1644Attributes and Application of IndicatorsINTRODUCTIONMicrobial water quality indicators are used in a variety of ways within publichealth risk assessment frameworks, including assessment of potential hazard, ex-posure assessment, contaminant source identification, and evaluating effective-ness of risk reduction actions. The most desirable indicator attributes, and there-fore the most appropriate indicators, naturally depend on their manner of use.This chapter describes desirable attributes of an indicator, typical applications ofindicators, indicator attributes that are appropriate for such applications, and pro-vides an assessment of whether current indicators and indicator approaches aremeeting the needs of each application. The chapter ends with a summary of itsconclusions and recommendations.INDICATOR ATTRIBUTESFor almost 40 years, Bonde’s (1966) attributes of an ideal indicator haveserved as an effective model of how a fecal contamination index for public healthrisk and treatment efficiency should function (Box 4-1). Three of Bonde’s at-tributes (1, 2, and 4) address the relationship between indictor organisms andpathogens of concern, while the remaining five describe desirable properties as-sociated with quantifying the indicator. However, Bonde’s attributes of an idealindicator must be refined to continue their relevance to public health protectionbecause the development and increasing availability of new measurement meth-ods necessitates the separation of criteria for evaluating indicators and detectionATTRIBUTES AND APPLICATION OF INDICATORS 165BOX 4-1Bonde’s (1966) Criteria for an Ideal IndicatorAn ideal indicator should1. Be present whenever the pathogens are present;2. Be present only when the presence of pathogens is an imminentdanger (i.e., they must not proliferate to any greater extent in the aque-ous environment);3. Occur in much greater numbers than the pathogens;4. Be more resistant to disinfectants and to the aqueous environ-ment than the pathogens;5. Grow readily on simple media;6. Yield characteristic and simple reactions enabling as far as pos-sible an unambiguous identification of the group;7. Be randomly distributed in the sample to be examined, or it shouldbe possible to obtain a uniform distribution by simple homogenizationprocedures; and8. Grow widely independent of other organisms present, when incul-cated in artificial media (i.e., indicator bacteria should not be seriouslyinhibited in their growth by the presence of other bacteria).methods. Historic definitions of microbial indicators, such as coliforms, havebeen tied to the methods used to measure them. Newly available methods (par-ticularly molecular methods; see Chapter 5 and Appendix C) allow more speci-ficity in the taxonomic grouping of microorganisms that are measured. More im-portantly, a variety of new methods are becoming increasingly available,providing several options for measuring each indicator group. Thus, separate cri-teria allow one to choose the indicator with the most desirable biological attributesfor a given application and then match this with a measurement method that bestmeets the need of the application. Box 4-2 lists desirable biological attributes ofindicators and Box 4-3 lists desirable attributes of methods. Biological AttributesThe most important biological attribute is a strong quantitative relationshipbetween indicator concentration and the degree of public health risk. This rela-tionship has been demonstrated primarily through epidemiologic studies for rec-reational exposures (Cabelli et al., 1979; Cheung et al., 1990; Seyfried et al.,1985a,b; Zmirou et al., 1987). An alternative means of demonstrating the rela-tionship to health risk is through correlation between prospective indicator con-centration and pathogen levels (Gerba et al., 1979; Labelle et al., 1980; Lipp et166 INDICATORS FOR WATERBORNE PATHOGENSBOX 4-2Desirable Biological Attributes of Indicators• Correlated to health risk• Similar (or greater) survival to pathogens°Ultraviolet exposure°Temperature°Salinity°Predation by indigenous flora°Desiccation°Freezing°Biologic survival mechanisms— Sporulation— Cyst and other latency mechanisms— Arrested metabolism (viable but non-culturable)— Shock proteins and other biochemical survival strategies°Response to disinfectants• Similar (or greater) transport to pathogens°Filtration°Sedimentation or settling°Adsorption to particles• Present in greater numbers than pathogens• Specific to a fecal source or identifiable as to source of originBOX 4-3Desirable Attributes of Methods• Specificity to desired target organism°Independent of matrix effects• Broad applicability• Precision• Adequate sensitivity• Rapidity of results• Quantifiable• Measures viability or infectivity• Logistical feasibility°Training and personnel requirements°Utility in field°Cost°Volume requirementsATTRIBUTES AND APPLICATION OF INDICATORS 167al., 2001a; Robertson, 1984; Seyfried et al., 1984). The latter approach is usedless frequently because assays for pathogens are specific to individual agents orclasses of agents (e.g., enteroviruses) and correlation with a single pathogen, orsubset of pathogens, does not establish a relationship with all illness-causingagents or their risks to human health (their health effects).The next two desirable biological attributes are similarity in survival andtransport characteristics of the indicator to those of the pathogen(s) of interest. Ifthere is differential transport or survival, the relationship between pathogen andindicator concentrations will change at varying distances from the source andover different times in the environment, making it difficult to select a criticalindicator concentration on which to make public health decisions (Griffin et al.,2001). For example, differences in viral and bacterial transport through soils andaquifers have been found to affect assessment of water quality impacts from sep-tic systems (Harden et al., 2003). If there is differential survival, it is generallypreferable that the indicator be more resilient than the pathogens so as to be pro-tective of public health. However, exceptionally long survival of potential indica-tors, such as spore-forming Clostridium perfringens, may render them too over-protective or nondiscriminatory because they may be present at concentrationsmistakenly considered to be indicative of a health risk long after the pathogenshave declined