Enterococci have been recognized as potential human pathogens for more than a century, but only in recent years have these organisms acquired prominence as important causes of nosocomial infections. The ability of enterococci to survive and/or disseminate in the hospital environment and to acquire antibiotic resistance determinants makes the treatment of some enterococcal infections in critically ill patients a difficult challenge. Enterococci were first mentioned in the French literature in 1899; the “entérocoque” was found in the human gastrointestinal tract and was noted to have the potential to produce significant disease. Indeed, the first pathologic description of an enterococcal infection dates to the same year. A clinical isolate from a patient who died as a consequence of endocarditis was initially designated Micrococcus zymogenes, was later named Streptococcus faecalis subspecies zymogenes, and would now be classified as Enterococcus faecalis. The ability of this isolate to cause severe disease in both rabbits and mice illustrated its potential lethality in the appropriate settings.
Enterococci are gram-positive organisms. In clinical specimens, they are usually observed as single cells, diplococci, or short chains (Fig. 45-1), although long chains are noted with some strains. Enterococci were originally classified as streptococci because organisms of the two genera share many morphologic and phenotypic characteristics, including a generally negative catalase reaction. Only DNA hybridization studies and later 16S rRNA sequencing clearly demonstrated that enterococci should be grouped as a genus distinct from the streptococci. Nonetheless, unlike the majority of streptococci, enterococci hydrolyze esculin in the presence of 40% bile salts and grow at high salt concentrations (e.g., 6.5%) and at high temperatures (46°C). Enterococci are usually reported by the clinical laboratory to be nonhemolytic on the basis of their inability to lyse the ovine or bovine red blood cells (RBCs) commonly used in agar plates; however, some strains of E. faecalis do lyse RBCs from humans, horses, and rabbits. The majority of clinically relevant enterococcal species hydrolyze pyrrolidonyl-β-naphthylamide (PYR); this characteristic is helpful in differentiating enterococci from organisms of the Streptococcus gallolyticus group (formerly known as S. bovis), which includes S. gallolyticus, Streptococcus pasteurianus, and Streptococcus infantarius, and from Leuconostoc species. Although at least 18 species of enterococci have been isolated from human infections, the overwhelming majority of cases are caused by two species, E. faecalis and Enterococcus faecium. Less frequently isolated species include Enterococcus gallinarum, Enterococcus durans, Enterococcus hirae, and Enterococcus avium.
Gram’s stain of cultured blood from a patient with enterococcal bacteremia. Oval gram-positive bacterial cells are arranged as diplococci and short chains. (Courtesy of Audrey Wanger, PhD.)
Enterococci are normal inhabitants of the large bowel of human adults, although they usually make up <1% of the culturable intestinal microflora. In the healthy human gastrointestinal tract, enterococci are typical symbionts that coexist with other gastrointestinal bacteria; in fact, the utility of certain enterococcal strains as probiotics in the treatment of diarrhea suggests their possible role in maintaining the homeostatic equilibrium of the bowel. Enterococci are intrinsically resistant to a variety of commonly used antibacterial drugs. One of the most important factors that disrupts this equilibrium and promotes increased gastrointestinal colonization by enterococci is the administration of antimicrobial agents. In particular, antibiotics that are excreted in the bile and have broad-spectrum activity (e.g., certain cephalosporins that target anaerobes and gram-negative bacteria) are usually associated with the recovery of higher numbers of enterococci from feces. This increased colonization appears to be due not only to the simple enterococcal replacement in a “biologic niche” after the eradication of competing components of the flora, but also (at least in mice) to the suppression—upon reduction of the gram-negative microflora by antibiotics—of important immunologic signals (e.g., by the lectin RegIIIγ) that help keep enterococcal counts low in the normal human bowel. Several studies have shown that higher levels of gastrointestinal colonization are a critical factor in the pathogenesis of enterococcal infections. However, the mechanisms by which enterococci successfully colonize the bowel and gain access to the lymphatics and/or bloodstream remain incompletely understood.
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