Abstract:

In this study, the virulence genes of four Escherichia coli O157:H7 strains were detected by multiplex polymerase
chain reaction (PCR). Then three selective media were compared for enumeration of E. coli O157:H7 and the injury of
E. coli O157:H7 under cold stress was also investigated with the appropriate selective medium. The results showed that
CICC 21530 strain was positive for all the three genes stx1, stx2 and eae, while NCTC 12900 and one beef isolate were
eae-positive, and two other isolates were negative for all the three virulence genes, which indicated that the pathogenicity of
four strains was different. The counts of all four strains on three selective media were lower than those on tryptose soya agar (TSA)
(P < 0.05) but there was no significant difference between them except for cefixime-tellurite sorbitol macconkey agar (CT-SMAC),
which indicated that CT-SMAC was not suitable for counting E. coli O157:H7 due to the greater inhibition on normal bacteria,
while SMAC or mEMB could be used. Then the injury of E. coli O157:H7 stored at 4 ℃ (cold stress) was investigated with
each of these two selective media. The results showed that the counts on SMAC, mEMB, or TSA were decreased during
the whole storage and some bacteria were injured or killed on the 10th day. In the current study, the selective media for accurate
enumeration of four E. coli O157:H7 strains with different virulence genes were finalized, which would provide the scientific basis
for quantitative assessment and risk control of E. coli O157:H7 in food safety evaluation.

Key words: Escherichia coli O157:H7, virulence genes, sub-lethal injury, cold stress, selective media