Enterococcus is a genus of bacteria that naturally resides in the gastrointestinal tract of humans and animals, forming part of the normal gut flora. While usually harmless, certain species can become opportunistic pathogens, causing severe and potentially life-threatening infections. These bacteria have garnered significant attention in healthcare settings because of their inherent ability to survive in harsh environments and their growing resistance to multiple classes of antibiotics. This dual nature elevates Enterococcus to a major public health concern, particularly in hospitals.
Defining Enterococci
Enterococci are classified as Gram-positive bacteria, meaning they retain the crystal violet stain used in the Gram staining procedure due to their thick peptidoglycan cell wall. These spherical or ovoid cells typically arrange themselves in pairs or short chains. The genus includes over 50 species, but two are overwhelmingly responsible for human infections: Enterococcus faecalis and Enterococcus faecium.
The biology of Enterococcus is defined by its hardiness, which allows it to thrive in diverse and challenging environments. They are facultative anaerobes, meaning they can survive in both oxygen-rich and oxygen-poor conditions. They can tolerate a wide range of temperatures and high concentrations of salt. This resilience enables them to persist on inanimate surfaces in healthcare environments, contributing significantly to hospital-acquired infections.
Common Infections Caused by Enterococci
When the natural balance of the body is disrupted, such as by illness, surgery, or the use of broad-spectrum antibiotics, enterococci can spread outside the gut and cause infection. They are considered opportunistic pathogens, primarily attacking individuals who are already immunocompromised or have underlying medical conditions. These bacteria are among the most frequent causes of hospital-acquired infections.
The most common manifestation of enterococcal infection is the Urinary Tract Infection (UTI), often associated with the use of urinary catheters. From the urinary tract or other sites of colonization, the bacteria can enter the bloodstream, leading to bacteremia and potentially sepsis. Enterococcal bacteremia is a serious condition with high mortality rates, particularly when caused by the more resistant species, E. faecium.
These organisms can also cause infective endocarditis, an infection of the inner lining and valves of the heart, which can be difficult to treat and often requires aggressive dual antibiotic therapy. Furthermore, enterococci are known to infect surgical sites, particularly abdominal and pelvic wounds, and contribute to complex intra-abdominal infections. The formation of biofilm on medical devices like catheters and prosthetic valves makes these infections particularly stubborn to clear.
The Threat of Antibiotic Resistance
The primary reason Enterococcus is a public health threat is its intrinsic ability to resist numerous antibiotics and its capacity to acquire new resistance genes. Enterococci are naturally less susceptible to many common antibiotics, including penicillins and cephalosporins, a trait that positions them to thrive when competing bacteria are eliminated. This intrinsic resistance is compounded by the bacteria’s ability to use mobile genetic elements, such as plasmids and transposons, to acquire external resistance genes from other bacteria.
This acquired resistance has led to the emergence of Vancomycin-Resistant Enterococci, or VRE, which represents one of the most serious challenges in modern medicine. Vancomycin is a powerful glycopeptide antibiotic historically reserved as a last resort for treating serious Gram-positive infections. VRE strains, particularly E. faecium, are often resistant to this drug, leaving clinicians with very limited treatment options.
The resistance mechanism is genetically encoded, most commonly by the vanA or vanB gene clusters. These genes fundamentally alter the building blocks of the bacterial cell wall, changing the peptidoglycan precursor structure. This molecular change prevents vancomycin from binding effectively to its target, rendering the drug ineffective. The prevalence of VRE is a serious concern, with some reports indicating that a high percentage of E. faecium isolates in healthcare settings are now vancomycin-resistant.
The difficulty in treating VRE infections is reflected in high mortality rates, especially for VRE bacteremia. When vancomycin is no longer effective, alternative antibiotics like linezolid and daptomycin must be used, but resistance to these drugs is also an emerging issue. The public health crisis posed by VRE is amplified by its ability to potentially transfer its resistance genes to other dangerous bacteria, such as Staphylococcus aureus.
Preventing Enterococcal Spread
Controlling the spread of Enterococcus, particularly VRE, relies heavily on rigorous infection control practices implemented within healthcare facilities. Since the bacteria are highly resilient and can persist on environmental surfaces, contact-based transmission is a major route of spread. Healthcare workers must adhere to strict hand hygiene protocols, using soap and water or alcohol-based hand rubs before and after all patient contact.
For patients known to be infected or colonized with VRE, contact isolation protocols are essential. This typically involves placing the patient in a private room or cohorting them with other VRE-positive patients. Healthcare personnel must wear gowns and gloves upon entering the patient’s room. Noncritical patient care equipment should be dedicated to the VRE-positive patient or thoroughly disinfected before use on another individual.
Environmental cleaning and disinfection are important, with a focus on high-touch surfaces in patient rooms. Responsible antibiotic stewardship plays a role in prevention by ensuring that antibiotics are used judiciously, which reduces the selective pressure that promotes the development and spread of resistant strains.