Table of Contents
- Classification
- Habitat
- Morphology
- Cultural Characteristics
- Biochemical Characteristics
- Virulence Factors
- Pathogenesis
- Clinical Manifestation
- Laboratory Diagnosis
- Treatment
- Antibiotic Resistance Profile
- Prevention
Enterococcus faecalis belongs to the Gram-positive, catalase-negative cocci category within the Enterococcus genus. It is non-motile and part of the Enterococcaceae family in the Lactobacillales order under the class Bacilli.
Introduction to Enterococcus faecalis:
- Found as a commensal bacterium in the human gastrointestinal (GI) and vaginal tracts, Enterococcus faecalis is commonly associated with nosocomial infections, including urinary tract infections, wound and soft-tissue infections, bloodstream infections, endocarditis, and meningitis.
- Discovered in 1899 by German microbiologist Fritz Schmalz, Enterococcus faecalis was initially classified as Streptococcus faecalis due to its resemblance to other Streptococcus bacteria. Formerly part of the Group D Streptococci, distinguished by their ability to thrive in bile, it underwent reclassification by the International Council of Microbiologists in 1984. This decision was based on variations in biochemical characteristics, cell wall structure, and genetic data from DNA hybridization studies. The name "Enterococcus" was proposed, derived from the Greek words "enteron" (intestine) and "kokkos" (berry or coccus), leading to the reclassification of the Streptococcus faecalis group as a distinct genus - Enterococcus, with the species S. faecalis reclassified as Enterococcus faecalis.
- Currently part of the 'Enterococcaceae family,' Enterococcus faecalis has emerged as a significant concern in healthcare settings due to its resistance to multiple antibiotics and its capacity to cause various nosocomial infections.
Classification of Enterococcus faecalis
Previously, Enterococcus faecalis was formally placed in the Streptococcaceae family within the Group D Streptococcus group. However, in the 1980s, a reclassification occurred, designating Group D Streptococci as a distinct family called 'Enterococcaceae,' and renaming the bacterium from Streptococcus faecalis to Enterococcus faecalis. Additionally, there is a debate regarding the phylum's name, with modern microbiologists advocating for 'Firmicutes,' while some still adhere to the old term 'Bacillota.'
|
Domain |
Bacteria |
|
Phylum |
Firmicutes (Previously Bacillota) |
|
Class |
Bacilli |
|
Order |
Lactobacillales |
|
Family |
Enterococcaceae |
|
Genus |
Enterococcus |
|
Species |
E. faecalis |
Habitat of Enterococcus faecalis
- Enterococcus faecalis demonstrates versatility in adapting and thriving across diverse environments and conditions. Its primary habitat is the intestinal tract of warm-blooded animals, including humans. Additionally, it colonizes the genitourinary tract, particularly the vaginal tract, along with the oral cavity and skin.
- In natural settings, Enterococcus faecalis can be found in soil, water, food, and decomposing vegetation. It is particularly abundant in soil and water contaminated with fecal matter.
Morphology of Enterococcus faecalis
- Enterococcus faecalis presents as Gram-positive cocci with a slightly oval shape when viewed at higher magnification.
- Typically ranging from 0.5 to 2 μm in diameter, the cells can elongate to 0.6 to 2.5 μm under certain growth conditions, giving them an ovoid appearance.
- Microscopically, these bacteria appear as oval-shaped entities, either as single cells, pairs, or chains.
- They are characterized as non-motile, non-spore-forming, and non-pigment-producing.
- Enterococcus faecalis has the ability to develop pili and form biofilm, enhancing its capacity to colonize and survive in challenging environmental conditions.
Cultural Characteristics of Enterococcus faecalis
- Enterococcus faecalis is a non-fastidious bacterial species, allowing it to thrive on a broad spectrum of general-purpose and selective culture media. As facultative anaerobes, they form visible colonies in temperatures ranging from 10°C to 45°C. These bacteria exhibit resilience to conditions like 6.5% NaCl concentration, bile salts, elevated pH up to 9.5, and even temperatures of 60°C for approximately half an hour. Despite these traits, Enterococcus faecalis is commonly cultivated aerobically at 35 (±2)°C.
- Primary isolation and characterization of Enterococcus faecalis in microbiology labs often employ blood agar medium. Other media such as Nutrient Agar, Bile Esculin Agar, Columbia CNA medium, and MacConkey Agar are also utilized for isolation and identification purposes.
- Selective media specifically designed for the rapid isolation and primary identification of Enterococcus spp., including E. faecalis, include CHROMagarTM Orientation, Brilliance UTI Agar, Enterococcosel agar, and mEnterococcus Agar (Slanetz and Bartley agar).
