The bacterium Enterococcus faecalis is classified as Gram-positive, a determination made through the Gram stain procedure. This organism is a spherical bacterium (coccus) that typically arranges itself in pairs or short chains. E. faecalis is a robust microbe that was reclassified from the Streptococcus family due to its unique resistance to harsh conditions. Its Gram classification is based entirely on the physical structure of its cellular envelope, which dictates how it interacts with the dyes used in the staining technique.
The Gram Stain Mechanism and Cell Wall Structure
The Gram stain is a differential staining technique that separates bacterial species into two groups based on their cell wall characteristics. The procedure involves four steps: applying a primary stain, using a mordant, a decolorization wash, and a counterstain. The initial step uses crystal violet, a purple dye that penetrates the cell walls.
Iodine is then added as a mordant to form large, insoluble crystal violet-iodine complexes within the cell. The next step is the alcohol wash, which serves as a decolorizer. The E. faecalis cell wall is composed of a thick, multilayered mesh of peptidoglycan.
The alcohol dehydrates this layer, causing it to shrink and tighten, effectively trapping the large dye complexes inside. Because the dye complex cannot escape, the bacterium retains the deep purple color of the primary stain. Gram-positive bacteria, like E. faecalis, lack the outer membrane structure found in Gram-negative bacteria. Their thick peptidoglycan layer can represent up to 90% of the cell envelope’s dry weight. The final step, counterstaining with safranin, has no effect on the already purple E. faecalis, confirming its Gram-positive status.
Normal Habitat and Commensal Role
Enterococcus faecalis is a common resident of the human and animal gastrointestinal tract, where it lives as a commensal organism. It is one of the most prevalent species in its genus found in humans, often making up 90 to 95% of the enterococci isolated from clinical samples. This bacterium colonizes the lower digestive system and is adapted to survive the challenging conditions within the gut.
E. faecalis is generally considered a neutral or beneficial member of the microbiota in this environment. Some strains are used as probiotics, suggesting a role in maintaining the balance of the gut flora. The organism’s ability to tolerate bile salts, high salt concentrations, and a wide range of temperatures (10°C to 45°C) allows it to thrive.
The presence of E. faecalis in the gut is normal, and it does not typically cause disease when confined there. It is also found in the oral cavity and the female genital tract. This existence as a harmless resident changes only when the organism gains access to normally sterile body sites or when the host’s defenses are impaired.
Infections and Pathogenesis
While E. faecalis is a normal gut resident, it is also a well-known opportunistic pathogen. It causes infection when the host’s immune system is compromised or when physical barriers are breached. The transition to an infectious agent often happens in hospital settings, where it is a leading cause of healthcare-associated infections.
One of the most frequent types of infection is the urinary tract infection (UTI), particularly in patients with indwelling urinary catheters. The bacterium can also cause severe bloodstream infections (bacteremia) and is a significant cause of infective endocarditis, an infection of the heart’s inner lining. Other sites of infection include surgical wounds, intra-abdominal abscesses, and meningitis.
The ability of E. faecalis to form biofilms on medical devices, such as catheters and prosthetic valves, is a major factor in its persistence and ability to evade the host immune response. Specific virulence factors, such as the production of cytolysin and aggregation substances, allow the organism to adhere to host tissues and damage cells. The organism is a threat primarily to those with underlying illnesses, the elderly, or patients undergoing invasive procedures.
The Challenge of Antibiotic Resistance
The Gram-positive classification of E. faecalis indicates that certain cell wall-targeting antibiotics should be effective. However, this bacterium possesses intrinsic and acquired mechanisms that allow it to resist multiple classes of antimicrobial drugs, presenting a significant clinical hurdle. E. faecalis is naturally tolerant to many beta-lactam antibiotics, like cephalosporins, due to the structure of its penicillin-binding proteins.
The most concerning development is the emergence of Vancomycin-Resistant Enterococci (VRE), which includes strains of E. faecalis that have acquired resistance to vancomycin. Vancomycin is a glycopeptide antibiotic reserved as a last-resort treatment for serious Gram-positive infections. It works by binding to D-alanyl-D-alanine precursors in the peptidoglycan layer, preventing cell wall construction.
VRE strains acquire genes, such as vanA or vanB, which alter the terminal structure of the peptidoglycan precursor to D-alanyl-D-lactate. This change causes a dramatic reduction in vancomycin’s binding affinity, rendering the drug ineffective. The ability of E. faecalis to acquire and transfer these resistance genes, often carried on mobile genetic elements like plasmids, contributes to the spread of VRE in hospital environments. Treatment options for VRE infections are limited, often relying on newer agents like linezolid or daptomycin. The presence of VRE is associated with higher mortality rates and prolonged hospital stays.