Type VI secretion drives reciprocal growth inhibition among symbiotic Vibrio fischeri strains

Bacterial cells are often in direct contact with one another and compete for nutrients and space. The type VI secretion system (T6SS) is a contact-dependent mechanism that enables bacteria to deliver toxic effectors into neighbouring cells. Vibrio fischeri is a bacterial symbiont of the Hawaiian bobtail squid Euprymna scolopes and encodes a T6SS on its second chromosome (T6SS2) that promotes interference competition among strains within the squid light organ. Effector diversity among T6SS2⁺ strains is poorly understood, which impedes predicting compatibility among strains within the light organ. Here, we report our investigation of strain ANM004 and how T6SS2 facilitates its interactions with other strains including the model T6SS2⁺ strain FQ-A001. We show that both strains exhibit reciprocal T6SS2-dependent antagonism. Comparative genomics reveals each strain encodes unique effectors within T6SS2 gene loci. Heterologous expression of an ANM004-encoded AHH nuclease effector in FQ-A001 inhibits kin growth in a T6SS2-dependent manner. This effect on FQ-A001 kin is abrogated by expression of the cognate immunity factor encoded downstream of the effector gene. Together, these results establish an experimental framework for investigating intercellular interactions between two T6SS2⁺ symbiotic strains.