Evolution of constitutive bacteriocin production and release

The production of the bacterial anti-competitor toxins, called bacteriocins, is widely described as spite, because the production of the toxins is costly and often requires cell death for release. Given these costs, it reasonable to think that bacteriocin production should be induced by the presence of unrelated competitor strains. Nonetheless, recent evidence in the insect-pathogenic bacterium Xenorhabdus koppenhoeffri showed that bacteriocin production occurs regardless of the presence of unrelated competitors. Could the constitutive production of bacteriocins be favored by natural selection? Here we use a mathematical model to examine this question, both within and between subpopulations. The model assumes that bacteriocin production is constitutive and costly, and that toxin release only occurs during natural cell death. We found that constitutive bacteriocin producers can outcompete non-producer sensitive strains within populations, and that it can spread in a metapopulation provided there is local competition for patches. Hence, the evolution of bacteriocin production does not require the detection of competitors or "suicidal" release of toxins.