Affiliation:
1. Department of Microbiology, University of Texas Health Science Center, San Antonio, Texas 78284-7758
Abstract
ABSTRACT
The transmembrane regulatory protein ToxR is required for expression of virulence factors in the human diarrheal pathogen
Vibrio cholerae
, including cholera toxin (CT) and the toxin coregulated pilus (TCP). ToxR is necessary for transcription of the gene encoding a second regulatory protein, ToxT, which is the direct transcriptional activator of CT and TCP genes. However, ToxR, independent of ToxT, directly activates and represses transcription of the outer membrane porins OmpU and OmpT, respectively. The genes encoding TCP and CT (and including ToxT) lie on horizontally acquired genetic elements, while the
toxR
,
ompU
, and
ompT
genes are apparently in the ancestral
Vibrio
chromosome. The contribution of ToxR-dependent modulation of outer membrane porins to cholera pathogenesis has remained unknown. We demonstrate that ToxR mediates enhanced bile resistance in a ToxT-independent manner. In both classical and El Tor biotypes of
V. cholerae
, a
toxR
mutant strain has a reduced minimum bactericidal concentration (MBC) of bile, the bile component deoxycholate (DC), and the anionic detergent sodium dodecyl sulfate (SDS) compared to both wild-type and
toxT
mutant strains. Classical and El Tor
toxR
mutant strains also exhibit reduced growth rates at subinhibitory concentrations of DC and SDS. Growth of either
V. cholerae
biotype in subinhibitory concentrations of bile or DC induces increased ToxR-dependent production of a major 38-kDa outer membrane protein, which was confirmed to be OmpU by Western blot. Measurement of transcription of a
ompU
p-
lacZ
fusion in both biotypes reveals stimulation (about two- to threefold) of ToxR-dependent
ompU
transcription by the presence of bile or DC, suggesting that ToxR may respond to the presence of bile. The
toxR
mutant strains of three additional human intestinal pathogenic
Vibrio
species,
V. mimicus
,
V. fluvialis
, and
V. parahaemolyticus
, display lower MBCs of bile, DC, and SDS and have altered outer membrane protein profiles compared to the parental wild-type strains. Our results demonstrate a conserved role for ToxR in the modulation of outer membrane proteins and bile resistance of pathogenic
Vibrio
species and suggest that these ToxR-dependent outer membrane proteins may mediate enhanced resistance to bile. We speculate that ToxR-mediated bile resistance was an early step in the evolution of
V. cholerae
as an intestinal pathogen.
Publisher
American Society for Microbiology
Subject
Infectious Diseases,Immunology,Microbiology,Parasitology