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This dissertation examines the evolution of a spiteful bacterial trait, bacteriocin production using insect-pathogenic bacteria in the genus Xenorhabdus. Bacteriocins are anticompetitor toxins produced by almost all known lineages of bacteria. Bacteriocin production is considered spiteful because it is costly to the producer; it also results in the killing of competiting strains of bacteria. Moreover, the costs of production are often too great to outweigh the direct benefits of competitor killing. Many costly traits in nature are maintained by being phenotypically plastic, and expressed only in environmental contexts where the benefits outweigh the costs. However, the role of competition-induced plasticity in the evolution of bacteriocin production has been largely untested. I empirically tested whether bacteriocin production is induced in response to the presence of non-self competitors, in a natural Xenorhabdus isolate (Chapter 1). Surprisingly, I found no evidence to support the plasticity hypothesis. This result was particularly puzzling because bacteriocin production in gram-negative bacteria like Xenorhabdus is often considered a ‘suicide mission’ where cell lysis is required for toxin release. Why would bacteriocin-producing cells commit ‘suicide’ in the absence of any competitors? To address this question, I used a modeling approach and examined the simplest conditions necessary for costly bacteriocin production to invade a metapopulation of faster-growing, sensitive cells (Chapter 2). Results show that bacteriocin release by lysis upon natural cell death can be sufficient for bacteriocin production to be favored; thus, competitor- induced plasticity is not necessary for bacteriocin production to be maintained. Next, I examined the consequences of spiteful bacteriocin production on disease. Owing to their substantial growth costs, spiteful behaviors are predicted to reduce within-host pathogen growth and thereby pathogen virulence, which is the degree of damage a pathogen causes to its host. Consistent with predictions, I found evidence for reduced levels of bacteriocin production in experimentally evolved lineages that show greater virulence (Chapter 3). Finally, I investigated the evolution of bacteriocin resistance and showed that the incorporation of live, heterospecific competitors in conjunction with bacteriocin doses can reduce the emergence of bacteriocin resistance in vitro (Chapter 4). In the face of the current antimicrobial resistance crisis, these results provide strong proof-of-concept for a novel approach to impede the emergence of resistance against alternative antimicrobials such as bacteriocins. Taken together, this dissertation provides novel insights into the evolution and maintenance of bacteriocins in pathogenic bacteria, and attests to the importance of social interactions in shaping pathogen evolution. |
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