R-type bacteriocins of determine the outcome of interspecies competition in a natural host environment.

species are bacterial symbionts of nematodes and pathogens of susceptible insects. Different species of nematodes carrying specific species of can invade the same insect, thereby setting up competition for nutrients within the insect environment. While species produce both diverse antibiotic compounds and prophage-derived R-type bacteriocins (xenorhabdicins), the functions of these molecules during competition in a host are not well understood. (), the symbiont of possesses a remnant P2-like phage tail cluster, 1, that encodes genes for xenorhabdicin production. We show that inactivation of either tail sheath () or tail fibre () genes eliminated xenorhabdicin production. Preparations of xenorhabdicin displayed a narrow spectrum of activity towards other and species. One species, (), was highly sensitive to xenorhabdicin but did not produce xenorhabdicin that was active against . Instead, produced high-level antibiotic activity against when grown in complex medium and lower levels when grown in defined medium (Grace's medium). Conversely, did not produce detectable levels of antibiotic activity against . To study the relative contributions of xenorhabdicin and antibiotics in interspecies competition in which the respective species produce antagonistic activities against each other, we co-inoculated cultures with both species. In both types of media outcompeted , suggesting that antibiotics produced by determined the outcome of the competition. In contrast, outcompeted in competitions performed by co-injection in the insect , while in competition with the xenorhabdicin-deficient strain (), was dominant. Thus, xenorhabdicin was required for to outcompete in a natural host environment. These results highlight the importance of studying the role of antagonistic compounds under natural biological conditions.