When suspensions of E. coli were heated at 52, 59 or 66 C, the
numbers of E. coli recovered on PCA or MAC declined progressively
until colonies could not be recovered on either agar. Differences in
mean Ct values for DNA extracted from samples treated with PMA,
or from samples taken from the same suspensions at the same
times and not treated with PMA were <1 cycle, even when viable
E. coli could not be recovered from the suspensions (Table 1). When
suspensions were heated at 72, 80 or 90 C for 1.5 min, the difference
in mean Ct values for DNA from cells treated or not treated
with PMA increased with increasing temperature although no
viable E. coli were detected in any of the heated suspensions.
The mean Ct values for DNA extracted from viable cells not
treated with deoxycholate or treated with deoxycholate at
concentrations 2.5% before treatment with PMA were not significantly
different (P > 0.05; Fig. 2). The mean Ct values for DNA
extracted from cells killed by heating at 52 C for 18 h increased
with increasing deoxycholate concentration up to 0.5%. The mean
Ct value for cells treated with 1% sodium deoxycholate was not
significantly different (P > 0.05) from the mean Ct value for cells
treated with 0.5% sodium deoxycholate, but the mean Ct value for
cells treated with 2.5% sodium deoxycholate was significantly less
(P < 0.05).
When suspensions of E. coli or milk inoculated with the
organism were incubated at 52, 59, 66 and 72 C, the numbers of
E. coli recovered on MAC were mostly less than the numbers
recovered on plates of PCA inoculated from the same samples.
However, the differences were generally small, with the mean and
standard deviation for the numbers recovered on MAC as percent
fractions of the numbers recovered on PCA from 22 samples of
heated suspensions or inoculated milk being 91 10%.
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