Sporulation Activated via σ Protects from a Tse1 Peptidoglycan Hydrolase Type VI Secretion System Effector.

Within bacterial communities, community members engage in interactions employing diverse offensive and defensive tools to reach coexistence. Extracellular-matrix production and sporulation are defensive mechanisms used by Bacillus subtilis cells when they interact with Pseudomonas chlororaphis strains expressing a type VI secretion system (T6SS). Here, we define Tse1 as the main toxin mobilized by the Pseudomonas chlororaphis T6SS that triggers sporulation in Bacillus subtilis. We characterize Tse1 as a peptidoglycan hydrolase that indirectly alters the dynamics and functionality of the cell membrane. We also delineate the response of cells to Tse1, which through the coordinated actions of the extracellular sigma factor σ and the cytoplasmic histidine kinases KinA and KinB, culminates in activation of the sporulation cascade. We propose that this cellular developmental response permits bacilli to defend against the toxicity of T6SS-mobilized Tse1 effector. The study of bacterial interactions is helping to define species-specific strategies used to modulate the competition dynamics underlying the development of community compositions. In this study, we deciphered the role of Pseudomonas T6SS when competing with and the mechanism by which a T6SS-toxin modifies physiology. We found that Pseudomonas triggers sporulation by injecting through T6SS a toxin that we called Tse1. We found that Tse1 is a hydrolase that degrades peptidoglycan and indirectly damages membrane functionality. In addition, we demonstrated the mechanism by which cells increase the sporulation rate upon recognition of the presence of Tse1. Interestingly, asporogenic cells are more sensitive to T6SS activity, which led us to propose sporulation as a last resort of bacilli to overcome this family of toxins.