1. ¹Savannah River Ecology Laboratory, University of Georgia, Aiken, SC, USA
2. ²Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, ME, USA
3. ³Department of Chemistry and Biomolecular Sciences, Macquarie University, Sydney, New South Wales, Australia

Correspondence: MS Wright, Current address: Flathead Lake Biological Station, University of Montana, 32125 Bio Station Lane, Polson, MT 59860, USA. E-mail: meredith.wright@umontana.edu

Received 24 August 2007; Revised 4 January 2008; Accepted 8 January 2008; Published online 14 February 2008.

Abstract

The acquisition of new genetic material via horizontal gene transfer allows bacteria to rapidly evolve. One key to estimating the contribution of horizontal gene transfer to bacterial evolution is to quantify the abundance of mobile genetic elements (MGEs) in bacterial communities under varying degrees of selective pressure. We quantified class 1 integrase (intI1) gene abundance in total community DNA extracted from contaminated and reference riverine and estuarine microhabitats, and in metal- or antibiotic-amended freshwater microcosms. The intI1 gene was more abundant in all contaminant-exposed communities indicating that relative gene transfer potential is higher in these communities. A second key to assessing the contributions of MGEs to bacterial evolution is to examine the structure and function of the MGE-associated gene pool. We determined that the gene cassette pool is a novel and diverse resource available for bacterial acquisition, but that contamination has no discernible effect on cassette richness. Gene cassette profiles were more similar within sites than among sites, yet bacterial community profiles were not, suggesting that selective pressures can shape the structure of the gene cassette pool. Of the 46 sequenced gene cassette products, 37 were novel sequences, while the 9 gene cassettes with similarity to database sequences were primarily to hypothetical proteins. That class 1 integrons are ubiquitous and abundant in environmental bacterial communities indicates that this group of MGEs can play a substantial role in the acquisition of a diverse array of gene cassettes beyond their demonstrated impact in mediating multidrug resistance in clinical bacteria.