INTRODUCTION

Horizontal gene transfer is a ubiquitous process that allows DNA sequences to be widely disseminated in natural microbial populations. During horizontal gene transfer, micro-organisms transfer genetic material to organisms other than their descendants, distinguishing the process from vertical gene transfer in which genetic material is passed to offspring via sexual or asexual reproduction. Horizontal gene transfer is most prevalent among bacteria but is not restricted to prokaryotes; highly unrelated organisms can participate in horizontal gene transfer including single- and multi-celled eukaryotes. Presently, little is known about the rates of horizontal gene transfer in both natural and engineered biofilm environments such as water distribution systems, water and wastewater treatment plants, wetlands and biologically active soils and sediments.

Evidence for horizontal gene transfer has been gathered steadily over the last 50 years. Its most prominent manifestation is the spread of antibiotic resistance among bacteria. The genes conferring antibiotic resistance can be quickly disseminated through bacterial populations via horizontal gene transfer, confounding the development of new drugs to treat bacterial infections. Another manifestation of horizontal gene transfer is tumour formation on the surfaces of plant roots after infection by Agrobacterium tumefaciens. This bacterium has been shown to transfer its DNA to plant cells, where the DNA directs the rapid growth of plant-cell tumours and the production of chemicals that nourish the bacteria. From these studies and others, several mechanisms of gene transfer have emerged, which appear to be highly conserved.

Greater understanding of horizontal gene transfer in the environment is needed for multiple purposes. In particular, there has been rapid progress in the creation of genetically engineered micro-organisms (GEMs) to be released into the environment.