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

Viable Escherichia coli cells were entrapped in agar gel layers to form art ificial biofilms. Sessile-like bacteria and planktonic (suspended) counterp arts were exposed for 28 d to natural (spring)-water at two temperatures (4 degrees C and 18 degrees C). Culturabilities on non-selective and selectiv e solid media were monitored over the exposure period, together with cell v iability that was determined by using a fluorescent viability probe and con focal scanning laser microscopy analysis. The number of planktonic organism s recovered on non-selective medium decreased by 3 logarithmic units during exposure at 4 degrees C and was reduced to an undetectable level at 18 deg rees C. Whatever the temperature of the spring water microcosm, however, th e immobilized-cell population did not decrease by more than one log unit ov er the exposure period. Cell counts on coliform-selective, lactose-deoxycho late agar plates confirmed the enhanced resistance of gel-entrapped cells t o 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 indi cated that planktonic cells rapidly died. These results show that the abili ty of microorganisms to colonize aquatic ecosystems in the fixed state (i.e . as biofilms) must be considered in studies evaluating cell survival in th ese environments. (C) 1998 Elsevier Science Ltd. All rights reserved.