Gel immobilization improves survival of Escherichia coli under temperature stress in nutrient-poor natural water

Viable Escherichia coli cells were entrapped in agar gel layers to form artificial biofilms. Sessile-like bacteria and planktonic (suspended) counterparts were exposed for 28 d to natural (spring)water at two temperatures (4°C and 18°C). Culturabilities on non-selective and selective solid media were monitored over the exposure period, together with cell viability that was determined by using a fluorescent viability probe and confocal scanning laser microscopy analysis. The number of planktonic organisms recovered on non-selective medium decreased by 3 logarithmic units during exposure at 4°C and was reduced to an undetectable level at 18°C. Whatever the temperature of the spring water microcosm, however, the immobilized-cell population did not decrease by more than one log unit over the exposure period. Cell counts on coliform-selective, lactose–deoxycholate agar plates confirmed the enhanced resistance of gel-entrapped cells to environment stress as compared to planktonic organisms: at the two tested temperatures, sublethal injury of suspended E. coli reached more than 80% whereas it did not exceed 65% for immobilized cells. Viability studies indicated that planktonic cells rapidly died. These results show that the ability of microorganisms to colonize aquatic ecosystems in the fixed state (i.e. as biofilms) must be considered in studies evaluating cell survival in these environments.