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

Enterococcus faecium is a Gram positive coccoid bacterium that is part of the commensal microbiota of the intestine of man and animals. They have also been found in oropharyngeal, vaginal, and skin secretions.

Being part of the human microbiota, it was believed to have low pathogenicity. However, it is currently considered an important pathogen in nosocomial infections, being the second most frequently isolated bacterial genus in hospitalized patients.

Among the pathologies in which it may be involved are severe urinary tract infections, wounds, bacteremia and endocarditis. Most infections originate from the endogenous microbiota, although it can be transmitted from person to person or through consumption of contaminated food or water.

E. faecium is capable of colonizing the respiratory tract and skin, and surviving on surfaces of inanimate objects for prolonged periods. It is less frequent in clinical isolates than E. faecalis.

While E. faecalis represents 80 to 90% of the microorganisms found in clinical samples, E. faecium only reaches 5 to 10% of isolates. However, E. faecium is more resistant to antimicrobials.

characteristics

They are facultative anaerobes, they ferment glucose and other carbohydrates obtaining lactic acid as the final product, but they do not produce gas.

It grows at 37 ° C in 24 hours of incubation in enriched media such as blood agar, chocolate and other special media for enterococcus.

Biochemical characteristics

Enterococcus faecium is capable of growing in nutritive broths with 6.5% NaCl and hydrolyzing esculin in the presence of 40% bile salts. It can withstand a pH of up to 9.6.

They produce leucine aminopeptidase (LAP) and give a negative reaction to the catalase test.

Likewise, Enterococcus faecium hydrolyzes L-pyrrolidonyl-β-naphthylamide (PYR) and according to the Lancefield classification it belongs to group D.

The aforementioned characteristics are fulfilled for the entire Genus Enterococcus.

However, the specific characteristics to identify the faecium species are: positive arabinose fermentation, negative tellurite, mobility and negative pigment.

Susceptibility to antibiotics

The recommended antimicrobials for Enterococcus faecium is vancomycin as the first choice.

However, E. faecium has a high capacity to acquire resistance genes for these antibiotics, so most of the isolates are resistant to vancomycin.

If it is a urine sample it is recommended to test nitrofurantoin, tetracycline and ciprofloxacin. In strains isolated from systemic infections such as bacteremia, endocarditis, etc., high-level gentamicin and streptomycin should also be tested and the presence of β-lactamase determined.

For strains resistant to vancomycin, it is recommended to try linezolid, teicoplanin, chloramphenicol, tetracycline, erythromycin, and rifampin.

Morphology

They are Gram positive cocci that are arranged in pairs or in short chains. They do not form spores and do not present flagella, therefore they are non-mobile.

In blood agar, the colonies of Enterococcus faecium are creamy, grayish-white in color and generally show Gamma hemolysis. Whereas in Esculin Bile Agar, the colonies blacken the medium around the colony.

On Slanetz-Bartley Agar (with 1% TTC) the colonies are red-brown in color and on KF Agar (with 1% TTC) the colonies are pink, surrounded by a yellow area.

Taxonomy

Domain: Bacteria.

Phylum: Firmicutes.

Class: Bacilli.

Order: Lactobacillales.

Family: Enterococcaceae.

Genus: Enterococcus.

Species: faecium.

Virulence factors

Among the most prominent virulence factors in this microorganism are:

-Hemolysins, with cytolytic action on some cells allowing invasion into the bloodstream.

-The surface protein and the collagen-binding adhesin, both promote colonization and therefore infection.

-Also, it has other virulence factors such as gelatinase, aggregation substance, lipase, hemagglutinins and its high capacity to acquire antibiotic resistance genes.

Pathologies

Most infections arise from the invasion of the individual's own microbiota, that is, it produces endogenous infections.

It is seen mainly in urinary tract infections, biliary tract infections, endocarditis, and septicemia.

Enterococcus are generally a rare cause of acute bacterial meningitis, accounting for only 0.3% to 4.9% of cases.

However, when it occurs, 10% are produced by the faecium species, 88% by faecalis and 2% by other Enterococcus species.

Endocarditis mainly occurs in the elderly or in patients with underlying vulvolopathies who are subjected to manipulations of the gastrointestinal or genitourinary tracts.

Treatment

Enterococcus faecium tends to be more resistant to antimicrobials than faecalis. Multi-resistance in these microorganisms represents a serious problem at the hospital level, and its control is a real challenge.

Vancomycin resistance is more common in E. faecium (50%) than in E. faecalis (5%).

So far, one of the antibiotics that has remained without resistance to Enterococcus faecium is linezolid (Oxazolidinone). This antibiotic has the advantage that it can be administered orally and has excellent bioavailability.

But there are also other alternatives such as Streptogramins (Quinupristin / dalfopristin) only for E. faecium, since it is not active against E. faecalis. As well as Lipopeptides: (Daptomycin) and Glycylcycline (Tigecycline).

However, before making any therapeutic decision it is necessary to have the laboratory results of the antibiogram in hand, in this way the available antibiotics can be better used.

Prevention

This bacterium is very difficult to control, however it will always be useful to comply with the standardized asepsis norms of health personnel and the correct sterilization and disposal of contaminated material.

Industrial benefits

Enterococcus faecium is used for its beneficial effects in animal production, specifically in rabbits.

The soy product fermented with this microorganism causes a reduction of 18.4% in total cholesterol and an increase of 17.8% in the HDL fraction.

Enterococcus faecium can also be used as a probiotic, incorporating it into the diet of young pigs to prevent diarrhea problems after weaning.

E. faecium is believed to modulate the immune response, allowing antibiotics to be reduced or dispensed with.

An additional benefit of the incorporation of E. faecium to low protein diets in pigs is the decrease in ammonia nitrogen in feces, this minimizes environmental contamination by reducing the characteristic odor of excreta.

References

1. Gutiérrez F, Cacho B, García G. Enterococcus faecium, etiological agent of acute bacterial meningitis: case report and review of the literature. Rev. Mex de Neuroc 2010; 11 (1): 107-110
2. 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.
3. Ortega L. Enterococci: update. Rev. Haban Cienc Méd. 2010; 9 (4): 507-515. Available in: scielo.
4. Wikipedia contributors. Enterococcus faecium. Wikipedia, The Free Encyclopedia. August 22, 2018, 16:38 UTC. Available at: en.wikipedia.org
5. Koneman E, Allen S, Janda W, Schreckenberger P, Winn W. (2004). Microbiological Diagnosis. (5th ed). Argentina, Editorial Panamericana SA