Conjugation, the most prevalent mechanism of horizontal gene transfer, has been extensively studied for its role in the dissemination of antibiotic resistance and catabolic genes among bacteria. Very little research on conjugation has been conducted in natural biofilm systems, although over 99% of bacteria in nature are attached to surfaces. Previous studies suggest that rates of conjugation on surfaces elevated compared to rates in liquid media. The goal of this research was to observe conjugation between bacteria growing in a biofilm reactor. Conjugation of the broad host range plasmid RP4 between two species of Pseudomonas occurred in the biofilm reactor at high frequencies. The most important environmental para-meter was the shear stress at the biofilm-liquid interface. Conjugation was only observed below a shear of 0.0851 N/m2, corresponding to a laminar flow regime. Increasing temperature from 15°C to 28°C increased conjugation frequencies 10,000-fold. Conjugation frequency was unchanged in experiments conducted with 3.5, 7, and 35 mg/l acetate, though total cell concentration in the biofilm increased as expected. These data suggest ways to manipulate environmental parameters to affect plasmid transfer rates among biofilm bacteria.

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