Enterococcus faecalis

Enterococcus faecalis. It is a Gram-positive, facultative anaerobic bacterium that is a common commensal organism in the gastrointestinal tracts of humans and other animals. Formerly classified as part of Group D Streptococcus, it was reclassified into its own genus in the 1980s. This organism is notable for its intrinsic hardiness, capable of surviving in harsh environments, and has emerged as a leading cause of hospital-acquired infections worldwide, particularly due to its increasing resistance to multiple antibiotics.

Characteristics and Identification

Enterococcus faecalis is a non-motile, catalase-negative coccus that typically occurs in pairs or short chains. It is distinguished by its ability to grow in 6.5% NaCl, at temperatures ranging from 10 to 45°C, and in the presence of 40% bile salts, characteristics utilized in selective media like Bile Esculin Agar. It hydrolyzes esculin and produces a distinctive blackening on such media. Key biochemical tests for identification include a positive PYR test and its ability to ferment L-arabinose, which helps differentiate it from the closely related Enterococcus faecium. In the clinical laboratory, automated systems like the VITEK 2 and MALDI-TOF mass spectrometry are commonly used for rapid and accurate speciation.

Pathogenesis and Virulence Factors

The pathogenicity of Enterococcus faecalis is multifactorial, involving an array of virulence determinants that facilitate colonization, immune evasion, and tissue damage. A critical initial step is adherence to host tissues, mediated by surface proteins like the aggregation substance and the enterococcal surface protein (Esp). The bacterium can form resilient biofilms on medical devices such as urinary catheters and heart valves. It produces cytolysin, a bacteriocin that lyses human cells and other bacteria, and the gelatinase enzyme, which degrades host tissues. Furthermore, it can acquire and transfer virulence genes and antibiotic resistance markers through plasmids and transposons, enhancing its adaptability and threat in clinical settings.

Clinical Significance and Infections

This bacterium is a major opportunistic pathogen, particularly in healthcare environments like the Mayo Clinic and Johns Hopkins Hospital. It is a common cause of urinary tract infections, often associated with instrumentation. It can lead to serious conditions such as bacteremia, endocarditis (notably affecting native and prosthetic valves), and intra-abdominal and pelvic infections. Post-surgical wound infections, particularly following procedures in the GI tract or biliary system, are also frequently attributed to this organism. Its ability to persist on environmental surfaces in hospitals, as documented by the Centers for Disease Control and Prevention, contributes to its role in nosocomial outbreaks.

Treatment and Antibiotic Resistance

Treatment of infections caused by Enterococcus faecalis is challenging due to its intrinsic and acquired resistance mechanisms. It exhibits natural low-level resistance to beta-lactams like penicillin and cephalosporins, and high-level resistance to aminoglycosides can develop, eliminating synergistic therapy options. Of grave concern is the rise of vancomycin-resistant enterococci (VRE), particularly strains carrying the vanA or vanB gene clusters. Linezolid, daptomycin, and tigecycline are often used as last-resort agents, but resistance to these, as reported in studies from the National Institutes of Health, is emerging. Combination therapy is frequently required, especially for severe infections like endocarditis.

Epidemiology and Prevention

Enterococcus faecalis is ubiquitously found in the environment, including soil, water, and food, but its primary reservoir is the intestinal flora of humans and animals. In hospitals, it is a common cause of outbreaks, with transmission occurring via the hands of healthcare workers, contaminated medical equipment, or environmental surfaces. Surveillance programs by agencies like the World Health Organization and the European Centre for Disease Prevention and Control track its spread. Prevention relies on strict adherence to infection control protocols, including hand hygiene with alcohol-based rubs, contact precautions for patients with multidrug-resistant strains, and prudent antibiotic stewardship to reduce selection pressure. Proper disinfection of endoscopes and other reusable medical devices is also critical to break the chain of transmission.

Category:Bacteria Category:Pathogenic bacteria