Persistence of Escherichia coli in freshwater periphyton: biofilm-forming capacity as a selective advantage.

Recent research has shown that Escherichia coli can persist in aquatic environments, although the characteristics that contribute to their survival remain poorly understood. This study examines periphytic E. coli populations that were continuously present in three temperate freshwater lakes from June to October 2008 in numbers ranging from 2 to 2 × 10(2)  CFU 100 cm(-2) . A crystal violet assay revealed that all tested periphytic E. coli isolates were superior biofilm formers and they formed, on average, 2.5 times as much biofilm as E. coli isolated from humans, 4.5 times as much biofilm as shiga-like toxin-producing E. coli, and 7.5 times as much biofilm as bovine E. coli isolates. Repetitive extragenic palindromic polymerase chain reaction (REP-PCR) DNA fingerprinting analysis demonstrated the genetically diverse nature of the periphytic isolates, with genetic similarity between strains ranging from 40% to 86%. Additionally, the role of curli fibers in biofilm formation was investigated by comparing biofilm formation with curli expression under optimal conditions, although little correlation (R(2)  = 0.095, P = 0.005) was found. The high mean biofilm-forming capacity observed in E. coli isolated from the periphyton suggests that selective pressures may favor E. coli capable of forming biofilms in freshwater environments.