Vibrio cholerae is a bacteria that causes cholera, a often fatal human diarrheal disease. Its site of colonization is the small intestine, where it expresses virulence factors, despite the presence of inhibitory substances such as bile, phospholipids, cholesterol and biliverdin pigment.
Outer membrane proteins (OMPs) are likely to play important roles in the bacterial survival and adaptation to different niches. Seven major OMPs have already been described in V. cholerae. Two of them, OmpU and OmpT, are critical for bile resistance in vitro, virulence factors expression and intestinal colonization.
Recently, a new porin highly expressed in inorganic phosphate (Pi) scarce media, in a PhoB/PhoR-dependent manner, has been described in V. cholerae classical strain 569B. It has been identified as the product of VCA1008, with high similarity to PhoE, a porin induced in E. coli by Pi starvation and specialized in the transport of anionic compounds. Moreover, VCA1008 has been shown to be essential for colonization.
In the present work we investigated a possible role for PhoEVc in bile resistance in classical and El Tor strains of V. cholerae.
We found that, besides 569B, other classical, O395, and a El Tor strain of V. cholerae O1 produced PhoEVc under Pi deprivation. Aditionally, incorporation of PhoEVc caused a reduction in the OmpU and OmpT in the outer membrane of the cells. When sodium deoxycholate (DOC), a constituent of bile salts, is added to low Pi medium, outer membrane of all strains presented an even higher amount of PhoEVc and further reduction in the quantity of other porins. Interestingly, PhoEVc shares high sequence similarity to V. cholerae OmpU, what suggests that, in addition to transport phosphate compounds, it could play a relevant role in the bacteria bile salt resistance.
Using gel shift assay, we demonstrate that PhoB binds a DNA strech containig the putative phoEVc promoter. A phoBR-lacZ fusion assay, on the other hand, showed that the increased expression PhoEVc is not the result of an greater transcription of PhoB/PhoR in the presence of DOC.