passes through the digestive tract, V. cholerae must cope with fluctuations in salinity. We analyzed the genome-wide transcriptional profile of V. cholerae grown at different NaCl concentrations and determined that the expression of compatible solute biosynthesis and transporter genes, virulence genes, and genes involved in adhesion and biofilm formation is differentially regulated. We determined that salinity modulates biofilm formation, and this response was mediated through the transcriptional regulators VpsR and VpsT. Additionally, a transcriptional regulator controlling an osmolarity adaptation response was identified. This regulator, OscR (osmolarity controlled regulator), was found to modulate the transcription of genes involved in biofilm matrix production and motility in a salinity-dependent manner. oscR mutants were less motile and exhibited enhanced biofilm formation only under low-salt conditions.The halotolerant bacterium Vibrio cholerae causes the disease cholera. V. cholerae inhabits aquatic environments, and its occurrence strongly correlates with salinity (15,24,33,37). Interannual climate variation is thought to have a significant impact on the environmental conditions governing V. cholerae occurrence and the incidence of cholera outbreaks (29,47,52). Studies have shown that cholera outbreaks occur seasonally and correlate with the monsoon in areas where cholera is endemic (11,20,46). Furthermore, seasonal fluctuations in rainfall as well as changes in sea surface height brought by tides and storm surges can cause wide variations in salinity concentrations, which affect the growth and distribution of the pathogen (15,20,57). Salinity is therefore a significant driving force in the occurrence of V. cholerae and cholera outbreaks.Microcosm experiments show that the growth and survival of V. cholerae are dependent on salinity, and the optimal salt concentration for growth is similar to salinities of brackish water and estuarine environments (38,56,57). Additional factors synergize with salinity to affect the growth and survival of V. cholerae. Increased concentrations of organic nutrients can compensate for a growth defect caused by high or low salinity concentrations (25,38,57). The organic carbon level was shown to be a key parameter in the growth of V. cholerae in natural freshwater (61) and seawater (43) microcosms. Additionally, lower temperatures (Յ15 to 20°C) were shown to slow or stop the growth of V. cholerae if salinity concentrations were above or below an optimal level (56). Adaptation to highsalinity conditions in many microorganisms involves the synthesis and transport of compatible solutes, which act to stabilize intracellular levels of water and turgor pressure without disturbing cellular function (6). V. cholerae can synthesize the compatible solute ectoine by using a four-gene operon which includes ectABC and a putative aspartokinase gene (49). V. cholerae can also import the compatible solutes prol...

The Virulence Transcriptional Activator AphA Enhances Biofilm Formation by Vibrio cholerae by Activating Expression of the Biofilm Regulator VpsT

“…V. cholerae biofilm formation requires a number of transcriptional regulators. The expression of one of the key regulators, VpsT, is known to be controlled by a number of regulatory components, such as quorum sensing (42), c-di-GMP (21), the biofilm activator VpsR (40), and osmolarity (31). To further investigate whether additional regulation is involved in vpsT transcription, we constructed a P vpsT -luxCDABE transcriptional fusion on a plasmid.…”

“…The regulation of VPS synthesis in V. cholerae is very complex. Environmental signals such as monosaccharides, nucleosides, indole, and osmolarity have been identified as being activators of vps gene transcription and biofilm formation (11,13,29,31). VpsT and VpsR are positive regulators of vps expression (1,34,39).…”

OscR, a New Osmolarity-Responsive Regulator in Vibrio cholerae

“…An implication of this control is that biofilm formation may be less adaptive to V. cholerae under low osmolarity conditions, although it is unclear at present why biofilm formation would not be equally beneficial to the bacterium at different salinities. Conversely, the expression of flagellin genes, e.g., flaB and flaD, and motility apparently are induced in the presence of OscR, but again, only under low-osmolarity conditions (29). These findings suggest that the bacterium senses and mounts a substantial and highly specific adaptational response to low osmolarity.…”

“…OscR is a member of the IclR family of transcriptional regulators, which are found in a variety of gram-negative and gram-positive bacteria and which function as repressors, activators, or both; these proteins contain a GAF domain, which binds cyclic nucleotides and sodium ions (1,20). Hypothetical proteins homologous to OscR are found in other Vibrio species and other gram-negative bacteria (29).…”

“…Understanding how the bacterium adapts to changes in salinity in its transitions between aquatic habitats, sources of drinking water, and the human intestine may therefore provide critical insight into the interrelated issues of the environmental dynamics of the bacterium and its pathogenic interactions with humans. In this issue, Shikuma and Yildiz (29) report a transcript profiling approach leading to identification of V. cholerae genes differentially regulated by salinity (osmolarity). Their identification and analysis of salinityresponsive genes, and in particular the identification of a new osmolarity-responsive regulator, OscR, open up new territory on how V. cholerae copes with and responds adaptively to changes in salinity during its transitions between the environment and the human intestine.…”