General Public
Water quality is monitored in order to protect human and environmental health.
Contamination of surface waters with faeces of human and non-human origin leads to
increased risk of human exposure to pathogenic microorganisms that can cause illness. People
may come into contact with these pathogens through drinking water supply, aquaculture, and
recreational activities. Microorganisms are incredibly diverse. There are many types – viruses,
bacteria, fungi, protozoa, even small invertebrates. Finding the microorganisms that can cause
disease in amongst all the others is technically challenging. Water quality standards are
currently monitored using faecal indicator organisms (FIO), typically the faecal indicator
bacteria, intestinal enterococci and E. coli, as it remains technically complicated and expensive
to monitor pathogens directly. FIO concentrations offer an indication of the presence of faecal
contamination and the potential risk to public health, but they do not determine contamination
sources.
Human faecal contamination is considered to be the greater risk to public health, largely
because viruses that commonly cause illness are highly host specific, but faeces from
nonhuman sources also pose a potential risk of infection by zoonotic pathogens (organisms
capable of causing disease in animal and human hosts). Recent research has suggested that the
risk to human health from animal faecal sources is varied. Since certain faecal sources pose a
greater risk to human health than others, reliance on FIO concentrations alone could result in
either an underestimation or overestimation of the potential risk. Legislative requirements, the
need to predict risk to human health, and the implications of source on the selection of
appropriate remediation measures, have led to the development of the field of microbial
source tracking (MST). MST encompasses techniques that aim to distinguish source(s) of faecal
contamination in surface and ground waters. MST is a rapidly advancing field, but there is
currently no single standardised method available to distinguish sources of faecal
contamination in surface waters in all situations. Many researchers agree that no single method
will be sufficient in all circumstances and that instead a ‘toolbox’ of multiple methods would be
advisable (Vogel et al., 2007; Plummer et al., 2009; Gourmelon et al., 2010).
MST is simple in theory, but not in practice. The water environment changes very quickly and is
very heterogeneous in time and space. A sample of water taken from a river could be different
depending on which side of a river bend it is taken from; two samples from the same site taken
an hour apart or upstream and downstream of a tributary are obviously different water.
Therefore samples must represent the river under many conditions. So, the theory of MST is
simple for any one sample in a laboratory, but using samples to explain river dynamics is very
difficult.
Most of the MST techniques which are used focus on indicator organisms. MST data needs to
be as specific as possible (e.g. found only in one host animal group and not in any other animal
group) and as sensitive as possible (e.g. found in all the individual host animals of that group,
not in just a few individuals). Each technique has strengths and weaknesses, in specificity,
sensitivity and in how easy (e.g. cheap) it is to perform in a laboratory. It can hopefully be seen
then that there are a series of links in a chain which make up MST data. Each MST technique
performs differently; each has different specificities, sensitivities and ease of use, each has to
link to an indicator group, each indicator group has to link to one or more pathogens (which are
in turn very diverse), and each sample processed has to make a link to describing the water
body (which changes rapidly).
There is a key difference between assessing water quality (spot samples, taken regularly over
fairly long time periods) and explaining the factors that have contributed to that water quality
(MST work – sampling effort is very different). Enough spot samples, taken once per week over
a bathing season, for many years, has given enough data for us to have a general picture of
water quality as assessed by FIOs. Explaining the specifics of that water quality – the role of
MST – is a lot more challenging. Overall – MST can provide useful information to us, but we
need to be aware of the strengths and weaknesses of that information when drawing
conclusions. MST is therefore theoretically simple, but practically, very complicated.