Physiological Heterogeneity Triggers Sibling Conflict Mediated by the Type VI Secretion System in an Aggregative Multicellular Bacterium

A hallmark of social microorganisms is their ability to engage in complex and coordinated behaviors that depend on cooperative and synchronized actions among many cells. For instance, myxobacteria use an aggregation strategy to form multicellular, spore-filled fruiting bodies in response to starvation. One barrier to the synchronization process is physiological heterogeneity within clonal populations. How myxobacteria cope with these physiological differences is poorly understood. Here, we investigated the interactions between closely related but physiologically distinct Myxococcus xanthus populations. We used a genetic approach to create amino acid auxotrophs and tested how they interact with a parental prototroph strain. Importantly, we found that auxotrophs were killed by their prototroph siblings when the former were starved for amino acids but not when grown on rich medium or when both strains were starved. This antagonism depended on the type VI secretion system (T6SS) as well as gliding motility; in particular, we identified the effector-immunity pair (TsxEI) as the mediator of this killing. This sibling antagonism resulted from lower levels of the TsxI immunity protein in the starved population. Thus, when starving auxotrophs were mixed with nonstarving prototrophs, the auxotrophs were susceptible to intoxication by the TsxE effector delivered by the T6SS from the prototrophs. Furthermore, our results suggested that homogeneously starving populations have reduced T6SS activity and, therefore, do not antagonize each other. We conclude that heterogeneous populations of M. xanthus use T6SS-dependent killing to eliminate starving or less-fit cells, thus facilitating the attainment of homeostasis within a population and the synchronization of behaviors. IMPORTANCE Social bacteria employ elaborate strategies to adapt to environmental challenges. One means to prepare for unpredictable changes is for clonal populations to contain individuals with diverse physiological states. These subpopulations will differentially respond to new environmental conditions, ensuring that some cells will better adapt. However, for social bacteria physiological heterogeneity may impede the ability of a clonal population to synchronize their behaviors. By using a highly cooperative and synchronizable model organism, M. xanthus, we asked how physiological differences between interacting siblings impacted their collective behaviors. Physiological heterogeneity was experimentally designed such that one population starved while the other grew when mixed. We found that these differences led to social conflict where more-fit individuals killed their less-fit siblings. For the first time, we report that the T6SS nanoweapon mediates antagonism between siblings, resulting in myxobacterial populations becoming more synchronized to conduct social behaviors.