The genus Enterococcus encompasses a group of bacteria ubiquitous in nature. These organisms are recognized as normal inhabitants, or commensals, of the gastrointestinal tract in both humans and animals. While they typically coexist harmlessly within the gut, Enterococcus species possess a unique resilience. Their ability to survive outside the body and acquire resistance to antibiotics has elevated them to a position of significant concern in contemporary healthcare settings.
Basic Characteristics and Habitat
Enterococcus species are Gram-positive bacteria, appearing as spherical or ovoid cells clustered in pairs or short chains. They are facultative anaerobes, meaning they can produce energy and survive in environments with or without oxygen. This metabolic flexibility contributes to their hardiness and widespread distribution.
The gastrointestinal tract is their primary natural reservoir, where they are constantly exposed to bile salts and fluctuating conditions. Enterococci demonstrate impressive environmental tolerance, capable of withstanding temperatures from 10°C to 45°C and growing in high-salt concentrations, such as a 6.5% sodium chloride solution. Enterococcus faecalis and Enterococcus faecium are the two species most frequently isolated from human infections. E. faecalis is the more common commensal, while E. faecium is strongly associated with higher levels of antibiotic resistance.
Clinical Significance and Infection Types
Enterococcus transitions from a harmless gut resident to a pathogen when a patient’s natural barriers are breached or their immune system is compromised. These bacteria are primarily opportunistic pathogens, posing the greatest threat in hospital settings, especially to individuals with underlying illnesses or indwelling medical devices like catheters or prosthetic heart valves. The presence of these foreign materials provides a surface for the bacteria to colonize and form protective biofilms.
Enterococcus causes several distinct infection types. These include urinary tract infections (UTIs), common in catheterized patients, and infections within the abdominal cavity, often following surgery or trauma that disrupts the bowel. Invasive infections include bacteremia, a bloodstream infection that can carry a significant mortality rate, and endocarditis, a severe infection of the heart valves.
The Challenge of Antibiotic Resistance
The challenge in treating enterococcal infections stems from the bacteria’s inherent and acquired resistance to numerous classes of antibiotics. Enterococcus possesses intrinsic resistance to several drugs, notably all cephalosporins, a common type of beta-lactam antibiotic. This natural resistance is due to the structure of their cell wall components, which do not bind effectively to these medications. They also exhibit low-level resistance to aminoglycosides, a class of antibiotics that normally requires combination therapy to be effective against enterococci.
The most concerning development is the emergence of acquired resistance to vancomycin, a powerful glycopeptide antibiotic. These strains are known as Vancomycin-Resistant Enterococci (VRE), representing a serious public health threat in healthcare facilities. VRE is often found in E. faecium strains, which are adept at acquiring and sharing resistance genes.
The mechanism of vancomycin resistance involves a set of genes, most commonly vanA or vanB, which are often located on mobile genetic elements that can easily be transferred between bacteria. These genes enable the bacteria to alter the terminal structure of the peptidoglycan precursors used to build the cell wall. Instead of the typical D-alanyl-D-alanine (D-Ala-D-Ala) structure, resistant bacteria synthesize a D-alanyl-D-lactate (D-Ala-D-Lac) precursor. Vancomycin requires the D-Ala-D-Ala structure to bind and inhibit cell wall synthesis. This small change in the cell wall effectively prevents the drug from attaching, rendering the antibiotic ineffective. The presence of VRE complicates patient management, often leading to prolonged hospital stays and increased mortality.
Diagnosis and Management Approaches
Identifying an Enterococcus infection begins with culturing samples from the site of infection, such as urine, blood, or a wound. Once the organism is isolated, clinical laboratories perform species identification and an Antibiotic Susceptibility Test (AST) to determine which antibiotics are effective against the specific strain. This testing is important to identify VRE strains and to detect high-level resistance to aminoglycosides.
The treatment of enterococcal infections must be tailored based on the species and its resistance profile. For serious infections like endocarditis or meningitis caused by susceptible strains, a combination of a cell wall-active agent, such as ampicillin, and an aminoglycoside is often used to achieve a synergistic, bactericidal effect. When VRE is confirmed, the therapeutic options become severely limited, often requiring the use of newer, “last-resort” antibiotics.
Linezolid, an oxazolidinone antibiotic, is a primary option for VRE infections, and daptomycin, a lipopeptide, is also frequently used. Infection control practices are equally important, focusing on strict hand hygiene and patient isolation protocols to prevent the spread of VRE to other vulnerable individuals within the hospital environment.