RP4 plasmid transfer among species of Pseudomonas in a biofilm reactor

Conjugation, the most prevalent mechanism of horizontal gene transfer, has been extensively studied for its role in the dissemination of antibiotic re sistance and catabolic genes among bacteria. Very little research on conjug ation has been conducted in natural biofilm systems, although over 99% of b acteria in nature are attached to surfaces. Previous studies suggest that r ates of conjugation on surfaces are elevated compared to rates in liquid me dia. The goal of this research was to observe conjugation between bacteria growing in a biofilm reactor. Conjugation of the broad host range plasmid R P4 between two species of Pseudomonas occurred in the biofilm reactor at hi gh frequencies. The most important environmental para-meter was the shear s tress at the biofilm-liquid interface. Conjugation was only observed below a shear of 0.0851 N/m(2), corresponding to a laminar flow regime. Increasin g temperature from 15 degrees C to 28 degrees C increased conjugation frequ encies 10,000-fold. Conjugation frequency was unchanged in experiments cond ucted 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 en vironmental parameters to affect plasmid transfer rates among biofilm bacte ria. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.