Enterococci are found in high concentrations in human feces, usually between 104 and 106 bacteria per gram wet weight (Layton, Walters, Lam, & Boehm, 2010; Slanetz & Bartley, 1957; Zubrzycki & Spaulding, 1962); see also Enterococcus Diversity, Origins in Nature, and Gut Colonization). Although enterococci usually represent less than 1% of the flora (Tendolkar, Baghdayan, & Shankar, 2003), they are usually present in the fecal consortium, but are outnumbered by other bacteria, including Escherichia coli, clostridia, and the Bacteroidales (Zubrzycki & Spaulding, 1962). Due to their ubiquity in human feces and persistence in the environment, enterococci have been adopted as indicators of human fecal pollution in water. More recently, their densities on human hands have been used as indicators of hand hygiene. The use of enterococci as indicators of human fecal pollution or contamination can be problematic, however, because enterococci are also found in animal feces (Harwood, Whitlock, & Withington, 2000; Layton, Walters, Lam, & Boehm, 2010), in soils (Byappanahalli & Fujioka, 2004; Goto & Yan, 2011), and on plants (Byappanahalli, Shively, Nevers, Sadowsky, & Whitman, 2003; Imamura, Thompson, Boehm, & Jay, 2011; Müller, Ulrich, Ott, & Müller, 2001). Although there is debate about the extent to which this happens in nature, there is evidence that enterococci are capable of replicating in extra-enteric environments, such as on beach sands (Bahirathan, Puente, & Seyfried, 1998; Zubrzycki & Spaulding, 1962) and in water containing kelp (Byappanahalli, Shively, Nevers, Sadowsky, & Whitman, 2003; Imamura, Thompson, Boehm, & Jay, 2011) and plankton (Mote, Turner, & Lipp, 2012). Identification of human-specific enterococcal species or genotypes could aid in the discrimination of human fecal contamination from other environmental sources of the organisms. Some data suggest that Enterococcus faecium and Enterococcus faecalis may be more prevalent in human feces than other enterococcal species, while Enterococcus casseliflavus and Enterococcus mundtii may be more abundant in environmental reservoirs (such as on plants) than other species (Bahirathan, Puente, & Seyfried, 1998; Ferguson, Moore, Getrich, & Zhowandai, 2005; Wheeler, Hartel, Godfrey, Hill, & Segars, 2002). However, a number of species of Enterococcus have been isolated from human feces (Layton, Walters, Lam, & Boehm, 2010); Enterococcus Diversity, Origins in Nature, and Gut Colonization), so it will be difficult to derive a single host-specific indicator. It has been suggested that E. faecium that contains the enterococcal surface protein (esp) gene may be human-specific (Scott, Jenkins, Lukasik, & Rose, 2005), but esp-containing E. faecium can also be found in select animal hosts (Layton, Walters, & Boehm, 2009; Whitman, Przybyla-Kelly, Shively, & Byappanahalli, 2007).
Fecal enterococci from the GI tract consortia of healthy humans are generally not virulent. Nevertheless, multidrug-resistant Enterococcus strains have emerged as leading causes of hospital-acquired infections (Tendolkar, Baghdayan, & Shankar, 2003). Vancomycin-resistant enterococci are particularly important pathogens (Willems, et al., 2005), as are esp-containing E. faecalis (Shankar, Baghdayan, Huycke, Lindahl, & Gilmore, 1999) and E. faecium (Willems, et al., 2001), as well as other types of E. faecalis and E. faecium (Wisplinghoff, Bischoff, Tallent, Seifert, Wenzel, & Edmond, 2004). It is estimated that there are 800,000 cases of enterococcal infection in the US each year, adding $500,000,000 to annual healthcare costs (Tendolkar, Baghdayan, & Shankar, 2003). Therefore, the presence of enterococci in the environment and on hands may have important direct health implications.