1. ¹NERC Centre for Population Biology, Imperial College London, Berkshire, UK
2. ²Molecular Microbial Ecology Group, Centre for Ecology and Hydrology, Oxford, UK

Correspondence: Dr RJ Ellis, NERC Centre for Population Biology, Imperial College London, Silwood Park Campus, Ascot, Berkshire SL5 7PY, UK. E-mail: r.ellis@imperial.ac.uk

³Current address: Molecular Microbiology, Pharmaceutical Science Research Division, Kings College London, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NH, UK.

⁴Current address: Department of Animal and Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield S10 2TN, UK.

⁵Current address: Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK.

Received 19 February 2007; Revised 21 February 2007; Accepted 21 February 2007.

Abstract

The conditions promoting the persistence of a plasmid carrying a trait that may be mutually beneficial to other cells in its vicinity were studied in structured and unstructured environments. A large plasmid encoding mercury resistance in Pseudomonas fluorescens was used, and the mercury concentration allowing invasion from rare for both plasmid-bearing and plasmid-free cells was determined for different initial inoculum densities in batch-culture structured (filter surface) and unstructured (mixed broth) environments. A range of mercury concentrations were found where both cell types could coexist, the regions being relatively similar in the two types of environment although density-dependent in the unstructured environment. The coexistence is explained in terms of frequency-dependent selection of the mutually beneficial mercury resistance trait, and the dynamics of bacterial growth under batch culture conditions. However, the region of coexistence was complicated by conjugation which increased plasmid spread in the mixed broth culture but not the structured environment.