Enterococcus faecalis is a Gram-positive bacterium that occupies a dual role in human health, functioning as both a common, harmless resident and a major disease-causing agent. As a member of the normal human gut flora, it peacefully coexists within the gastrointestinal tract. However, this organism is an opportunistic pathogen, meaning it can cause severe infections when the body’s defenses are weakened or when it gains access to parts of the body where it does not normally reside. The bacterium has become a significant concern in healthcare settings due to its tenacity and its increasing ability to resist antimicrobial treatments.
Basic Biology and Natural Environment
Enterococcus faecalis is characterized as a non-motile, facultative anaerobe, which means it can generate energy and survive in environments with or without oxygen. This organism was historically classified as part of the Group D Streptococcus bacteria until the 1980s, when genetic and biochemical differences led to its reclassification into the separate genus Enterococcus. The name itself reflects its primary habitat, with “entero” referring to the intestine.
A defining feature of this bacterium is its remarkable hardiness, allowing it to survive in extremely harsh conditions that would destroy most other microbes. It can tolerate high salt concentrations and can survive across a wide temperature range, including brief exposure to 60°C. This resilience is why E. faecalis is commonly found in the gastrointestinal tracts of humans and animals, but also persists in the environment, including soil and water.
Within the human body, the gastrointestinal tract is the primary reservoir for E. faecalis, but it also colonizes the oral cavity and the female genitourinary tract. The colonization is typically benign, with the bacterium acting as a commensal organism. Problems arise when an underlying condition, such as a weakened immune system or the presence of a medical device, allows the organism to leave its natural environment and invade sterile body sites.
Sites and Types of Infection
When E. faecalis transitions from a commensal to a pathogen, it is responsible for a variety of serious clinical diseases. The most common infection caused by this organism is the Urinary Tract Infection (UTI), often associated with the use of urinary catheters that provide a surface for the bacteria to colonize. These infections can range from simple cystitis to more severe conditions like pyelonephritis, which affects the kidneys.
The bacterium is also a frequent cause of life-threatening bloodstream infections (bacteremia or sepsis), which can occur when the organism enters the blood, often through a breakdown in the gastrointestinal barrier or through an indwelling intravenous line. From the bloodstream, E. faecalis can travel to the heart, where it is a significant cause of infective endocarditis. E. faecalis is one of the most common bacteria involved in this serious heart condition, especially in healthcare-associated cases.
The organism is frequently implicated in intra-abdominal and pelvic infections, especially those occurring after surgery or trauma to the gastrointestinal tract. The bacterium can cause abscesses and peritonitis, often in combination with other types of bacteria present in the gut. Its ability to form a biofilm allows it to persist on surfaces like medical implants, which contributes to chronic and difficult-to-clear infections.
In the dental setting, E. faecalis is notorious for its role in persistent root canal infections. It is found in a high percentage of teeth that require retreatment. The bacterium’s robust survival mechanisms, including its tolerance to alkaline conditions and its biofilm-forming capacity, allow it to endure the challenging environment within the dental pulp space.
Addressing Antibiotic Resistance
The treatment of E. faecalis infections is complicated by the organism’s inherent and acquired resistance to many common antibiotics. The species possesses intrinsic resistance to several classes of drugs, including cephalosporins, quinolones, and most \(\beta\)-lactam antibiotics, which limits the initial selection of effective therapies.
E. faecalis also exhibits a low level of intrinsic resistance to aminoglycosides, a class of antibiotics that is often used in combination with other drugs for severe infections. However, the bacterium can acquire high-level resistance through the uptake of mobile genetic elements, such as plasmids. This acquired resistance is particularly problematic because it eliminates the synergistic effect needed for successful treatment of serious infections like endocarditis.
The most significant public health concern is the emergence of Vancomycin-Resistant Enterococci (VRE). Vancomycin is a powerful antibiotic often reserved for treating serious Gram-positive infections, and resistance to it creates a major therapeutic challenge. E. faecalis strains have developed this resistance, which further narrows the available treatment options.
The mechanism for acquired resistance often involves the organism gaining new genetic material from other bacteria, which allows it to produce enzymes that inactivate the antibiotic. The ability of E. faecalis to easily share and acquire these resistance genes, combined with its environmental hardiness, underscores why it is responsible for rising rates of antibiotic-resistant, healthcare-associated infections globally.