Meeting Abstract
Acquisition of beneficial symbionts from the environment is found throughout biology, yet little is known about how potential colonizers interact with one another during symbiotic initiation. We use the vibrio-squid symbiosis as a model system to understand how symbionts interact during the infection process to ultimately impact community structure in the host. Although a single species of bioluminescent bacteria (Vibrio fischeri) is found in the light organ of the squid, Euprymna scolopes, this light organ community is comprised of multiple, genetically distinct V. fischeri strains. Several “founder cells” can initiate colonization and expand their populations within the light organ, however certain strain combinations are incompatible and result in a one strain dominating within the host. We hypothesized that such strain-specific interactions are driven by genetic differences that allow symbiotic populations to compete for a narrow niche: the light organ. We developed a culture-based, co-incubation assay to explore strain-specific interactions of light organ symbionts and we observed two outcomes: coexistence where two different populations grew together without apparent inhibition, and competition where one population killed the other. Comparative genomics and targeted mutagenesis indicate symbionts use the type VI secretion system (T6SS), which acts as a molecular syringe, to inject lethal toxins directly into neighboring cells. Interestingly, these squid symbionts do not kill more distantly-related marine bacteria, but rather target other members of the Vibrionaceae family, which are the primary colonizers of this host-associated niche in fish and squid. These findings are the first evidence that symbionts actively kill competitor cells using a mechanism to specifically target populations that share the same niche.
