Vibrio fischeriOuter Membrane Protein OmpU Plays a Role in Normal Symbiotic Colonization

Abstract

The nascent light-emitting organ of newly hatched juveniles of the Hawaiian sepiolid squid Euprymna scolopes is specifically colonized by cells of Vibrio fischeri that are obtained from the ambient seawater. The mechanisms that promote this specific, cooperative colonization are likely to require a number of bacterial and host-derived factors and activities, only some of which have been described to date. A characteristic of many host-pathogen associations is the presence of bacterial mechanisms that allow attachment to specific tissues. These mechanisms have been well characterized and often involve bacterial fimbriae or outer membrane proteins (OMPs) that act as adhesins, the expression of which has been linked to virulence regulators such as ToxR in Vibrio cholerae. Analogous or even homologous mechanisms are probably operative in the initiation and persistence of cooperative bacterial associations, although considerably less is known about them. We report the presence in V. fischeri of ompU, a gene encoding a 32.5-kDa protein homolog of two other OMPs, OmpU of V. cholerae (50.8% amino acid sequence identity) and OmpL of Photobacterium profundum (45.5% identity). A null mutation introduced into the V. fischeri ompU resulted in the loss of an OMP with an estimated molecular mass of about 34 kDa; genetic complementation of the mutant strain with a DNA fragment containing only the ompU gene restored the production of this protein. The expression of the V. fischeri OmpU was not significantly affected by either (i) iron or phosphate limitation or (ii) a mutation that renders V. fischeri defective in the synthesis of a homolog of the OMP-regulatory protein ToxR. The ompU mutant grew normally in complex nutrient media but was more susceptible to growth inhibition in the presence of either anionic detergents or the antimicrobial peptide protamine sulfate. Interestingly, colonization experiments showed that the ompU null mutant initiated a symbiotic association with juvenile light organ tissue with only about 60% of the effectiveness of the parent strain. When colonization did occur, it proceeded more slowly and resulted in an approximately fourfoldsmaller bacterial population. Surprisingly, there was no evidence that in a mixed infection with its parent, the ompU-defective strain had a competitive disadvantage, suggesting that the presence of the parent strain provided a shared compensatory activity. Thus, the OmpU protein appears to play a role in the normal process by which V. fischeri initiates its colonization of the nascent light organ of juvenile squids.

Role for the major outer-membrane protein from Vibrio anguillarum in bile resistance and biofilm formation

“…One complete ORF was identified and named ompU. The deduced OmpU protein sequence is 330 aa long and is 75 % identical to OmpU from V. cholerae (Sperandino et al, 1996), 59 % identical to OmpU of V. fischeri (Aeckersberg et al, 2001) and 47 % identical to OmpL of Photobacterium profundum (Welch & Bartlett, 1996).…”

“…In addition, a null mutation in the V. fischeri ompU gene also showed an increased sensitivity to membrane-disrupting agents such as bile. However, this observation may indicate the importance of OmpU in maintaining membrane integrity as opposed to survival within the intestinal environment (Aeckersberg et al, 2001).…”

“…In Vibrio cholerae and Vibrio fischeri, OmpU homologues play a role in protecting the bacterium from bile salts (Aeckersberg et al, 2001;Provenzano & Klose, 2000). In V. cholerae, bile resistance is suggested to be an early step in the evolution of this bacterium as an intestinal pathogen.…”

Role for Phosphoglucomutase in Vibrio fischeri-Euprymna scolopes Symbiosis

“…When a sufficient cell density is reached, the bacteria luminesce, providing the counterillumination ability believed to be an antipredation mechanism for the squid host (see reference 49 for a review). Various studies characterizing the Vibriosquid symbiosis have identified several genetic determinants involved in establishing this association (2,14,48,50,52); however, much remains to be learned about how these symbiotic partners interact.…”

“…More recently identified colonization factors include RscS, a sensor kinase hypothesized to regulate symbiosis-specific genes (52), and OmpU, an outer membrane protein (2). Because colonization appears to require active participation on the part of both the bacterium and the squid host (36), it is likely that this exclusive association depends upon numerous additional host and bacterial genes.…”