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Chapter 8*: SANITARY INSPECTION AND MICROBIOLOGICAL WATER QUALITY - page 13 / 52

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the ratio of coprostanol to 24-ethylcoprostanol in human and herbivore (sheep and cow) faeces (Leeming et al., 1996) and comparing these to ratios obtained for water samples (Leeming et al., 1998). Other animals that are ubiquitous in urban areas such as dogs and birds, either do not have coprostanol in their faeces or have it in trace amounts only (Leeming et al., 1994).

Table 8.1 Examples of faecal sterol biomarkers

Systematic name

Common name

Comments

C27 sterols

5β-cholestan-3α- Coprostanol ol

Human faecal biomarker; high relative amounts indicate fresh human faecal contamination

24-ethyl-5β- cholestan-3β-ol

24- ethylcoprostanol

Herbivore faecal biomarker; high relative amounts indicate herbivore faecal contamination

24-ethyl-5β- cholestan-3α-ol

24-ethyl-epi- coprostanol

Present in some herbivore faeces

5β-cholestan-3α ol

cholest-5-en-3β- ol

5α-cholestan-3β

ol

C29 sterols

- Epi-coprostanol

Present in sewage sludges; high relative amounts suggest older faecal contamination

Cholesterol

C27 precursor to 5α- and 5β-stanols

- Cholestanol

The thermodynamically most stable isomer is ubiquitous; if the ratio of coprostanol to cholestanol is < 0.5, origin of 5β- stanols may not be faecal

24-ethylcholest- 5-en-3β-ol

24-ethylcholesterol C27

precursor to 5α- and 5β-stanols

The thermodynamically most stable isomer is ubiquitous

24-ethyl-5α-

24-

cholestan-3β-ol

ethylcholestanol

Faecal sterols generally associate with particulate matter, and can be concentrated from 1-10 litres of water by simply filtering the water through a glass fibre filter (such as type OFF, Whatman). The lipids are extracted by acetone, concentrated, derivatised and quantified by gas chromatography. Thus the method requires a suitable chemistry laboratory and may cost ten times more than that for the analysis of E. coli and enterococci. Nonetheless, it is an appropriate method for specific studies investigating the proportion of human and animal faecal contamination.

    • 8.4

      Analytical methods

      • 8.4.1

        Most Probable Number

Most Probable Number (MPN) analysis is a statistical method based on the random dispersion (Poisson) of micro-organisms per volume in a given sample. Classically, this assay has been performed as a multiple-tube fermentation test. Although the technique

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