Enterococcus includes Gram-positive bacteria common in the human gastrointestinal tract. These organisms usually live harmlessly as part of the normal gut microbiota, assisting in metabolic functions. However, they can act as opportunistic pathogens when the host’s defenses are compromised, especially in healthcare settings. The two species responsible for the vast majority of human enterococcal infections are Enterococcus faecalis and Enterococcus faecium. Understanding their distinct biological, clinical, and resistance profiles is necessary for effective diagnosis and treatment.
Commensal Roles and Natural Habitat
These bacteria are natural components of the human gut, where they primarily colonize the lower gastrointestinal tract. E. faecalis is typically the dominant species in healthy individuals, accounting for an estimated 90% to 95% of enterococcal isolates in the gut. Its population is generally found at high concentrations, ranging from \(10^5\) to \(10^7\) bacteria per gram of feces.
E. faecium is naturally much less abundant, making up only about 5% to 10% of the total enterococcal population. As commensals, both species contribute to maintaining the internal environment by helping to regulate intestinal pH and metabolizing various nutrients, including carbohydrates. Their presence also plays a role in modulating the host’s immune system.
Biological Divergence: Core Differences in Structure and Metabolism
Despite their similar names, E. faecalis and E. faecium are genetically distinct organisms with key metabolic differences. These inherent biological traits establish the foundational divergence between the two species, particularly in their fermentation patterns when exposed to certain carbohydrates.
E. faecalis strains ferment both sorbitol and pyruvate but are unable to utilize arabinose. Conversely, E. faecium strains often ferment arabinose but lack the ability to ferment sorbitol. A further distinction is that E. faecalis possesses a cytochrome-like respiratory system and a catalase enzyme, which are not present in E. faecium.
These differences extend to their genomes. Comparative genomic analysis shows that E. faecium strains that cause disease often have significantly smaller genomes than their non-clinical counterparts. This suggests a specific genomic adaptation in E. faecium that allows them to thrive in the stressful environment of the hospital.
Clinical Significance: Infection Types and Virulence Factors
The two species differ in their capacity to cause disease and the types of infections they commonly trigger. E. faecalis is responsible for 85% to 90% of all enterococcal infections, strongly associated with urinary tract infections (UTIs) and intra-abdominal wound infections. Its pathogenicity is largely driven by a suite of secreted virulence factors.
Specific factors like gelatinase (GelE) and cytolysin are prevalent in E. faecalis and enable it to break down host tissues. Gelatinase is a zinc metalloendopeptidase that hydrolyzes gelatin, collagen, and hemoglobin, causing tissue damage. Cytolysin is a toxin that dissolves host cells and contributes to more severe infections.
E. faecium, while causing fewer overall infections, is disproportionately associated with severe, hospital-acquired infections (HAIs), particularly bacteremia and endocarditis. E. faecium has an increased ability to form biofilms on medical devices, such as catheters. This protective community allows the bacteria to persist in the host, leading to prolonged systemic infections.
The Challenge of Treatment: Distinct Antibiotic Resistance Profiles
The most significant clinical difference lies in their inherent and acquired resistance to antibiotics. Both species possess intrinsic resistance to certain drugs, such as all cephalosporins, because of their unique cell wall structure. However, E. faecium is intrinsically less susceptible to \(\beta\)-lactam antibiotics, including ampicillin, due to low-affinity penicillin-binding proteins (PBPs).
This natural resistance is compounded by the ease with which both species acquire mobile genetic elements, such as plasmids, that carry additional resistance genes. E. faecium has become far more challenging to treat because of its high propensity to develop multidrug resistance, often showing resistance to a greater number of antibiotic classes than E. faecalis.
The clinical danger of Vancomycin-Resistant Enterococcus (VRE) is overwhelmingly dominated by E. faecium strains. The widespread use of broad-spectrum antibiotics in hospitals has selected for these highly resistant strains, making them a major public health concern. Treating these infections requires alternative agents, such as linezolid or daptomycin.