Enterococci are bacteria commonly found in the digestive tracts of humans and animals, as well as in the environment (soil and water). They belong to the genus of lactic acid bacteria; Enterococcus faecalis and Enterococcus faecium are the two species most frequently isolated from human samples. Microbiologists use Gram staining, a fundamental diagnostic technique, to classify bacteria like Enterococci. This procedure distinguishes bacteria based on their unique cell wall structure, categorizing them into one of two major groups.
The Gram Stain Status of Enterococci
Enterococci are classified as Gram-Positive bacteria, a determination made using the Gram stain. This differential staining process begins by treating the bacteria with a crystal violet dye. Gram-positive bacteria possess a thick, mesh-like layer of peptidoglycan, a polymer of sugars and amino acids, which effectively traps this initial violet stain.
When an alcohol wash is applied, the thick peptidoglycan layer prevents the crystal violet from being easily rinsed away. Consequently, Gram-positive bacteria retain the deep purple color of the stain. In contrast, Gram-Negative bacteria have a thin peptidoglycan layer situated between two membranes, which cannot retain the stain and appear pink or red after a counterstain is applied.
Natural Habitat and Non-Pathogenic Role
The name Enterococcus derives from their primary habitat: the gastrointestinal tract, where they are a normal part of the human gut flora. They are hardy organisms, able to survive in diverse and harsh environments outside the body, such as soil, water, and on plants. This resilience allows them to tolerate a wide range of temperatures (10°C to 45°C), high salt concentrations, and various pH levels.
In healthy individuals, Enterococci exist as commensal organisms, living in harmony with their host without causing harm. They play a role in maintaining intestinal health by helping to crowd out more harmful pathogens and influencing homeostasis. Although they make up less than 1% of the total adult intestinal microbiota, their presence is a characteristic feature of the digestive tract.
When Enterococci Cause Serious Infection
Despite their usual harmless role in the gut, Enterococci are classified as opportunistic pathogens. They primarily cause infection when they move outside their normal habitat or when the host’s defenses are weakened. These bacteria are a leading cause of infections acquired in healthcare settings (nosocomial infections). At-risk populations include the elderly, immunocompromised individuals, or those with serious underlying illnesses.
Infections often occur when Enterococci translocate from the gastrointestinal tract to sterile body sites, frequently following surgical procedures or the use of medical devices. They commonly cause urinary tract infections (UTIs), especially those associated with catheter use. They are also significant causes of bacteremia (bloodstream infection) and infective endocarditis (infection of the heart lining or valves).
The Challenge of Vancomycin Resistance
A major complication in treating Enterococcus infections is their ability to resist multiple antibiotics, making them a significant public health concern. The most challenging strains are Vancomycin-Resistant Enterococci (VRE), which resist vancomycin, a powerful drug historically used for severe Gram-positive infections. VRE emerged in the mid-1980s and has spread rapidly, particularly within hospitals.
The resistance mechanism is often encoded by genes, such as vanA or vanB, carried on mobile genetic elements like plasmids or transposons. These mobile elements allow resistance genes to be easily exchanged between different Enterococcus strains and other types of bacteria. The genes modify the cell wall structure, altering the final amino acid in the bacterial cell wall precursor from D-Ala-D-Ala to D-Ala-D-Lac.
This change prevents the vancomycin molecule from binding effectively to its target, rendering the drug inactive. Since VRE infections are difficult to treat, often requiring alternative and more toxic antibiotics like linezolid or daptomycin, strict infection control measures are paramount. Hand hygiene and contact precautions are critical to prevent the patient-to-patient spread of these highly resistant organisms.