dilution series of a pure culture. In the case ofEscherichia coligrowing in
brain/heart infusion broth, incubated at 37 1 C, it is possible to detect the
presence of one or two viable cells in five or six hours.
By obtaining detection times using samples where the microbial
population is known, calibration curves relating detection time and
microbial numbers can be drawn so that count data can be derived from
detection times. One such example forSalmonellaEnteritidis is presented
in Figure 10.3. Some claim that the only value in converting detection
times to counts is that the food microbiologist derives a sense of security
from having data in a familiar form. Since electrical methods measure
microbial activity directly, detection time may be a more appropriate
measure of the potential to cause spoilage than a viable plate count.
In the food industry the potential for simultaneously testing many
samples makes electrical methods a useful means for assessing the quality
of raw materials and products. They have the additional advantage that
the worse the microbiological quality, the shorter is the detection time, and
the sooner the manufacturer knows that there may be a problem. In
modern instruments, which can accommodate more than 500 samples, the
results can be displayed using an unambiguous quality colour code of
acceptable (green), marginally acceptable (orange), and unacceptable (red).
Figure 10.3 A calibration curve forSalmonellaEnteritidis using a Bactometer
Chapter 10 385