Enterococcus faecalis is a bacterium that commonly resides in the human gastrointestinal tract, where it functions as a normal member of the intestinal flora. Its presence is usually benign. Under certain conditions, however, this bacterium can become an opportunistic pathogen. When the body’s defenses are compromised or it spreads to other areas, it can cause urinary tract infections, bloodstream infections, and wound infections.
Cellular Morphology and Gram Stain
A single cell of Enterococcus faecalis is a coccus, which means it has a spherical or ovoid shape. These cells are quite small, typically measuring between 0.5 to 2.5 micrometers. Their slightly elongated, oval appearance becomes more apparent under high-power magnification. This bacterium is also non-motile, meaning it does not have the structures, such as flagella, that would allow it to move on its own.
When subjected to the Gram stain procedure, E. faecalis is classified as Gram-positive. Its thick peptidoglycan cell wall retains the crystal violet dye from the staining process, causing the cells to appear purple or blue-violet under a microscope.
The Arrangement of Enterococcus Faecalis
Observing Enterococcus faecalis microscopically reveals that individual cocci are not usually found in isolation. Instead, they are characteristically arranged in pairs, known as diplococci, or in short chains. This grouping is a direct result of cell division occurring along a single plane, where daughter cells often remain connected after division is complete.
This pattern of arrangement helps differentiate it from other Gram-positive cocci. For instance, bacteria in the genus Staphylococcus divide along multiple random planes, resulting in their characteristic grape-like clusters. In contrast, Streptococcus also divides along a single plane but forms much longer chains than those seen with Enterococcus.
Microscopic Identification in the Laboratory
In a clinical laboratory, identifying bacteria from a patient sample often begins with a Gram stain and microscopic examination. Observing Gram-positive cocci arranged in pairs and short chains provides a strong presumptive identification of an Enterococcus species. This visual evidence is a first step that guides the subsequent testing strategy.
This microscopic view, while informative, is not sufficient for a definitive identification of Enterococcus faecalis. Other related bacteria can have a similar appearance under the microscope. Therefore, microbiologists must perform additional tests to confirm the identity of the organism.
To specifically identify E. faecalis, laboratories use a series of biochemical tests. These tests assess the bacterium’s ability to grow in specific conditions, such as high salt concentrations, and its capacity to metabolize certain compounds. These metabolic fingerprints, combined with the initial microscopic observations, allow for a conclusive identification.