EFFECT OF TRANSPOSON-INDUCED MOTILITY MUTATIONS ON COLONIZATION OF THE HOST LIGHT ORGAN BY VIBRIO-FISCHERI

Vibrio fischeri is found both as a free-living bacterium in seawater a nd as the specific, mutualistic light organ symbiont of several fish a nd squid species. To identify those characteristics of symbiosis-compe tent strains that are required for successful colonization of the nasc ent light organ of juvenile Euprymna scolopes squids, we generated a m utant pool by using the transposon Mu dI 1681 and screened this pool f or strains that were no longer motile. Eighteen independently isolated nonmotile mutants that were either flagellated or nonflagellated were obtained. In contrast to the parent strain, none of these nonmotile m utants was able to colonize the juvenile squid light organ. The flagel lated nonmotile mutant strain NM200 possessed a bundle of sheathed pol ar flagella indistinguishable from that of the wild-type strain, indic ating that the presence of flagella alone is not sufficient for coloni zation and that it is motility itself that is required for successful light organ colonization. This study identifies motility as the first required symbiotic phenotype of V. fischeri.