Evolution of horizontal transfer in E. coli plasmids
For many parasites, a fundamental conflict should exist between modes of horizontal (infectious) and vertical (intergenerational) transmission. Parasite activities that increase infectious transmission are presumed to generally reduce host fitness (growth rate). In turn, reduced host fitness impedes vertical transmission of the parasite and thereby causes a tradeoff between transmission routes.
We use conjugative plasmids and their E. coli hosts as a model to examine the evolution of parasite transmission. One experiment allowed plasmids to evolve for hundreds of generations in environments that contained different densities of bacterial hosts (Turner et al. 1998, Evolution 52:315-329). The plasmid’s rate of conjugative (infectious) transfer increased at the expense of host fitness, indicating a tradeoff between horizontal and vertical modes of transmission. Surprisingly, susceptible host density had no significant effect on which mode of transfer was selectively favored. Continued research focuses on other environmental factors mediating the evolution of conjugation rates, epistatic interactions between genes for antibiotic-resistance and conjugative-transfer, and phenotypic plasticity in traits governing plasmid transmission.
Evolution of E. coli-Phage Interactions
Filamentous phages such as M13 generally do not kill their bacterial hosts. Rather, these viruses replicate within the cytoplasm, and can be inherited during cell division (vertical transfer) or can move between cells (horizontal transfer). Our ongoing work concerns effects of filamentous phage on the growth of their E. coli hosts, and whether their symbiotic interaction can switch from parasitism to mutualism depending on ecological conditions, such as presence of antibiotics. Also, we are using experimental evolution of E. coli-phage interactions to test whether viruses can move freely along the parasitism-mutualism continuum, versus being evolutionarily ‘locked-in’ to one type of symbiosis that prevents them from switching to a new evolved strategy.
Phage therapy is the use of bacteria-specific viruses to treat bacterial infections, rather than using antibiotic drugs. Widespread failure of antibiotics suggests that phage therapy and other alternatives may become increasingly important in combatting multi-drug resistant bacteria. We are using experimental evolution and molecular microbiology to determine how phage-therapy candidates may evolutionarily improve in their ability to attack deadly bacterial pathogens, such as E. coli O157:H7. In addition, we are using mathematical modeling to examine how virus characteristics such as mutation rate should affect abilities for phages to coevolve with bacterial pathogens (Kysela and Turner 2007, J. Theor. Biol. 249:411-421).
Evolution of mutualisms
– John Lennon and Paul McCartney (A Little Help from my Friends, 1967)
Mutualisms (cooperative interactions) have traditionally received less attention than parasitic interactions. This is surprising given that cooperation among individuals is extremely common in nature. For instance, the stability of bacterial communities may often rely upon cross-feeding, whereby certain bacteria excrete metabolites (nutrients) that are essential for the persistence of other strains in the local environment.
We are currently using E. coli bacteria to examine the ecology and evolution of mutualisms. In particular, an earlier study found that stable genetic polymorphisms can evolve even when bacteria are cultured in simple habitats containing only the single limiting resource glucose (Turner et al. 1996, Ecology 77:2119-2129). Ongoing projects concern the stability of the coexistence in different laboratory environments, the metabolites that contribute to the observed cross-feeding, and the vulnerability of mutualisms to invasion by exploitative genotypes and parasites.