ENTEROCOCCUS FAECALIS: CHARACTERISTICS, MORPHOLOGY, PATHOGENESIS - BIOLOGY - 2025

Enterococcus faecalis, formerly called Streptococcus faecalis, is a microorganism that is part of the microbiota of the intestinal and bile ducts. It can also be found as a microbiota of the vagina and male urethra, as well as in the intestinal tract of animals such as poultry, cattle, dogs, pigs, horses, goats, and sheep.

It can also be detected in soil, water or food, indicating fecal contamination, with the exception of fermented foods such as cheeses, raw sausages and meats where its presence is normal.

Source: Photograph taken by the author MSc. Marielsa gil

E. faecalis belonged to the Group D genus Streptococcus, but was recently classified into its own genus called Enterococcus. They are a frequent source of infections at the hospital and community level.

Currently they have taken clinical relevance due to their antimicrobial resistance against penicillin, cephalosporins, aminoglycosides, trimethoprim-sufamethoxazole and vancomycin. Infections can become severe and fatal due to their multi-resistance.

Enterococcus faecalis occupies 80 to 90% of human enterococcal isolates.

characteristics

Scanning electron micrograph of Enterococcus faecalis

They are facultative anaerobic microorganisms, immobile, catalase negative or weakly positive, with the capacity to ferment glucose and other carbohydrates with production of lactic acid, but without gas. It also has the ability to form biofilms.

Enterococcus differ from Strectococcus in that they can grow in a temperature range of 10ºC to 45ºC. They are more resistant to adverse environmental changes, being able to tolerate concentrations of 6.5% NaCl, develop at pH 9.6 and withstand temperatures of 60 ° C for up to half an hour.

Taxonomy

Enterococcus faecalis belongs to Domain Bacteria, Phylum Firmicutes, Class Bacilli, Order: Lactobacillales, Family: Enterococcaceae, Genus: Enterococcus, Species faecalis.

Morphology

Enterococcus faecalis are cocci of size 0.6-2.0 × 0.6-2.5 µm, Gram positive that are distributed in short chains or in pairs. They do not form spores.

Virulence factors

E. faecalis is non-pathogenic in immunocompetent patients, therefore it behaves as an opportunistic pathogen.

Unlike other microorganisms, its virulence factors are not well defined. However, the following are known:

Cytolysin

Some strains can produce a cytolysin with cytotoxic action against certain eukaryotic cells while acting as a hemolysin against human erythrocytes and from various animals such as rabbit, horse, and bovines.

Aggregation substance

An aggregation substance (AS) of protein origin, bound to the surface of the bacteria, has been described that facilitates the accumulation of microorganisms to favor the exchange of plasmids, which is crucial for the acquisition of resistance genes.

It is believed that this substance is also involved in the adherence of the bacteria to the cells of the renal, cardiac and intestinal epithelia.

Pheromone production

Enterococcus faecalis produces pheromones, which are peptide substances that stimulate the transfer of plasmid DNA by conjugation between strains.

It also acts as chemotactic substances attracting polymorphonuclear cells (PMN), favoring the inflammatory process.

Lipoteichoic acids

For their part, the lipoteichoic acids present in the cell wall (group D antigen) induce the production of tumor necrosis factor and interferon gamma by modulating the immune response.

Production of bacteriocins, enzymes and superoxide ion

An interesting fact is that some strains of Enterococcus faecalis can produce bacteriocins that have the ability to lyse a wide variety of Gram positive and Gram negative bacteria.

E. faecalis is also known to produce various enzymes such as hyaluronidase and gelatinase. Both are extracellular.

Finally they are capable of producing a large amount of superoxide ion (O ₂ ^-). This property suggests to be an efficient mechanism to survive macrophage phagocytosis.

Pathogenesis / symptoms

It is believed that for there to be an Enterococcus faecalis infection there must first be colonization of the bacteria in the mucosa. It binds to target cells through adhesins.

After colonization, the microorganism can invade other anatomical regions until it reaches the lymphatic or circulatory system. In this way it can produce various pathologies.

Virulent strains of enterococci that colonize the intestinal mucosa can move from the intestinal lumen to the lymph nodes, liver, and spleen, after being endocyted by cells of the ileum, colon, or intestinal macrophages.

