The sensitivity and robustness of the PCR-based assay
were investigated by spiking different environmental samples
with decreasing numbers of stationary phase E. coli
O157:H7 and subjecting spiked material to a primary and
secondary enrichment prior to DNA extraction and PCR
ampli®cation. The assay was extremely sensitive, capable of
detecting initial E. coli O157:H7 populations 610 cfu g±1
or ml±1 in all of the environmental matrices tested (Table 1).
Indeed, after enrichment, it was possible to detect E. coli
O157:H7 populations as few as 1 cfu ml±1 in private
drinking water. The assay was robust in contrasting soils,
as soil properties such as humic content have been reported
to inhibit PCR (Tebbe and Vahjen 1993). PCR ampli®cation
in the different soils used in this study, may therefore
have arisen from dilution of PCR-inhibiting soil compounds
in the secondary enrichment, while at the same
time, enrichment served to increase numbers of the target
bacteria.