8.3.2 Faecal streptococci and enterococci
Faecal streptococci have received widespread acceptance as useful indicators of faecal pollution in natural aquatic ecosystems. These organisms show a close relationship with health hazards (mainly for gastrointestinal symptoms) associated with bathing in marine and freshwater environments, (Cabelli et al., 1982, 1983; Dufour, 1984; Kay et al., 1994; WHO, 1998). They are not as ubiquitous as coliforms (Borrego et al., 1982), they are always present in the faeces of warm-blooded animals (Volterra et al., 1986), and it is believed that they do not multiply in sewage-contaminated waters (Slanetz and Bartley, 1965). Enterococci, however, have been shown to grow in freshly stored urine (Höglund et al., 1998). Nonetheless, their die-off rate is slower than the decline in coliforms in seawater (Evison and Tosti, 1980; Borrego et al., 1983) and persistence patterns are similar to those of potential water-borne pathogenic bacteria (Richardson et al., 1991). Reviews of all these aspects have been carried out by Sinton et al., (1993a,b).
The group called faecal streptococci includes species of different sanitary significance and survival characteristics (Gauci, 1991; Sinton and Donnison, 1994). In addition, the proportion of the species of this group is not the same in animal and human faeces (Rutkowski and Sjogren, 1987; Poucher et al., 1991). The taxonomy of this group, comprising species of two genera Enterococcus and Streptococcus (Holt et al., 1993), has been subject to extensive revision in recent years (Ruoff, 1990; Devriese et al., 1993; Janda, 1994; Leclerc et al., 1996). Although several species of both genera are included under the term enterococci (Leclerc et al., 1996), the species most predominant in polluted aquatic environments are Enterococcus faecalis, E. faecium and E. durans (Volterra et al., 1986; Sinton and Donnison, 1994; Audicana et al., 1995).
Enterococci, a term commonly used in the USA, includes all the species described as members of the genus Enterococcus that fulfil the following criteria: growth at 10°C and 45°C, resistance to 60°C for 30 minutes, growth at pH 9.6 and at 6.5 per cent NaCl, and the ability to reduce 0.1 per cent methylene blue. The most common environmental species fulfil these criteria and thus in practice the terms faecal streptococci, enterococci, intestinal enterococci and Enterococcus group can be considered synonymous.
8.3.3 Alternative faecal indicators
The lack of a strong relationship between faecal indicators and health outcomes in a number of epidemiological studies in warm tropical waters may, in part, relate to the inappropriate nature of E. coli or faecal streptococci as indices of waterborne pathogens in these recreational waters. In this context an alternative index group, sulphite-reducing clostridia or spores of Clostridium perfringens, have been proposed and are used in Hawaii (Anon, 1996).
Spores of C. perfringens are largely faecal in origin (Sorensen et al., 1989), they are always present in sewage (about 104-105 colony forming units (cfu) per 100 ml), they are highly resistant in the environment and appear not to reproduce in aquatic sediments (which appears to be the case with thermotolerant coliforms) (Davies et al., 1995). It is interesting to note, however, that dog faeces may have some 9 × 108 cfu C. perfringens per gram dry weight (dw), whereas pig faeces are similar to humans (4.8 × 105 cfu C. perfringens per gram dw). C. perfringens is generally less common or absent in other warm blooded animals. Hence, although dogs have a similar number of thermotolerant