(Borrego, 1994). While faecal streptococci are suggested as the recommended indicator for salt water, either faecal streptococci or Escherichia coli can be used for monitoring freshwaters. Additional variables can be investigated if they are considered relevant, such as the spores of Clostridium perfringens in tropical waters where the traditional indicators may increase in number in soil and water (Hardina and Fujioka, 1991; Anon, 1996). Staphylococci are generally assumed to serve as indicators of water pollution deriving from bathers themselves (i.e. by shedding from the body surface). The epidemiological significance of the recovery of Staphylococci remains unclear.
8.3.1 Thermotolerant coliforms and E. coli
Thermotolerant (faecal) coliforms constitute the subset of total coliforms that possess a more direct and closer relationship with homeothermic faecal pollution (Geldreich, 1967). These bacteria conform to all the criteria used to define total coliforms (all are aerobic and facultatively anaerobic, Gram-negative, non-spore forming rod-shaped bacteria that ferment lactose with gas and acid production in 24-48 hours at 36 ± 1°C), but in addition they grow and ferment lactose with production of gas and acid at 44.5 ± 0.2°C within the first 48 hours of incubation. For this reason, the term “thermotolerant coliforms” rather than “faecal coliforms” is a more accurate name for this group (WHO, 1993). The physiological basis of the elevated temperature phenotype in the thermotolerant coliforms has been described as a thermotolerant adaptation of proteins to, and their stability at, the temperatures found in the enteric tracts of animals (Clark, 1990). Thermotolerant coliforms include strains of the genera Klebsiella and Escherichia (Dufour, 1977). The thermotolerant coliform definition is not based on strictly taxonomic criteria, but on specific biochemical reactions or on the appearance of characteristic colonies on selective and/or differential culture media. Certain Enterobacter and Citrobacter strains are also able to grow under the conditions defined for thermotolerant coliforms (Figueras et al., 1994; Gleeson and Gray, 1997). E. coli is, however, the only biotype of the family Enterobacteriaceae that is almost always faecal in origin (Bonde, 1977; Hardina and Fujioka, 1991). Therefore, the thermotolerant coliform group when used should ideally be replaced by E. coli as an indicator of faecal pollution. For the purpose of water testing, most E. coli can be confirmed by a positive indole test and by their inability to use citrate (as the only carbon source) in the culture medium. Alternatively, E. coli can be distinguished easily enzymatically by the lack of urease or presence of β-glucuronidase enzymes. The enzymes can be recognised easily using culture media that contain specific substrata (Gauthier et al., 1991; Brenner et al., 1993; Walter et al., 1994).
However, several studies have indicated the limitation of both the thermotolerant coliform group and E. coli as ideal faecal indicators or pathogen index organisms. Several thermotolerant Klebsiella strains have been isolated from environmental samples with high levels of carbohydrates in the apparent absence of faecal pollution (Dufour and Cabelli, 1976; Knittel et al., 1977; Niemi et al., 1997). Similarly, other members of the thermotolerant coliform group, including E. coli, have been detected in some pristine areas (Rivera et al., 1988; Ashbolt et al., 1997) and have been associated with regrowth in drinking water distribution systems (Lechevallier, 1990). The principal disadvantages of this organism as an indicator in water are: (i) its detection in other environments without faecal contamination (Hazen and Toranzos, 1990; Hardina and Fujioka, 1991), and (ii) its low survival capability in aquatic environments when compared with faecal pathogens (Borrego et al., 1983; Cornax et al., 1990).