A 15-h stationary phase culture of E. coli O157:H7 strain
3704 was serially diluted in sterile, quarter-strength
Ringer's solution (Fisher Scienti®c UK Ltd). Dilutions
were plated on Luria Bertani (LB) agar and incubated at
37°C for 18 h and cell numbers calculated from the mean
number of colony forming units (cfu) obtained from
triplicate plates at the appropriate dilution. Soil (10 g
fresh weight) or water (10 ml) was then inoculated with
0á8 ml aliquots of this dilution series. To ensure even
distribution of added cells, soil and inocula were thoroughly
mixed using a sterile spatula and water samples
mixed by gentle, end over end inversion. After mixing,
1 g soil (fresh weight) or 1 ml of water was added to
sterile 50-ml plastic incubation tubes (Greiner Labortechnik
Ltd, Stonehouse, UK) containing 20 ml of Tryptone
Soya Broth (TSB) (Oxoid Ltd, Basingstoke, UK). Inoculated
tubes were incubated at 37°C with continuous
shaking (200 rev min±1) in an orbital shaker (Stuart
Scienti®c, Redhill, UK) for 15 h. Following incubation,
2 ml of primary enrichment was added to 20 ml of TSB
and incubated for 6 h under the conditions described.
Secondary enrichment cultures were then centrifuged at
10 000 g for 15 min. After centrifugation, supernatants
were discarded and cell pellets were frozen at ±20°C prior
to molecular analysis.