All bacteria compete for growth niches and other limited resources in both the environment and within eukaryotic hosts. These existential battles are waged at several levels, but one common strategy entails the transfer of growth-inhibitory protein toxins directly between competing bacterial cells. Toxic antibacterial effectors are invariably encoded together with immunity proteins, which specifically neutralize toxin activity to prevent auto-inhibition. Antibacterial effector-immunity protein pairs are extraordinarily diverse in sequence, and each immunity protein only provides protection against its cognate toxin. Thus, toxin-immunity protein polymorphism underpins an important mechanism of self/nonself discrimination in bacteria. The Hayes lab is interested in the structure, function and physiology of specialized secretion systems that mediate inter-bacterial conflict. Because these toxin-delivery systems readily penetrate bacterial cell envelopes, they are of considerable interest for the development of novel approaches to antimicrobial therapy.