Canadian recreational water quality guidelines - Indicators of fecal contamination: E. coli and enterococci in recreational waters

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• Significance of E. coli in recreational water areas
• Significance of enterococci in recreational water areas

Significance of E. coli in recreational water areas

Indicators can be used for various purposes as part of a recreational water quality management plan. Fecal indicators signal the likely presence of fecal contamination. Common fecal indicators include E. coli and enterococci, as well as source-specific fecal indicators, such as the HF183 Bacteroides genetic marker for detecting human sewage (Harwood et al., 2014). E. coli or enterococci are used as the primary indicators of fecal material in fresh waters, with enterococci the preferred indicator for marine waters. Source-specific fecal indicators are used when microbial source tracking is recommended.

Description

E. coli is a member of the coliform group of bacteria and part of the family Enterobacteriaceae. It is a facultative anaerobic, Gram-negative, non-spore-forming, rod-shaped bacterium that can ferment lactose and grow over a broad temperature range (7-45°C) with an optimal growth temperature of 37°C (Ishii and Sadowsky, 2008; Percival and Williams, 2014). Coliform bacteria are also often defined by their ability to express the enzymes β-galactosidase and β-glucuronidase. E. coli is found in high numbers in the intestinal tract and feces of humans and warm-blooded animals. It can also be found in numerous cold-blooded animal species (Tenaillon et al., 2010; Gordon, 2013; Frick et al., 2018). Some strains of E. coli can adapt to live independently of fecal material and become naturalized members of the microbial community in environmental habitats. Naturalized strains can grow and maintain their population if favourable conditions exist (Ashbolt et al., 1997; Ishii and Sadowsky, 2008; Jang et al., 2017).

E. coli is present in human feces at a concentration of approximately 10⁷to 10⁹ cells per gram and comprises about 1% of the total biomass in the large intestine (Edberg et al., 2000; Leclerc et al., 2001). In 2 separate studies, it was detected in 94% and 100% of the human subjects tested (Finegold et al., 1983; Leclerc et al., 2001). These values are significantly higher than those reported for other members of the coliform group and were matched or exceeded only by enterococci and certain species of anaerobic bacteria (Bacteroides, Eubacterium). E. coli comprises about 97% of the coliform organisms in human feces, with Klebsiella spp. comprising 1.5% and Enterobacter and Citrobacter spp. together comprising another 1.7%. In raw sewage, E. coli generally declines relative to other coliforms, representing less than 30% of coliforms at sewage plant influents (Ashbolt et al., 2001). Nonetheless, there are stress-resistant E. coli that appear to persist, if not grow, within sewage treatment works and are released in treated effluent (Zhi et al., 2016). Sewage effluents may also contribute antibiotic resistant bacteria in surface waters downstream of wastewater treatment plants (Day et al., 2019; Logan et al., 2020). In animal feces, E. coli numbers can vary considerably, but typically fall within the range from 10³ to 10⁹ cells per gram (Ashbolt et al., 2001; Tenaillon et al., 2010; Yost et al., 2011; Ervin et al., 2013). In domestic animals, E. coli has been shown to represent between 90% and 100% of all coliforms in feces (Dufour, 1977).

Although the vast majority of E. coli types are harmless, some strains of this bacterium can cause GI, as well as more serious health complications (for example, hemorrhagic colitis, hemolytic uremic syndrome, kidney failure) and urinary tract infections. Nonetheless, even during outbreaks, fecal concentrations of the typical non-pathogenic E. coli will be greater in water sources than those of the pathogenic strains (Degnan, 2006; Soller et al., 2010a).

E. coli can be rapidly and easily enumerated in recreational waters and epidemiological studies have demonstrated a link between E. coli in fresh waters and the risk of GI among swimmers (Dufour, 1984; Wade et al., 2003; Wiedenmann et al., 2006; Marion et al., 2010). In Canada, most recreational water quality guidelines for natural fresh waters use E. coli as a fecal indicator for making public health decisions. Enterococci are also beginning to be used more frequently.