Cultural Characteristics of Enterococcus faecalis on Various Culture Media:
Blood Agar Medium:
- Colonies of E. faecalis are minute (pinpoint), exhibiting a smooth, gray or grayish-white appearance. They are non-hemolytic (γ-hemolytic), with occasional strains showing a narrow zone of β- or α-hemolysis around colonies.
Bile Esculin Agar Medium:
- E. faecalis, being bile salt-tolerant, thrives in 40% bile salt concentration. The colonies are small, convex, transparent, and slightly brownish, often accompanied by a brown-black halo around them. In some cases, blackening of the medium may occur.
Nutrient Medium:
- On nutrient agar, E. faecalis forms small, spherical, smooth, opaque, creamy colonies.
MacConkey Agar Medium:
- On MAC agar without crystal violet, E. faecalis produces small, smooth colonies with pink coloring, indicating its ability to ferment lactose.
Columbia CNA Agar Medium:
- This selective medium for Gram-positive cocci yields small, convex, mostly non-hemolytic, opaque circular colonies with a faint bluish coloration in certain cultures.
Brilliance UTI Agar:
- In Brilliance UTI agar, E. faecalis develops small, circular, smooth colonies with a turquoise blue color.
CHROMagarTM Orientation:
- On CHROMagarTM Orientation, E. faecalis forms small circular colonies with a turquoise blue color.
M-Enterococcus Medium (Slanetz and Bartley Agar):
- This selective medium, commonly used for Enterococcus spp. isolation, results in small, circular, smooth, convex colonies with a pink or white color.
Biochemical Characteristics of Enterococcus faecalis
General Biochemical Test Results
|
General Biochemical Characteristics |
Enterococcus faecalis |
|
Arginine dihydrolase Test |
Positive
(+) |
|
40% Bile-salt Tolerance |
Positive
(+) |
|
Bile Solubility Test |
Insoluble |
|
Capsule |
Negative
(-) |
|
Catalase |
Negative
(-) |
|
Citrate (Simmons) |
Negative
(-) |
|
Coagulase |
Negative
(-) |
|
Deoxyribonuclease (DNase) |
Negative
(-) |
|
Esculin Hydrolysis |
Positive
(+) |
|
Gram Staining |
Gram
Positive Cocci |
|
Gas (in MRS broth) |
Negative
(-) |
|
Hemolysis |
γ-
hemolytic (rarely α-and β-) hemolytic |
|
H2S (Hydrogen Sulfide) |
Negative
(-) |
|
LAP (Leucine amino peptidase) Test |
Positive
(+) |
|
Motility |
Non-motile |
|
6.5% NaCl |
Positive
(+) |
|
Nitrate Reduction |
Positive
(+) |
|
Oxidase |
Negative
(-) |
|
OF (Oxidative Fermentation) |
Facultative
Anaerobes |
|
PYR (Pyrrolidonyl Aryl amidase) Test |
Positive
(+) |
|
Pyruvate Broth Test |
Positive
(+) |
|
Spore |
Negative
(-) |
|
Growth at 4°C |
Negative
(-) |
Carbohydrate Fermentation Tests
|
General Biochemical Characteristics |
Enterococcus faecalis |
|
N-Acetylglucosamine |
Positive
(+) |
|
Adonitol |
Negative
(-) |
|
L-Arabinose |
Negative
(-) |
|
D-Arabitol |
Negative
(-) |
|
L-Arabitol |
Negative
(-) |
|
Arbutin |
Negative
(-) |
|
Cellobiose |
Positive
(+) |
|
Dulcitol |
Negative
(-) |
|
D-Fructose |
Positive
(+) |
|
D-
and L-Fucose |
Negative
(-) |
|
Galactose |
Positive
(+) |
|
D-Glucose |
Positive
(+) |
|
Glycerol |
Positive
(+) |
|
Glycogen |
Negative
(-) |
|
Inulin |
Negative
(-) |
|
Lactose |
Positive
(+) |
|
Maltose |
Positive
(+) |
|
Mannitol |
Positive
(+) |
|
D-Mannose |
Positive
(+) |
|
D-Raffinose |
Negative
(-) |
|
Rhamnose |
Variable |
|
Ribose |
Positive
(+) |
|
Sorbitol |
Positive
(+) |
|
Sorbose |
Negative
(-) |
|
Sucrose |
Positive
(+) |
|
D-Tagatose |
Positive
(+) |
|
Trehalose |
Positive
(+) |
|
Tartrate |
Negative
(-) |
|
L-Xylose |
Negative
(-) |
|
D-Xylose |
Variable |
Enzymatic Hydrolysis Tests
|
General
Biochemical Characteristics |
Enterococcus
faecalis |
|
Arginine
Dehydrolase |
Positive
(+) |
|
Caseinase |
Negative
(-) |
|
DNase |
Negative
(-) |
|
Esculin
Hydrolysis |
Positive
(+) |
|
α-Galactosidase |
Negative
(-) |
|
Gelatinase |
Positive
(+) |
|
Lipase |
Positive
(+) |
|
Lysine
Decarboxylase |
Negative
(-) |
|
Leucine
Arylamidase |
Positive
(+) |
|
Ornithine
Decarboxylase |
Negative
(-) |
|
Phenylalanine
Deaminase |
Negative
(-) |
|
Pyrrolidonyl
Aminopeptidase |
Positive
(+) |
|
Tryptophanase |
Positive
(+) |
Virulence Factors of Enterococcus faecalis
Adhesins:
These are surface-associated proteins or structures facilitating E. faecalis cell attachment to host cells or tissues. E. faecalis produces various adhesins such as pili (fimbriae), collagen-binding adhesins, enterococcal surface proteins (ESPs), and aggregation substances (Agg). These adhesins enable attachment to host surfaces, biofilm development, colonization promotion, and protection against flushing actions in the urinary tract.
