Enterococcus refers to a group of bacteria, specifically a large genus within the phylum Bacillota. These microorganisms are characterized as Gram-positive cocci, meaning they appear as spherical cells that retain a purple stain in a common laboratory test. They often arrange themselves in pairs or short chains, making them visually similar to some types of streptococci. While numerous species exist within this genus, two are most frequently associated with humans: Enterococcus faecalis and Enterococcus faecium.
Normal Role in the Body and Environment
Enterococcus species are commonly found in various environments, including soil, water, plants, and food. They are also natural inhabitants of the gastrointestinal tract in both humans and animals. In humans, E. faecalis accounts for approximately 90-95% of enterococcal species in the intestines, with E. faecium making up a smaller proportion of 5-10%.
These bacteria live harmlessly within the gut as commensal organisms, a normal component of the healthy gut microbiome that does not lead to illness. Enterococci are known for their resilience, capable of surviving a wide range of conditions, including extreme temperatures, varying pH levels, and high salt concentrations.
When Enterococcus Causes Infection
While often harmless, Enterococcus species can become opportunistic pathogens, causing infections when they spread beyond their usual habitats or when an individual’s defenses are weakened. These bacteria are a common cause of healthcare-associated infections, contributing to approximately 7.4% of all such cases. Infections can arise in individuals with compromised immune systems, those undergoing prolonged hospital stays, or patients who have had medical procedures.
Common types of infections caused by Enterococcus include urinary tract infections (UTIs), particularly in catheterized patients. They can also lead to wound infections, especially in chronic wounds like decubitus ulcers or diabetic foot ulcers. More severe infections include bacteremia (bacteria in the bloodstream) and infective endocarditis (infection of the heart valves). Less commonly, Enterococcus can cause intra-abdominal infections or meningitis.
Understanding Antibiotic Resistance
Antibiotic resistance in Enterococcus species is a concern in healthcare, stemming from both intrinsic and acquired mechanisms. Intrinsic resistance means these bacteria are naturally resistant to certain antibiotics without prior exposure. For instance, Enterococci are naturally resistant to cephalosporins, clindamycin, and trimethoprim-sulfamethoxazole. This natural resistance limits the initial range of effective treatment options.
Acquired resistance develops when bacteria gain new resistance genes, often through mobile genetic elements like plasmids or transposons. This allows them to become resistant to antibiotics they were once susceptible to. A major concern is vancomycin-resistant Enterococcus (VRE), a substantial challenge in clinical settings. VRE strains have acquired genes that alter their cell wall structure, preventing vancomycin from binding and exerting its effect.
The prevalence of VRE has increased considerably, with a significant proportion of healthcare-associated infections now involving vancomycin-resistant strains. E. faecium isolates demonstrate a significantly higher proportion of vancomycin resistance compared to E. faecalis, with rates of vancomycin resistance reaching approximately 80% for E. faecium and around 10% for E. faecalis. This high level of resistance in E. faecium makes infections caused by this species particularly difficult to treat, often requiring combinations of antibiotics or newer agents.
Diagnosis and Treatment Strategies
Diagnosing Enterococcus infections begins with culturing samples from the suspected site of infection. This involves collecting specimens such as urine, blood, or wound fluid and allowing the bacteria to grow in a laboratory setting. Once isolated, the bacteria can be identified using various methods, including biochemical tests or advanced techniques.
Following identification, antibiotic susceptibility testing is performed to determine which antibiotics will be effective against the specific strain of Enterococcus. This testing is important given widespread antibiotic resistance, especially to vancomycin. The results guide clinicians in selecting the most appropriate treatment, as an ineffective antibiotic could lead to treatment failure and prolonged infection. Treatment approaches are tailored based on the type and severity of the infection, as well as the unique resistance profile of the isolated bacteria.