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Indicators for Waterborne Pathogens
BOX 4-1 Bonde’s (1966) Criteria for an Ideal Indicator
An ideal indicator should
Be present whenever the pathogens are present;
Be present only when the presence of pathogens is an imminent danger (i.e., they must not proliferate to any greater extent in the aqueous environment);
Occur in much greater numbers than the pathogens;
Be more resistant to disinfectants and to the aqueous environment than the pathogens;
Grow readily on simple media;
Yield characteristic and simple reactions enabling as far as possible an unambiguous identification of the group;
Be randomly distributed in the sample to be examined, or it should be possible to obtain a uniform distribution by simple homogenization procedures; and
Grow widely independent of other organisms present, when inculcated in artificial media (i.e., indicator bacteria should not be seriously inhibited in their growth by the presence of other bacteria).
methods. Historic definitions of microbial indicators, such as coliforms, have been tied to the methods used to measure them. Newly available methods (particularly molecular methods; see Chapter 5 and Appendix C) allow more specificity in the taxonomic grouping of microorganisms that are measured. More importantly, a variety of new methods are becoming increasingly available, providing several options for measuring each indicator group. Thus, separate criteria allow one to choose the indicator with the most desirable biological attributes for a given application and then match this with a measurement method that best meets the need of the application. Box 4-2 lists desirable biological attributes of indicators and Box 4-3 lists desirable attributes of methods.
Biological Attributes
The most important biological attribute is a strong quantitative relationship between indicator concentration and the degree of public health risk. This relationship has been demonstrated primarily through epidemiologic studies for recreational exposures (Cabelli et al., 1979; Cheung et al., 1990; Seyfried et al., 1985a,b; Zmirou et al., 1987). An alternative means of demonstrating the relationship to health risk is through correlation between prospective indicator concentration and pathogen levels (Gerba et al., 1979; Labelle et al., 1980; Lipp et
