During the past few decades, enterococci have emerged as important healthcare-associated pathogens (Arias & Murray, 2012; Austin, Bonten, Weinstein, Slaughter, & Anderson, 1999; Boyce, et al., 1994; Benenson, et al., 2009; Goossens, 1998; Handwerger, et al., 1993). The continuing progress of modern medical care toward more intensive and invasive medical therapies for human disease has undoubtedly contributed to the increased prevalence of these remarkable opportunistic pathogens. This trend has also been attributed to the increasing antibiotic resistance among clinical isolates of enterococci. The rapid spread of enterococci with resistance to vancomycin (VRE) has been of particular concern. Many healthcare-associated strains that are resistant to vancomycin also show resistance to penicillin, as well as high-level resistance (HLR) to aminoglycosides. Finally, as has historically been the case with enterococci, resistance is emerging to newer agents used to treat VRE infections, such as linezolid, quinupristin/dalfopristin, and daptomycin (Chow, Donahedian, & Zervos, 1997; Herrero, Issa, & Patel, 2002; Sabol, Patterson, Lewis II, Aaron, Cadena, & Jorgensen, 2005).
Over the past two decades, Enterococcus faecium has emerged as a leading cause of multidrug-resistant enterococcal infection in the United States (Hidron, et al., 2008). E. faecium is intrinsically more antibiotic-resistant than E. faecalis, with more than half of its pathogenic isolates expressing resistance to vancomycin, ampicillin, and high-levels of aminoglycosides. Treating infections caused by this species can be difficult, and the magnitude of the problem is vast. Approximately 40% of medical intensive care units in a recent National Healthcare Safety Network report found that the majority of device-associated infections (namely, infections due to central lines, urinary drainage catheters, and ventilators) were due to vancomycin- and ampicillin-resistant E. faecium (80% and 90.4%, respectively) (Hidron, et al., 2008). Although they were often resistant to high-level aminoglycosides and some macrolides, healthcare-associated infections in these units due to E. faecalis remained largely susceptible to vancomycin and ampicillin (93.1% and 96.2%, respectively) for reasons that are not entirely known. Other enterococcal species are rarer causes of human infection, including E. durans, E. avium, E. casseliflavus, E. hirae, E. gallinarum, E. raffinosus, and E. muntdii (Gordon, et al., 1992).
In the following sections, the common human infections caused by enterococci are briefly described. The epidemiology of antibiotic-resistant enterococci in healthcare settings is summarized, including the role of colonization pressure and host factors on the emergence of VRE in clinical settings. Finally, the current challenges facing clinicians who treat antibiotic-resistant enterococcal infections are reviewed.