No abstract

bThe protein RpoS is responsible for mediating cell survival during the stationary phase by conferring cell resistance to various stressors and has been linked to biofilm formation. In this study, the role of the rpoS gene in Escherichia coli O157:H7 biofilm formation and survival in water was investigated. Confocal scanning laser microscopy of biofilms established on coverslips revealed a nutrient-dependent role of rpoS in biofilm formation, where the biofilm biomass volume of the rpoS mutant was 2.4-to 7.5-fold the size of its rpoS ؉ wild-type counterpart in minimal growth medium. The enhanced biofilm formation of the rpoS mutant did not, however, translate to increased survival in sterile double-distilled water (ddH 2 O), filter-sterilized lake water, or unfiltered lake water. The rpoS mutant had an overall reduction of 3.10 and 5.30 log 10 in sterile ddH 2 O and filter-sterilized lake water, respectively, while only minor reductions of 0.53 and 0.61 log 10 in viable counts were observed for the wild-type form in the two media over a 13-day period, respectively. However, the survival rates of the detached biofilm-derived rpoS ؉ and rpoS mutant cells were comparable. Under the competitive stress conditions of unfiltered lake water, the advantage conferred by the presence of rpoS was lost, and both the wild-type and knockout forms displayed similar declines in viable counts. These results suggest that rpoS does have an influence on both biofilm formation and survival of E. coli O157:H7 and that the advantage conferred by rpoS is contingent on the environmental conditions.

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.

KT (2012) Persistence of Escherichia coli in freshwater periphyton: biofilm-forming capacity as a selective advantage. FEMS Microbiol Ecol 79: 608-618. Fig. 2. Relatedness of Escherichia coli isolates from various sources based on an analysis of REP-PCR DNA fingerprints using Pearson correlation. Individual strains are identified based on a letter indicating source grouping (C, bovine; H, human; P, periphytic; O157, STEC serogroup O157; O111, STEC serogroup O111; O26, STEC serogroup O26) and a unique strain number. K12 represents the K-12 standard E. coli strain.