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.