Plasmid transfer betweenPseudomonasspp. within epilithic films in a rotating disc microcosm

A microcosm using rotating slate discs in a chemostat was used to study bacterial population dynamics and genetic interactions in river epilithon. Populations of all introduced donor and recipient Pseudomonas spp. decreased with time but all the bacteria survived better on the slate discs than in the liquid phase. Conjugal transfer of an epilithic plasmid encoding mercury resistance (pQM1) occured with transfer frequencies of 1.4 × 10−6 to 3.6 × 10−3 per recipient, which were about 100-fold lower than in standard membrane filter mating experiments.