CHAPTER 10
Methods for the Microbiological
Examination of Foods
10.1 INDICATOR ORGANISMS
It is frequently necessary to conduct a microbiological examination of
food to determine its quality. This may be necessary to estimate its shelf-
life, its suitability for human consumption or to confirm that it meets
some established microbiological criterion (see Chapter 11). The total
mesophilic plate count is widely used as a broad indication of microbio-
logical quality, although it is unsuitable for this purpose in fermented
foods which contain large numbers of organisms as a natural conse-
quence of their preparation. In other foods, knowledge of the total count
may be useful but it is often of more value to obtain an estimate of the
numbers of a particular component of the total flora. Examples of this
could be moulds in a cereal, psychrotrophic bacteria in a chilled food,
anaerobes in a vacuum-packed food, or yeasts in a fruit beverage.
A quite different reason for a microbiological examination of a food
may be to identify the cause of spoilage or the presence of a pathogen
where a food has been implicated in foodborne illness. The methods for
determining an estimate of the total mesophilic count are very different
from those required for demonstrating the presence of a pathogen, or its
isolation for further study.
The isolation of specific pathogens, which may be present in very low
but significant numbers in the presence of larger numbers of other
organisms, often requires quite elaborate procedures, some of which
are outlined in Chapter 7. It may involve enrichment in media which
encourage growth of the pathogen while repressing the growth of the
accompanying flora, followed by isolation on selective diagnostic media,
and finally the application of confirmatory tests (see, for example,
Section 7.12.4).