Competition in dense bacterial biofilms through killing and replication

Typically, biofilms feature many different microbial species and strains. This diversity presents a challenge, as individuals must compete for resources and space. In response, microbial species have evolved antagonistic mechanisms such as the Type VI secretion system (T6SS). This is a contact-dependent toxin-delivery system that enables the injection of fatal toxins into other bacteria as well as eukaryotic cells. Previous studies have shown that such an antagonistic one-on-one interaction causes an initially well-mixed culture to phase separate, providing protection by number. We study biofilms consisting of V. cholerae, which express T6SS, as an experimentally controllable and practically relevant system of mutual killers. While we observe phase separation, the typical size of clonal patches stops changing much earlier than expected. In experimental and numerical studies of mutual killing strains of V. cholerae, we show that a protective layer of debris forms at the interface between patches, which slows down further killing. This poses major consequences for the role of antagonistic interactions and the survival dynamics in biofilms.