The bead-beating recovery technique was performed as previously
described with a slight modification (Lindsay & Von Holy, 1997). Each
stainless steel coupon was placed in a 50 ml conical flask containing
10ml of peptone saline solution (0.85%wv−1 NaCl and 0.1%wv−1 peptone
(Oxoid))with 20 g of glass beads (5mmdiameter) and agitated on
a shaker (150 rpm) at 25 °C for 10min, afterwhich the cell numberwas
determined by plate counting.
2.6. Statistical analysis
To establish the calibration of each strain, 6 or 7 calibration points
were carried out in triplicate and three experiments were performed,
separately in time. For the comparison of the two methods, triplicate
stainless steel coupons were separately treated with either the impedance
method or the bead-beating recovery technique. The consistency
of different data setwas analysed using t-test. Data thatwere statistically
significant appeared with a P-value b 0.05.
3. Results
The impedance measuring system used in this study measured two
values-the M-value and the E-value. Our previous results showed that
the M-value gave sharp and reproducible growth curves for all
Y. enterocolitica strains (data not shown), whereas the E-value curves
were mostly out-of-shape and poorly reproducible. The M-value was
therefore chosen to establish the calibrations. The calibration equations
(Table 1) showed good correlations comparing the plate counts to the
impedance results (r2 N 0.95).