Vibrio fischeri outer membrane protein OmpU plays a role in normal symbiotic colonization

The nascent light-emitting organ of newly hatched juveniles of the Hawaiian sepiolid squid Euprymma scolopes is specifically colonized by cells of Vib rio fischeri that are obtained from the ambient seawater. The mechanisms th at promote this specific, cooperative colonization are likely to require a number of bacterial and host-derived factors and activities, only some of w hich have been described to date. A characteristic of many host-pathogen as sociations 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 ad hesins, the expression of which has been linked to virulence regulators suc h as ToxR in Vibrio cholerae. Analogous or even homologous mechanisms are p robably operative in the initiation and persistence of cooperative bacteria l associations, although considerably less is known about them. We report t he presence in V. fischeri of ompU, a gene encoding a 32.5-kDa protein homo log of two other OMPs, OmpU of V. cholerae (50.8% amino acid sequence ident ity) 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 m utant 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 compl ex nutrient media but was more susceptible to growth inhibition in the pres ence of either anionic detergents or the antimicrobial peptide protamine su lfate. Interestingly, colonization experiments showed that the ompU null mu tant initiated a symbiotic association with juvenile light organ tissue wit h only about 60% of the effectiveness of the parent strain. When colonizati on did occur, it proceeded more slowly and resulted in an approximately fou rfold-smaller bacterial population. Surprisingly, there was no evidence tha t in a mixed infection with its parent, the ompU-defective strain had a com petitive disadvantage, suggesting that the presence of the parent strain pr ovided a shared compensatory activity. Thus, the OmpU protein appears to pl ay a role in the normal process by which V. fischeri initiates its coloniza tion of the nascent light organ of juvenile squids.