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 B 0 and T7 0, a mutant bacterial clone which is resistant to T7 0, denote B 1 and a host range mutant phage, T7 1 which is capable of growth on both B 0 and B 1. 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, B 2, which is resistant to both T7 0 and T7 1. In phage—free competition, the wild—type B 0 bacterial clone has a marked advantage relative to both B 1 and B 2 while no difference is detected between B 1 and B 2. When competing for a B 0 host, the wild—type T7 0 phage clone has a marked advantage over T7 1. 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.