The transfer of genetic material among bacteria in the environment has been linked to a range of important phenomena related to bacterial adaptation and evolution, including bioremediation capability, metal tolerance, antibiotic resistance by pathogens in the environment, and gene fl ow from genetically modifi ed microorganisms. Transfer of genetic material can occur among microorganisms present in both the planktonic and attached states. Given the propensity of organisms to exist in sessile communities under oligotrophic conditions, and that such conditions typify the subsurface, study of subsurface gene transfer phenomena should include processes and kinetics for a range of sessile community structures, from reversibly attached single cells to mature biofi lms, as well as planktonic communities. This study very briefl y reviewed horizontal, primarily conjugative, gene transfer in natural porous media, and the kinetics used to date to describe conjugative gene transfer in both planktonic (aqueous suspension) and sessile (surface associated) communities. The mathematics so far used to describe the kinetics of conjugation have developed largely from experimental observations of planktonic gene transfer, and are absent of time lags that occur between gene transfer events or plasmid stability that appear experimentally. We develop a novel formulation of delay-difference equations for gene transfer for attached-state microbes using an exposure-time approach to account for lags.

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