Hemolysin/Cytolysin:
E. faecalis produces hemolysin, responsible for host cell lysis. This protein aids bacterial invasion of host tissues, evasion of immune cells, and damage to host tissues.
Biofilm Formation:
Biofilm formation is a crucial virulence factor for E. faecalis. It enables the bacterium to evade the immune system, resist changing environments and antibiotics, and colonize various surfaces, including medical devices and host tissues.
Gelatinase:
This extracellular enzyme, produced by E. faecalis, contributes to the destruction of extracellular matrix components and host tissue. This facilitates bacterial spread within a host, acquisition of essential nutrients, and invasion of host tissues.
Pathogenesis of Enterococcus faecalis Infections
Enterococcus faecalis, functioning as both a commensal and opportunistic pathogen, is responsible for various infections such as urinary tract infections, soft tissue infections, wounds, bloodstream infections, endocarditis, and more. The pathogenesis is underpinned by virulence factors like adhesins, extracellular enzymes, biofilm-forming ability, antimicrobial resistance, and others. While the disease's pathogenesis varies based on the type and site of infection, a general overview of the steps in the pathogenesis of E. faecalis infection can be outlined as follows:
Adherence and Colonization:
E. faecalis, a commensal and opportunistic pathogen, initiates infection by adhering to the host's cell surface using various adhesins. Once attached, rapid multiplication leads to colonization at the adhered site.
Biofilm Formation:
The bacterium has the capability to form biofilms over catheters, implanted medical devices, and host tissues, providing protection against immune responses and antimicrobials, thereby sustaining persistent infections.
Release of Toxins and Extracellular Enzymes:
Following adherence and colonization, E. faecalis releases toxins and extracellular enzymes, including hemolysin, gelatinase, lipase, proteases, etc. These substances facilitate the destruction of host tissue components, dissemination, and survival.
Invasion and Evasion:
Toxins and enzymes enable E. faecalis to invade host epithelial cells while evading immune responses.
Host's Immune Response (Inflammatory Response):
The presence of the bacterium triggers inflammatory and other immune responses in the host's body. Inflammation recruits neutrophils and macrophages to the infected site, leading to tissue damage, swelling, pain, watery discharge, and other associated symptoms.
Clinical Manifestation of Enterococcus faecalis
Enterococcus faecalis exhibits a diverse range of clinical manifestations in humans, predominantly appearing in healthcare-associated infections and seldom in community-acquired infections. As opportunistic pathogens, these bacteria primarily target individuals with compromised or weakened immune systems. The following are common diseases caused by Enterococcus faecalis in humans:
Urinary Tract Infections (UTIs):
UTIs represent the most prevalent clinical manifestation of E. faecalis. Typically observed in healthcare-associated UTIs, especially among catheterized and immunocompromised patients, E. faecalis frequently leads to cystitis and pyelonephritis.
Intra-abdominal Infections:
Despite being commensals in the human gastrointestinal tract, E. faecalis can cause intra-abdominal infections such as peritonitis and the formation of intestinal abscesses. Approximately 30% of intra-abdominal infection cases are associated with E. faecalis.
Endocarditis:
Accounting for 5% to 15% of cases, Enterococcus faecalis ranks as the third most common cause of infective endocarditis. This condition is primarily attributed to the dissemination of bacteria through the bloodstream following infection in the genitourinary tract.
Bloodstream Infections:
E. faecalis is frequently implicated in bloodstream infections, leading to bacteremia and sepsis. While most infections are nosocomial, community-acquired bloodstream infections by E. faecalis are also reported.
Surgical Site Infections:
E. faecalis has the potential to cause infections at surgical sites post-procedures, resulting in wound infections, abscess formation, or deep tissue infections.
Other Infections:
In addition to the mentioned manifestations, E. faecalis is rarely associated with ocular infections, respiratory infections, and nervous system infections such as meningitis.