Occurrence in the aquatic environment

Once shed from a human/animal host, survival of E. coli in the recreational water environment is dependent on many factors, including temperature, exposure to sunlight, available nutrients, water conditions such as pH and salinity and competition from and predation by other microorganisms (Korajkic et al., 2015). Numerous authors have reported on the ability of beach sand, sediments and aquatic vegetation to prolong the survival, replication and accumulation of fecal microorganisms (Whitman and Nevers, 2003; Whitman et al., 2003; Ishii et al., 2006; Olapade et al., 2006; Kon et al., 2007a; Hartz et al., 2008; Byappanahalli et al., 2009; Heuvel et al., 2010; Verhougstraete et al., 2010; Whitman et al., 2014; Devane et al., 2020). These environments are thought to provide more favourable conditions of temperature and nutrients than the adjacent waters and to offer protection from certain environmental stressors such as sunlight. As mentioned earlier, some strains of E. coli can become naturalized and grow in environmental habitats (Power et al., 2005; Byappanahalli et al., 2006; Ishii et al., 2006; Kon et al., 2007b; Byappanahalli et al., 2012b). Growth of E. coli in the environment is a limitation to its use as an indicator of fecal contamination. However, although E. coli is not exclusively associated with recent fecal wastes, it is accepted that E. coli is predominantly of fecal origin and remains a valuable indicator for determining recreational water quality.

Association with pathogens

The occurrence of fecal pathogens in recreational waters, including enteric bacteria, enteric viruses and parasitic protozoa, is strongly dependent on the fecal sources that impact the swimming area. The presence and numbers of fecal pathogens in the environment can be sporadic and highly variable. Therefore, monitoring for fecal indicators is used in place of directly monitoring for pathogens. The presence of E. coli in water indicates the potential for the presence of fecal pathogens that could result in an increased health risk to swimmers.

The association between E. coli and individual enteric pathogens is highly variable. Several early studies found that the survival rate for E. coli was similar to the survival rate for enteric bacterial pathogens (Rhodes and Kator, 1988; Korhonen and Martikainen, 1991; Chandran and Mohamed Hatha, 2005). In addition, one study reported that the probability of detecting Salmonella or Shiga-toxin producing E. coli (STEC) steadily increased as the concentration of E. coli increased, although no single sample could provide absolute assurance of the presence or absence of these pathogens (Yanko et al., 2004). Other studies have reported increased odds of detecting enteric pathogens (Campylobacter, Cryptosporidium, Salmonella and E. coli O157:H7) when densities of E. coli exceed 100 cfu/100 mL (Van Dyke et al., 2012; Banihashemi et al., 2015; Stea et al., 2015). In agricultural watersheds across Canada, Edge et al. (2012) generally found that higher numbers of waterborne pathogens were associated with higher levels of E. coli. However, they cautioned against using low levels of E. coli to infer no waterborne pathogen occurrence. Numerous studies have also reported on the lack of a correlation between E. coli concentrations and the presence of enteric viruses and protozoa in surface waters, reflecting different fecal sources and long persistence of these pathogens over fecal indicator bacteria (Griffin et al., 1999; Denis-Mize et al., 2004; Hörman et al., 2004; Dorner et al., 2007; Edge et. al., 2013; Prystajecky et al., 2014). Overall, although E. coli is limited in that it does not reliably predict the presence of specific fecal pathogens (Wu et al., 2011; Edge et al., 2013; Lalancette et al., 2014; Banihashemi et al., 2015; Krkosek et al., 2016), it can be used to indicate an increased potential for pathogens to be present.

Significance of enterococci in recreational water areas

Like E. coli, enterococci are used as a primary indicator of fecal contamination. Elevated numbers detected in either fresh or marine waters indicate the potential presence of fecal material and thus the possible presence of fecally sourced pathogenic bacteria, viruses and protozoa.

Description

Enterococci are members of the genus Enterococcus. The genus was created to include the more fecal-specific species of the genus Streptococcus, formerly considered as group D streptococci. In practice, the terms enterococci, fecal streptococci, Enterococcus and intestinal enterococci have been used interchangeably (Bartram and Rees, 2000). Enterococci are Gram-positive, round-shaped bacteria that meet the following criteria: growth between temperatures of 10 °C and 45 °C, resistance to heat exposure at 60 °C for 30 minutes, growth in the presence of 6.5% sodium chloride and at pH 9.6 and the ability to reduce 0.1% methylene blue (Bartram and Rees, 2000; APHA et al., 2017). They are also often defined by their ability to express the enzyme β-glucosidase.

