Bacteria can transfer DNA from one cell to another, even if they are distantly related. This is called horizontal gene transfer, and it helps bacteria adapt to new contaminants and environmental conditions. It also facilitates the spread of traits such as antibiotic resistance. In the soil, one mechanism of horizontal gene transfer can occur when bacteria contact DNA adsorbed to soil surfaces. Soil surfaces provide places for the DNA to sorb so that they can interact/"infect" with groundwater transported microbial cells. It is important to understand this process because it contributes to the development and spread of both positive (degrading contaminants) and negative (antibiotic resistance) bacterial traits.

Cell movement, or motility, affects how often cells associate with surfaces, and therefore is likely to affect the frequency of interaction between the cells and adsorbed DNA. This project investigates the relationships among cell motility, cell attachment and transport, and gene transfer by DNA adsorbed to soil. The research goal is to identify what controls how fast bacteria are transformed; this is expected to depend on how long they reside on the surface. Cells are expected to transform only when they are actually stuck on the soil surface, not when they are trapped near, but not on, soil surfaces. Thus transformation rates of both motile and non-motile cells will depend on their residence-time on surfaces coated with DNA. The ability of motile cells to swim allows them to approach surfaces independent of the chemistry of surface interactions. Consequently motile cells should exhibit greater frequency of transformation than non-motile cells. These hypotheses will be tested through specific experimental and modeling objectives involving determination of attachment mechanisms in a radial stagnation point flow (RSPF) system, of residence time distributions and spatial distribution in a micromodel system, and of attachment-detachment and gene transfer kinetics in batch and column systems. The experiments will involve both motile and non-motile bacterial strains and will use surfaces coated with DNA. Results will be used in the development and testing of models of bacterial transport and horizontal gene transfer in the soil environment. As groundwater is a major source of drinking water and irrigation water, its vulnerability to biological and chemical contaminants is a major public health concern. The research results will help risk assessment of groundwater contamination, as related processes of microbial transport and microbial evolution are studied together. Graduate and undergraduate students participating in this proposed project will receive an interdisciplinary education, including experience in outreach. The investigators will continue their commitment to recruit women and minorities and to train undergraduate students through Engineers without Borders (EWB) at U of I and at UC Davis and Women Engineering Link at UC Davis.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Thomas Torgersen
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University of California Davis
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