AbstractsBiology & Animal Science

Bacterial viruses (i.e. phages) are ubiquitous intracellular parasites of bacteria, that along with protist grazers account for majority of bacterial mortality in nature. Phages impose strong selection for bacterial phage-resistance, which is often coupled with fitness costs on bacterial traits such as growth ability, virulence or motility. Traditionally phage-host interactions have been studied with two species systems in the laboratory, neglecting the complex web of interactions present in natural communities. The ability of phages to selectively kill bacteria has ignited an interest on phages as alternative antibacterials. However, in order to develop phage therapy, understanding of phage-host interactions in the eco-evolutionary context is essential. In this thesis I studied the implications of lytic phages on opportunistic pathogenic bacteria, as opportunists often have the ability reproduce and reside in outside-host environments, where they are predisposed to a variety of selection pressures. The role of phages in top-down control of bacterial biomass and the evolution of bacterial phage-resistance were studied in the presence of protist predators with differing feeding modes, in low-resource systems mimicking natural pond environment. Hypothesis of coincidental evolution suggests that virulence is a by-product of selection for traits that maximize bacterial fitness in environmental reservoirs. Yet, disease outbreaks by opportunists are relatively rare, suggesting that something constrains the selection for virulence. To assess the role of lytic phages on the evolution of virulence, bacteria were cultured in low-resource environment, accompanied with changes in temperature regime or changes in composition of the community of interacting bacterial enemy species, and the virulence of bacteria was measured in vivo. To study whether the potential phage-resistant bacteria surviving phage therapy would be coupled with lowered virulence, due to costs associated with phage-resistance, a clinical bacterial isolate was exposed to phage cocktails and the virulence of the phage-resistant bacteria was measured in vivo. given the strong selection for phage- resistance, the prospects of phage therapy depend a great deal on whether new phages infecting pathogenic bacteria can be readily isolated from environment. To address this, an attempt was made to isolate phages against clinical bacterial isolates harboring resistance genes to multiple antibiotics. A single lytic phage was shown to be a non- efficient top-down regulator of bacterial biomass. Rapidly emerging phage-resistant bacteria took over the bacterial populations after initial lysis by phages and protist grazers accounted for most of the long-term negative trophic effects on bacterial biomass. The presence of protist predators selected for bacteria that were less susceptible to infection by lytic phages, which suggests an overlap in the bacterial defense against a parasite and predatory protists. In general, the presence of lytic phages selected for lowered…