The genus Enterococcus is thought to comprise more than 30 species classified into 5 to 6 major groups (E. faecalis, E. faecium, E. avium, E. gallinarum, E. italicus, and E. cecorum) (Svec and Devriese, 2009; Byappanahalli et al., 2012a). E. faecalis and E. faecium occur in significant quantities in both human and animal feces and are the species most frequently encountered in fecally polluted aquatic environments (Bartram and Rees, 2000). Other species commonly isolated from fecal material, but in lower numbers, include E. durans, E. hirae, E. gallinarum, and E. avium (Pourcher et al., 1991; Moore et al., 2008; Staley et al., 2014). Enterococci are present in high concentrations in human and animal feces, with concentrations reported on the order of 10⁶/g to 10⁷/g (Sinton, 1993; Edberg et al., 2000). Human fecal microbiota studies reported by Leclerc et al. (2001) demonstrated that Enterococcus species were present in 100% of the subjects tested.

Enterococci have been used to indicate fecal contamination in fresh and marine waters and have been associated with the risk of GI among swimmers (Cabelli, 1983; Kay et al., 1994; Pruss, 1998; WHO, 1999; Wade et al., 2003, 2006, 2008; Napier et al., 2017).

Occurrence in the aquatic environment

Enterococci have been detected in water samples from diverse environmental habitats (Yamahara et al., 2009; Byappanahalli et al., 2012a; Staley et al., 2014). They are also routinely isolated from marine and fresh recreational waters known to be impacted by human and animal fecal sources. In general, enterococci tend to be present at concentrations approximately 1-fold to 3-fold lower than those of E. coli in feces and municipal wastes (Sinton, 1993; Edberg et al., 2000). Compared with other indicator microorganisms (for example, E. coli, thermotolerant coliforms), enterococci may have greater resistance to certain environmental stresses in recreational waters, such as conditions of sunlight and salinity. They have also demonstrated greater resistance to wastewater treatment practices, including chlorination, and prolonged survival in marine and freshwater sediments (Davies et al., 1995; Desmarais et al., 2002; Ferguson et al., 2005). The source of the enterococci may also affect their persistence, with enterococci from cattle out-persisting those from sewage (Korajkic et al., 2013). Enterococci has also been reported to survive and grow in organic-rich environments, such as on mats of the green algae species Cladophora (Byappanahalli et al., 2003; Whitman et al., 2003; Verhougstraete et al., 2010) and in some environmental habitats (for example, sand, sediments, soils) (Ran et al., 2013; Staley et al., 2014).

As in the case of E. coli, the existence of environmental habitats as potential sources of enterococci is a limitation when interpreting monitoring data (Whitman et al., 2003; Byappanahalli et al., 2012a), but it is accepted that enterococci detected in water samples are predominantly of fecal origin and they remain a valuable indicator for determining recreational water quality.

Association with pathogens

Direct correlations between fecal indicator concentrations and the concentration of any specific pathogen should not be expected. Within a watershed, indicators and pathogens may come from multiple different sources, and once discharged into water sources, they experience different dilution, transport and inactivation rates (Wilkes et al., 2009). Although individual studies have occasionally observed a correlation between the presence of enterococci and the detection of a specific pathogen, the relationships are generally weak (Brookes et al., 2005, Wilkes et al., 2009). In 1 study, a survey of surface waters collected from various watersheds in southern California, showed a good predictive ability with PCR detection of STEC and enterococci using culture-based methods (Yanko et al., 2004). It was reported that above an enterococci concentration of 100 most probable number (MPN)/100 mL, the probability of detection of STEC was approximately 60% to 70%. Like E. coli, enterococci are not predictive of the presence of viruses and protozoa (Griffin et al., 1999; Schvoerer et al., 2000, 2001; Jiang et al., 2001; Jiang and Chu, 2004).

The presence or absence of enterococci in an individual sample should not be interpreted to mean that specific enteric pathogenic microorganisms are also present or absent in the same sample. Enterococci are regarded as general indicators of fecal contamination and are routinely monitored, as epidemiological studies have shown that increased concentrations in recreational areas indicate an increased risk of adverse health impacts.