Coevolution of the Listeria Phage-Host System and Intraspecies Antagonism of Listeria monocytogenes

Listeria monocytogenes is a deadly bacterial foodborne pathogen that continues to have global impacts on human health and the economy. Listeria bacteriophages have been studied since the 1940s for their application as a biocontrol and detection tool against L. monocytogenes. Listeria phages have been shown to contribute to genetic diversity of Listeria and can select for phage-resistant variants over time. Some phages can integrate into the genomes of Listeria and influence gene regulation, particularly during Listeria infection of macrophages. Although much has been learned regarding the Listeria phage- host system since the 1940s, there is a lack of research surrounding the co-adaptation ofListeria phages in response to phage-resistance, and the influence of integrated prophage genes on the Listeria phage-host system. In this dissertation, comparative genomics, standard phage characterization protocols, and molecular biology techniques were employed to investigate phage-host and intraspecies interactions. Whole genome sequencing and assembly methods were used to contribute complete bacterial and phage genomes for further comparative analysis. Coevolution of strictly lytic phages with L. monocytogenes coupled with variant analysis revealed genetic mutations that contributed to phage-resistance in L. monocytogenes and altered host range of phages. Genomic analysis was used to demonstrate that recombination occurred between closely related phages during coinfection. This study showed that coevolution can be used to procure Listeria phages with desired host-range changes that target phage-resistant variants of L. monocytogenes. Prophage and monocin gene clusters were found to be prevalent in a panel of Listeria strains and were subjected to induction and host-range analysis to assess their activity. Prophage induction as evidenced by plaque formation was observed from only three of seventeen strains, while all induced strains exhibited the ability to produce zones of inhibition on the Listeria strains tested, consistent with previously reported monocin activity. Monocin gene expression from induced L. monocytogenes was verified using qRT-PCR, and the inhibitory activity of the monocin was confirmed by cloning and expression of the monocin locus from Bacillus subtilis. This study showed that prophage and monocins are active participants in the Listeria phage-host system, and that monocins are involved in intraspecies antagonism.