Diverse Phage-Encoded Toxins and Their Role in Bacterial Ecology

Bacteriophages are the most abundant and diverse microbial entity on the planet. Found in every environment, the influence of phages extends beyond their bacterial host and has a variety of impacts within an ecosystem, including controlling bacterial abundance, affecting community composition, and even influencing biogeochemical cycling; they are also key players in microbial evolution. Lysogenic phages commonly introduce beneficial genes that drive evolution and promote adaptation. The effects of lysogeny are as broad reaching as phages themselves, imparting such benefits as enhanced bacterial fitness, phenotypic plasticity, biofilm formation, antimicrobial compounds or resistance, as well as virulence factors. While pathogens benefit from a multitude of virulence factors through lysogeny, phage-encoded toxins and toxin-conversion are most well-characterized insofar as their contribution to bacterial pathogenesis and their overall impact on the human or animal host. The toxins’ effects on hosts are typically detrimental or even lethal; sometimes the expression of the toxin results in lethality to the bacterial host itself. However, not all phage-encoded toxins have a negative effect on the environment in which the toxin is produced. The protection against parasitoid wasps that exists in some forms of aphids is a result of a complex interplay between the aphid, their facultative endosymbiont Hamiltonella defensa, and the APSE phage-encoded toxins it produces. Understanding the contributions of phage-encoded toxins to their bacterial hosts and the impacts they have on the various ecosystems where they are expressed is paramount to deciphering the complexity and driving force for the diversity and exchange of phage-encoded toxins within bacterial communities. This chapter gives an overview of the various impacts that prophages and subsequent lysogenic conversion confer to their bacterial hosts, with a focus on toxin production by several medically important pathogens and the subsequent impacts on the human hosts they infect. The beneficial relationship between phage-encoded toxins and their impact on insects is also explored.