Enterococcus faecalis is a common bacterium frequently found residing within the human body, particularly in the gastrointestinal tract. While it often exists as a harmless component of the normal microbial community, it possesses the capacity to cause significant health issues under certain conditions. Understanding its dual nature, both as a benign inhabitant and a potential threat, is important for comprehending its impact on human health.
Understanding Enterococcus faecalis
Enterococcus faecalis is a Gram-positive, facultatively anaerobic bacterium found in the intestines of humans and other warm-blooded animals. Beyond the digestive system, E. faecalis can also colonize other areas, including the oral cavity and the female genital tract.
This bacterium is known for its remarkable resilience, allowing it to survive in various harsh environments. It can tolerate a wide range of temperatures, from 10 to 45 degrees Celsius, and can withstand highly alkaline pH levels up to 9.6. Its ability to endure conditions such as high salt concentrations and bile salts further contributes to its persistence both inside and outside the host. This hardiness enables E. faecalis to persist on surfaces for extended periods, ranging from days to several months.
While often a commensal, E. faecalis is also an opportunistic pathogen, meaning it can cause infections when it spreads from its usual habitat or when a person’s immune system is weakened. It is one of the most common species of Enterococci, responsible for approximately 80% of human enterococcal infections.
Infections Caused by Enterococcus faecalis
Enterococcus faecalis can cause a range of infections when it moves beyond its usual habitats or when a person’s immune system is compromised. These infections are particularly prevalent in healthcare settings, where patients may have weakened defenses or invasive medical devices.
Urinary tract infections (UTIs) are among the most common conditions caused by E. faecalis, especially in individuals with indwelling catheters or other urinary obstructions. These bacteria can adhere to medical devices and form biofilms, which helps them evade both the immune system and antibiotic treatments. E. faecalis UTIs can sometimes progress to more severe conditions like pyelonephritis, an infection of the kidney, which can then serve as a source for bloodstream infections.
E. faecalis is also a frequent cause of wound infections, including those in surgical sites, diabetic ulcers, and burns. The bacteria’s ability to form biofilms on wound beds makes these infections particularly difficult to treat, contributing to delayed healing. In some cases, E. faecalis wound infections can modulate the host’s immune response, leading to persistent infection and impaired tissue repair.
When E. faecalis enters the bloodstream, a condition known as bacteremia can develop. This can occur from an untreated UTI, a wound infection, or during surgery. Bacteremia caused by E. faecalis carries a significant mortality rate, often leading to systemic inflammatory responses like sepsis. If left untreated, bacteremia can further progress to endocarditis, a serious infection of the heart’s inner lining and valves.
Endocarditis caused by E. faecalis is a severe infection, accounting for 5% to 15% of all infective endocarditis cases. This condition typically arises when bacteria from the bloodstream attach to damaged heart valves or prosthetic heart devices. The formation of biofilms on heart valves makes E. faecalis endocarditis particularly challenging to treat, often requiring prolonged and aggressive antibiotic therapy.
The Challenge of Antibiotic Resistance
Antibiotic resistance represents a significant challenge in treating Enterococcus faecalis infections. This bacterium possesses both intrinsic and acquired mechanisms that allow it to withstand the effects of various antibiotics.
Intrinsic resistance means that E. faecalis is naturally less susceptible to certain antibiotics, even before exposure. For example, E. faecalis exhibits intrinsic resistance to many beta-lactam antibiotics, such as cephalosporins, and to some aminoglycosides. This inherent resistance is often due to the presence of low-affinity penicillin-binding proteins (PBPs) or an inability of the antibiotic to penetrate the bacterial cell. E. faecalis is also intrinsically resistant to clindamycin and quinupristin-dalfopristin, often through the activity of efflux pumps encoded by genes like lsa.
Acquired resistance, on the other hand, develops when bacteria gain new genetic material or undergo mutations that confer resistance. E. faecalis is particularly adept at acquiring resistance genes through horizontal gene transfer, where genetic elements like plasmids and transposons move between bacteria. These mobile genetic elements can carry genes that encode enzymes that inactivate antibiotics or modify antibiotic targets. This adaptability contributes to the rising rates of multi-drug resistant strains.
A prime example of this challenge is Vancomycin-Resistant Enterococci (VRE), which includes vancomycin-resistant strains of E. faecalis. VRE are a major concern in healthcare settings because vancomycin is a powerful antibiotic often reserved for serious infections. Resistance to vancomycin typically arises from the acquisition of specific genes, such as vanA or vanB, which alter the bacterial cell wall structure, preventing vancomycin from binding effectively. This alteration in the peptidoglycan synthesis pathway means the antibiotic can no longer disrupt cell wall formation.
The emergence of VRE means that infections caused by these strains are much harder to treat, leading to prolonged hospital stays, increased healthcare costs, and higher mortality rates. VRE can spread easily in hospitals through direct contact and contaminated surfaces, posing a substantial threat to vulnerable patients.
Current Treatment Approaches and Prevention
The initial step in treatment involves identifying the specific strain and its resistance profile through susceptibility testing. This testing helps guide healthcare providers in selecting the most effective antibiotics.
For infections caused by susceptible E. faecalis strains, ampicillin is often the preferred antibiotic. For more severe infections, such as endocarditis or bacteremia, combination therapy is commonly employed to achieve a bactericidal effect. This frequently involves combining a cell wall-active agent like ampicillin with an aminoglycoside such as gentamicin or streptomycin. The cell wall-active agent helps the aminoglycoside penetrate the bacterial cell, enhancing its effectiveness.
In cases of Vancomycin-Resistant Enterococci (VRE), treatment options become more limited. Linezolid and daptomycin are often used for VRE infections, particularly for bloodstream infections. For uncomplicated urinary tract infections caused by VRE, options like nitrofurantoin or fosfomycin may be considered. The duration of therapy varies significantly, ranging from 7-14 days for UTIs to 4-6 weeks for severe conditions like endocarditis.
Preventing E. faecalis infections relies heavily on robust infection control practices, especially in healthcare settings.
Prevention Strategies
Hand hygiene: Healthcare workers must diligently wash their hands or use alcohol-based sanitizers before and after patient contact.
Environmental cleaning: Proper cleaning and disinfection of environmental surfaces and medical equipment are crucial to limit the spread of these resilient bacteria.
Antibiotic stewardship: Programs promote the appropriate use of antibiotics, ensuring they are prescribed only when necessary, at the correct dosage, and for the appropriate duration. Reducing unnecessary antibiotic use helps to slow the development and spread of resistant strains, preserving the effectiveness of existing treatments for E. faecalis and other challenging pathogens.