Laboratory Diagnosis of Enterococcus faecalis:
In the laboratory setting, Enterococcus faecalis is primarily isolated from clinical samples such as urine and pus/swab, with no specific considerations required for sample collection, transportation, and processing.
1. Cultural Characters and Morphological Identification:
Blood agar medium (BA), Columbia CNA agar medium, and Bile Esculin Agar medium are commonly employed for culturing samples suspected to contain Enterococcus spp. The appearance of colonies in these mediums, or any other used culture medium, is observed and compared with the colonial characteristics of E. faecalis.
Microscopic observation after Gram-staining reveals Gram-positive cocci with elongated oval-shaped cells, typically arranged in pairs or chains.
2. Biochemical Characterization:
Following confirmation of Gram-positive cocci, a series of biochemical tests is conducted for E. faecalis confirmation. Commonly used tests include the catalase test, oxidase test, LAP test, PYR test, Bile Esculin test, growth on 6.5% NaCl, arginine dihydrolase test, and carbohydrate fermentation tests (pyruvate, mannitol, and raffinose).
3. Molecular Diagnosis:
For enhanced speed and accuracy in identification, molecular diagnostic tools such as PCR and sequencing, rRNA sequencing, and Mass spectrometry utilizing matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF MS) are preferred. However, these methods are rarely employed in routine clinical diagnosis.
Treatment of Enterococcus faecalis Infections
For urinary tract infections (UTIs), a range of antibiotics such as tetracycline, nitrofurantoin, quinolones, ampicillin, and tetracycline are commonly prescribed. In cases of systemic and potentially lethal infections, a combination of penicillin (Penicillin-G), ampicillin, or vancomycin with an aminoglycoside like gentamycin or streptomycin is frequently administered. More recent antimicrobials, linezolid and daptomycin, are occasionally employed. In instances of multidrug-resistant (MDR) E. faecalis infections, chloramphenicol and other antibiotics may be considered. Vancomycin is specifically effective against beta-lactamase-producing E. faecalis strains. However, antimicrobial resistance is a growing concern, rendering some antibiotics less effective. Presently, combination therapy involving cell-wall active agents (penicillin, ampicillin, or vancomycin) along with an aminoglycoside is widely employed in the treatment of E. faecalis infections.
Antibiotic Resistance Profile of Enterococcus faecalis:
- Enterococci exhibit intrinsic and inherited resistance to a broad spectrum of commonly used antimicrobial agents, including aminoglycosides, beta-lactams, and glycopeptides. Reports indicate resistance to widely used therapeutics, including newer options like linezolid, daptomycin, and vancomycin. This extensive antimicrobial-resistant profile poses challenges in treating E. faecalis infections with single antibiotics, necessitating combination therapy and prolonged antibiotic consumption.
- E. faecalis inherently resists several penicillin derivatives. While initially sensitive to ampicillin, the acquisition of beta-lactamase-producing genes and alterations in penicillin-binding proteins (PBPs) have diminished ampicillin's efficacy when used alone or in combination.
- Similarly, E. faecalis has developed resistance against aminoglycosides, macrolides, tetracyclines, and quinolones through the production of aminoglycoside-modifying enzymes, rRNA methylase enzymes, formation of efflux pumps, and alterations in DNA gyrase and topoisomerase-IV, respectively.
Vancomycin-Resistant Enterococci (VRE)/Vancomycin-Resistant E. faecalis (VRE):
- Vancomycin, a reserved antibiotic for E. faecalis treatment, is the drug of choice for beta-lactam-resistant and aminoglycoside-resistant strains. However, global reports indicate emerging resistance to vancomycin by E. faecalis.
- The primary resistance mechanism in E. faecalis involves the acquisition of resistance genes, predominantly vanA or vanB genes. These genes are transferable between various bacterial strains and species, often carried on mobile genetic components like plasmids or transposons.
- The extensive use of vancomycin, particularly in healthcare settings, has exerted selective pressure, leading to the development of Vancomycin-Resistant Enterococci (VRE). Infections caused by VRE strains pose a significant threat, as treatment choices are limited, often requiring combination therapy or newer antibiotics such as linezolid and daptomycin, consequently increasing treatment costs.
Prevention of Enterococcus faecalis Infections:
Enterococcus faecalis, as an opportunistic pathogen primarily associated with healthcare-related infections, demands stringent preventive measures. The following procedures are essential to curtail the spread of E. faecalis:
- Implementation of proper hand hygiene practices.
- Adequate disinfection of equipment.
- Thorough environmental cleaning protocols.
- Isolation of individuals who are either infected or colonized by E. faecalis.
In healthcare settings, it is crucial to establish and adhere to strict infection control procedures, surveillance systems, and antibiotic stewardship programs. These comprehensive measures collectively contribute to the prevention and containment of Enterococcus faecalis infections.
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