The objective of this research is to elucidate the mechanisms by which animals acquire bacterial symbionts anew during each generation. To reach this goal I will take advantage of the natural association between the Hawaiian bobtail squid, Euprymna scolopes, and its specific, luminescent symbiont, Vibrio fischeri. The morphology of the symbiosis facilitates direct access to the site of infection, allowing for imaging of live bacteria as they colonize nascent squid. <P>The experiments proposed combine molecular genetic approaches with confocal laser-scanning microscopy to address the following specific aims: (1) identification of factors in V. fischeri that mediate the initiation of the symbiosis, and (2) examination of the role played by host-derived mucus in establishing the symbiosis and determining bacterial species-specificity. Colonization of mucosal surfaces by pathogenic Vibrio species is a critical process for infection.<P> This benign infection provides a natural environment in which to study these processes with minimal perturbation, which may also yield insights into how pathogenic infections are mediated in a variety of hosts.