Strategies for recovering of planktonic and sessile cells of Escherichia coli O157:H7 from freshwater environment

摘要

The experiments were performed with Escherichia coli O157:H7 EDL 933 in freshwater microcosms at 12 degrees C. At 35, 45, and 70 days, samples were taken and filtered through 0.45 mu m membrane filters. The following alternatives were tested to evaluate the recovery percentage of injured cells:(1) selective media CHROMagar(TM)O157 and chromID(TM)O157:H7 agar, at 37 degrees C for 24 h; (2) tryptic soy agar supplemented with yeast extract (TSAE), incubated at 25 degrees C for 2 or 4 h, then transferred to CHROMagar(TM)O157 or chromID(TM)O157:H7 agar at 37 degrees C (TSAE2h-CHROM, TSAE4h-CHROM and TSAE2h-ID, TSAE4h-ID); (3) thin agar layer (TAL) method, TSAE was overlaid on CHROMagar(TM)O157 or chromID(TM)O157:H7 agar (TALCHROM and TALID, respectively) and incubated at 37 degrees C for 24 h; and (4) TALCHROM at 25 degrees C for 4 h, then continued up to complete 24 h at 37 degrees C (TALCHROM4h). Furthermore, the recovery of E. coli O157:H7 cells adhering to glass coverslips were evaluated to mimic biofilm conditions. The recovery percentages obtained from each alternative were calculated relative to TSAE counts. After 70 days, TSAE4hCHROM and TALCHROM4h showed the highest recovery percentage (>90 %) from water microcosms. Despite the improved recovery of cell adhering to glass surfaces, the percentages obtained with TSAE4hCHROM were low. Further studies for the recovery of biofilm-forming E. coli O157:H7 are required. Preincubation on TSAE at 25 degrees C for 4 h, combined with CHROMagar(TM)O157, or by thin agar layer method (TALCHROM) enhanced significantly the recovery of viable cells of E. coli O157:H7 after prolonged stay in water microcosms.