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Case Study 2.1 Environmental Health Laboratory (Middlesborough,
England)
Some forms of environmental contamination are not, by their very nature, imme-
diately apparent. This is particularly true when it is of microbial origin. The
Environmental Health Laboratory of the Public Health Laboratory Service (PHLS)
at The James Cook University Hospital, Middlesborough, is responsible for envi-
ronmental microbial analysis. Materials most commonly examined are from water
originating from the sea, rivers, spa pools, swimming pools and drinking water. They
also analyse food in cases of suspected food poisoning and swabs from a variety
of possibly contaminated surfaces. On receipt of samples at the PHLS, they are
recorded with full details of time and place of sampling and are given identification
codes. The laboratory insists that the specimens are transported in such a manner as
to prevent bacterial multiplication. For some bacteria a resuscitation step is included
to aid the recovery of damaged organisms. This can be important to ensure that
all microbes present in the original material remain viable and culturable at least
until testing has been accomplished. The total concentration of bacteria is deter-
mined by transferring a measured volume of the sample onto a petri dish containing
growth medium solidified in agar. After these ‘plates’ have been incubated, usually
overnight, colonies are visible with the naked eye, each one originating from a
single bacterium. These colonies are counted and the number of organisms in each
millilitre of original sample, calculated. Thereare many specific tests which can be
used to identify the bacteria. The decision of which to use routinely, has been deter-
mined partly by constraints of time since a speedy warning is essential if pathogens
or a particularly high microbial load is present. In addition, the tests are designed
to look for the microbes which are specially problematic in given circumstances
such as Pseudomonads in swimming and spa pools, or which are indicative of a
particular contamination. An example of the latter are faecal coliforms, such as
Escherichia coli, which can indicate the presence of sewage. A rapid test for these
involves incubation of a measured volume of sample with the colourless chemical
orthonitrophenol galactoside (ONPG). If the enzymeβ-galactosidase, expressed by
these coliforms, is present, galactose is hydrolysed from ONPG which produces
a yellow colour. This is one of many rapid tests which rely on colour change in
response to the activity of an enzyme diagnostic of a group of organisms, and pro-
vides for a simple and reproducible method of scoring for the presence or absence
of contamination. If these initial tests prove positive within the guidelines set forContinued on page 48