Influence of cold stress on the preliminary enrichment time needed for detection of enterohemorrhagic Escherichia coli in ground beef by PCR.
The influence of cold stress at 4 and 0 degree C on the detection time as assessed by impedance technology (Bactometer; Biomérieux, Marcy l'Etoile, France) of different enterohemorrhagic Escherichia coli (EHEC) strains was determined. Although there is some variation in susceptibility among EHEC strains, prolonged exposure of EHEC to cold stress, i.e., 4 and 5 days at 4 and 0 degree C, respectively, in general significantly increased their detection time. This reflects an increase of the lag-phase time caused by cold stress. Two EHEC strains were selected to determine the minimum preliminary enrichment time that would ensure a positive PCR detection of low numbers of verotoxin-producing E. coli (VTEC; 2 to 2 x 10(5) CFU/25 g) inoculated into ground beef (25 g) and stored at 4 or -20 degrees C for 8 and 14 days, respectively. Incubation times of 6 and 9 h of 1 to 10 CFU/g and 1 to 10 CFU/25 g, respectively, were sufficient for PCR detection of VTEC in ground beef when analysis was performed immediately after inoculation (no cold stress). When cells are exposed to cold stress (4 or -20 degrees C) a 24-h enrichment period is recommended. Restriction of enrichment time to 9 h under these circumstances decreases the sensitivity of PCR detection to 80 CFU/g. Hence, to obtain maximum sensitivity, PCR detection of VTEC in naturally contaminated ground beef should be performed after 24 h of enrichment.
Uyttendaele
M
M
Department of Food Technology and Nutrition, Faculty of Agricultural and Applied Biological Sciences, University of Ghent, Belgium.
Grangette
C
C
Rogerie
F
F
Pasteau
S
S
Debevere
J
J
Lange
M
M
D000818:Animals; D002417:Cattle; D003080:Cold Temperature; D004269:DNA, Bacterial / Q000032:analysis; D004926:Escherichia coli / Q000302:isolation & purification*; D008460:Meat / Q000382:microbiology*; D016133:Polymerase Chain Reaction; D013997:Time Factors
D004269:DNA, Bacterial