Introduction to Enterococcus faecalis
The genus *Enterococcus* represents a significant group of bacteria within clinical and environmental microbiology, having been reclassified from the Group D streptococci. Among the enterococci, *Enterococcus faecalis* is the most frequently isolated species, accounting for a large percentage of human enterococcal infections. It is a Gram-positive, non-spore-forming coccus that typically presents in pairs or short chains. While it is a common commensal organism of the gastrointestinal tracts of humans and animals, it is also a major opportunistic pathogen, particularly in hospital settings, where it is a leading cause of nosocomial infections such as endocarditis, urinary tract infections, and surgical wound infections. The ability of *E. faecalis* to cause widespread disease is compounded by its intrinsic resistance to various antibiotics and its capacity to acquire resistance to vancomycin, making its accurate and rapid identification via a comprehensive panel of biochemical tests critically important for effective clinical management and epidemiological tracking. The identification process relies on a combination of physiological characteristics that define the genus, followed by specific tests that differentiate *E. faecalis* from other *Enterococcus* species, such as *E. faecium*.
Initial Genus Identification: Key Physiological Characteristics
The first step in identifying *Enterococcus faecalis* involves confirming its membership in the *Enterococcus* genus. Enterococci are characterized by a set of robust physiological properties. They are typically facultative anaerobes but can also grow under aerobic conditions. They are highly resilient organisms, demonstrating the ability to grow in a wide temperature range, generally from 10°C to 45°C, and they can survive harsh environmental conditions, including resistance to 30 minutes at 60°C. Crucially, *E. faecalis* is non-motile, although some non-standard strains may exhibit motility, and it is a non-sporing, non-capsulated organism. Two of the most fundamental tests that distinguish *Enterococcus* from *Staphylococcus* and *Micrococcus* (other Gram-positive cocci) are the Catalase and Oxidase tests, both of which yield negative results for *E. faecalis*.
The Bile Esculin Hydrolysis Test
The Bile Esculin test is a cornerstone for the presumptive identification of the *Enterococcus* genus. The test utilizes Bile Esculin Agar (BEA), a medium that is both selective and differential. Its selective nature comes from the inclusion of bile salts, specifically oxgall, which inhibits the growth of most Gram-positive bacteria other than enterococci and some Group D streptococci. The differential component is esculin. If an organism possesses the enzyme esculinase (or bile-tolerant beta-glucosidase), it will hydrolyze the esculin into two products: glucose and esculetin. The esculetin then reacts with ferric citrate, an iron salt included in the medium, to produce a characteristic dark brown to black precipitate that diffuses into the agar. *Enterococcus faecalis* is positive for Bile Esculin Hydrolysis, and a positive result is essential for the initial categorization of the isolate as an enterococcus or a Group D streptococcus.
Growth in 6.5% Sodium Chloride (NaCl)
A further test required to differentiate enterococci from non-enterococcal Group D streptococci (like *Streptococcus bovis*) is the 6.5% NaCl tolerance test. *Enterococcus faecalis* is notably salt-tolerant and is capable of thriving in brain-heart infusion (BHI) broth containing 6.5% sodium chloride. This test is based on the organism’s inherent physiological resistance to high osmotic pressure. The ability to grow in the presence of 6.5% NaCl, combined with a positive Bile Esculin test, provides a definitive biochemical profile for classifying an isolate as belonging to the genus *Enterococcus*.
The Pyrrolidonyl Arylamidase (PYR) Test
The PYR test is considered one of the most reliable and rapid tests for the definitive identification of *Enterococcus* species, including *E. faecalis*, and for distinguishing them from *Streptococcus* species. This test detects the presence of the enzyme L-pyrrolidonyl-β-naphthylamide hydrolase (often simply referred to as PYRase). The substrate, L-pyrrolidonyl-β-naphthylamide (PYR), is hydrolyzed by the enzyme to L-pyrrolidone and β-naphthylamine. The addition of a color developer (such as cinnamaldehyde reagent) to the hydrolyzed product causes a bright red color to appear within minutes, indicating a positive result. *E. faecalis* is PYR-positive, making this a crucial, rapid test for its identification.
Differentiating E. faecalis through Species-Specific Fermentation Tests
Once an isolate is confirmed to be an *Enterococcus* (PYR+, Bile Esculin+, 6.5% NaCl+), a panel of carbohydrate fermentation tests is employed to distinguish between the most common species, particularly *E. faecalis* and *E. faecium*. The general carbohydrate profile for *E. faecalis* shows that it is a strong fermenter of many common sugars, producing acid but typically no gas. Key fermentation results for *E. faecalis* include:
– **Glucose:** Positive (+), as a baseline characteristic of most fermentative bacteria.
– **Lactose:** Positive (+), which is often visualized on differential media like MacConkey agar.
– **Sucrose:** Positive (+), a useful differentiator.
– **Maltose:** Positive (+).
– **Mannitol:** Positive (+), which is a key differentiator from species like *E. faecium* in older identification schemes, though variability exists in modern strains of *E. faecium*.
– **Sorbitol:** Positive (+), although some strains may show variability in this test.
Conversely, the organism is typically negative (-) for the fermentation of L-arabinose and D-raffinose. The fermentation profile—specifically the combination of positive results for mannitol and sorbitol alongside negative results for arabinose and raffinose—is highly characteristic of *Enterococcus faecalis* and is used as the basis for most clinical and commercial identification keys.
Other Diagnostic Biochemical Reactions
A few additional biochemical tests provide further context and confirmation, although some may exhibit variability based on the specific strain or methodology used:
– **Voges-Proskauer (VP) Test:** The result for *E. faecalis* is often positive (+), indicating the production of acetoin from glucose metabolism, though some sources report variable or inconclusive results.
– **Indole Test:** Negative (-), meaning it does not break down the amino acid tryptophan to produce indole.
– **Citrate Utilization:** Negative (-), indicating it cannot use citrate as a sole carbon source.
– **Urease Test:** Negative (-), meaning it does not possess the urease enzyme to hydrolyze urea.
– **Nitrate Reduction:** This test is highly variable in the literature, with some strains showing a positive result (reduction of nitrate to nitrite) and others negative. Due to its inconsistency, it is generally not relied upon as a primary identifier in routine clinical use for this species.
In conclusion, the gold standard for the conventional identification of *Enterococcus faecalis* involves a stepwise process that leverages its unique biochemical fingerprint. This starts with Gram-stain and morphology, is confirmed at the genus level by a negative Catalase, positive Bile Esculin, positive 6.5% NaCl tolerance, and positive PYR test, and is ultimately defined at the species level by its characteristic carbohydrate fermentation pattern, particularly the positive fermentation of mannitol and sorbitol combined with the negative fermentation of arabinose and raffinose. This panel of tests remains indispensable for differentiating this common, and often drug-resistant, human pathogen.