Pathologies

Enterococcus faecalis has been isolated from urinary tract infections, bacteremia, endocarditis, intradominal, pelvic infections, soft tissue infections, wounds, neonatal sepsis, and rarely meningitis.

It has also been associated with cystitis, pyelonephritis, prostatitis and perinephritic abscesses, attributable to structural abnormalities or instrumental interventions in the urinary tract.

It is frequently involved in mixed infections. For example, in infections with anaerobes and other facultative bacteria, especially in soft tissues.

Contagion

E. faecalis can be spread by fecal-oral transmission, by contact with contaminated fluids or surfaces.

Most bacteremia originate from urinary infections, peritonitis, wounds, decubitus ulcers, catheters, or other intravenous devices, as well as complications from caesarean sections, endometritis, or acute pelvic inflammatory disease.

The conditioning factors for the acquisition of Enterococcus faecalis infections are generally due to immunosuppression states. For example:

• Patients with long periods of hospitalization,
• Patients with deep-seated malignancies and infections,
• Diabetics, among others.

Also the use of broad spectrum antibiotics with little or no action against this microorganism favors its proliferation.

Diagnosis

It is done through the cultivation and isolation of the microorganism in the laboratory.

Colorless to gray colonies of 2-3 mm in diameter are observed on blood agar, and may present alpha, beta or gamma hemolysis, depending on the strain and type of blood used.

Biochemical tests are used to identify them, including the PYR test (L-pyrrolindonyl β-naltylamide), the leucine aminopeptidase (LAP) test and the hydrolysis of esculin.

Treatment

Due to the multi-resistance that is frequently found in this species, the treatment of infections can be somewhat complicated.

The normal treatment for this bacteria is amoxicillin or ampicillin alone or in combination with gentamicin or streptomycin.

But because Enterococcus faecalis has registered resistance to penicillins, cephalosporins, and especially high-level resistance to aminoglycosides, this combination is sometimes not possible, so the ideal treatment was vancomycin.

However, there are currently strains of E. faecalis that are vancomycin resistant (VRE) with various phenotypes (VanA to VanE). This darkens the therapeutic landscape. Teicoplanin is also an option but is also resistant at times.

In uncomplicated urinary tract infections, nitrofurantoin and fosfomycin may be useful, and in urinary tract infections associated with prostatitis, the combination of nitrofurantoin with rifampin can be used.

There are new drugs with sensitivity for E. faecalis VRE such as linezolid and daptomycin useful for cases of bacteremia.

Prevention

To prevent colonization by virulent strains of this bacterium, it is necessary to comply with the aseptic standards (disinfection and sterilization) of places or objects contaminated with this microorganism, especially in the hospital environment.

References

1. Girón-González J and Pérez –Cano R. Treatment of Enterococcal infections. Rev. Clin Exp 2003; 203 (10): 482-485.
2. Fernández F, Fuente J, Rubianes M, Pérez S, Álvarez A, Nodar A, Sopeña B, Martínez C. Bacteremia due to Enterococcus faecalis. Rev Clin Esp 2004, 204: 244-250.
3. Wikipedia contributors. Enterococcus faecalis. Wikipedia, The Free Encyclopedia. July 31, 2018, 17:04 UTC. Available at: en.wikipedia.org/ Accessed September 6, 2018.
4. Caicedo E, Urrutia J, Fernández D, Guío S, Méndez Y. Treatment of bacteremia due to vancomycin-resistant enterococcus with daptomycin versus linezolid: systematic review and meta-analysis. IATREIA 2017; 30 (1): 5-20.
5. Díaz M, Rodríguez C, Zhurbenko, R. Fundamental aspects of the genus Enterococcus as a highly important pathogen today. Rev Cubana Hig Epidemiol, 2010; 48 (2): 147-161.
6. Comerlato CB, from Resende MCC, Caierão J, d 'Azevedo PA. Presence of virulence factors in Enterococcus faecalis and Enterococcus faecium susceptible and resistant to vancomycin. Memories of the Oswaldo Cruz Institute. 2013; 108 (5): 590-595.
7. Van Tyne D, Martin MJ, Gilmore MS. Structure, Function, and Biology of the Enterococcus faecalis Cytolysin. Toxins. 2013; 5 (5): 895-911.