A Complex Community in a Simple Habitat: An Experimental Study with Bacteria and Phage

Abstract

Continuous culture populations of the bacterium especially coli and its virulent virus T7 have been studied as a model of a predator—prey in a simple habitat. These organisms maintain apparently stable states of coexistence in: (1) a phage—limited situation where all of the bacteria are sensitive to the coexisting virus and the sole, and potentially limiting carbon source, glucose, is present in excess; and (2) a resource—limited situation where the majority of the bacteria are resistant to these phage and in which there is little free glucose. The composition of these interacting populations is examined in detail and evidence indicating that this simple experimental culture system can support relatively complex communities is presented. In the predator—limited situation, two populations at each of two trophic levels can be maintained; the wild—type bacterial and phage strains, denoted B0 and T70, a mutant bacterial clone which is resistant to T70, denote B1 and a host range mutant phage, T71 which is capable of growth on both B0 and B1. In the resource—limited situation, three populations of bacteria and two populations of phage can coexist. The include the above described clones and a third bacterial strain, B2, which is resistant to both T70 and T71. In phage—free competition, the wild—type B0 bacterial clone has a marked advantage relative to both B1 and B2 while no difference is detected between B1 and B2. When competing for a B0 host, the wild—type T70 phage clone has a marked advantage over T71. The fit of these observations to some previously developed theory of resource—limited growth, competition and predation is discussed and a mechanism to account for the persistence of these communities is presented. The latter assumes that their stability can be attributed solely to intrinsic factors, i.e., the population growth and interaction properties of the organisms in this continuous culture habitat.

Modeling the role of bacteriophage in the control of cholera outbreaks

“…An alternative hypothesis would suppose that the bacteria develop phage resistance during the epidemic (15,16). Epidemics may provide increased opportunity for the required mutations to arise during exponential growth occurring within infected individuals.…”

Coevolution can reverse predator–prey cycles

“…Key to our approach is the assumption that clones differ in their life history traits and that there are costs for low vulnerability and high offense. Trade-offs between species traits have been observed in predators (31,32) and prey (13,33,34). In the clonal model, low-vulnerability prey clones are consumed at a lower rate than high-vulnerability clones at the cost of a lower growth rate (i.e., a lower intraspecific competitive ability).…”

Spatial Structure Mitigates Fitness Costs in Host-Parasite Coevolution

“…Recent studies of bacterium-phage coevolution have found that infectivity range is correlated with the number of amino acid changes in tail fibers relative to the ancestral genotype , providing further support for the GFG framework. However, coevolutionary arms races are unlikely to be maintained indefinitely as fitness costs associated with generalism (usually in the form of lower growth/infectivity rates) can reduce selection for broad ranges (Chao et al 1977;Webster and Woolhouse 1999;Sasaki 2000;Bohannan et al 2002;Lopez-Pascua and Buckling 2008;Poullain et al 2008). Sasaki (2000) predicted that fitness costs will lead to fluctuations between specialism (narrow range) and generalism (broad range), but empirical observations suggest that fitness costs may instead lead to fluctuating selection among genotypes with similar ranges .…”