Enterococcus is a genus of bacteria found in the gastrointestinal tracts of humans and other animals, where they are a normal part of the gut microbiome. Under certain circumstances, however, they can become opportunistic pathogens capable of causing infections. Their resilience allows them to survive in a variety of environments, contributing to their presence in both community and hospital settings.
Microscopic Appearance
The individual cells are cocci, meaning they are spherical or slightly ovoid in shape, with diameters ranging from 0.6 to 2.5 micrometers. The arrangement of these cells is an identifying feature. Enterococci are most frequently observed in pairs, known as diplococci, or in short chains. This arrangement results from their pattern of cell division, which occurs along a single plane.
A fundamental technique for bacterial identification is the Gram stain. Enterococci are Gram-positive, retaining the crystal violet stain and appearing purple or blue-violet due to a thick peptidoglycan layer in their cell wall. These bacteria are non-motile, as they lack flagella, and they do not produce spores.
Colony Characteristics on Culture Media
Populations of enterococci become visible as colonies on an agar plate. These colonies are small, round, and appear creamy-white or grayish-white. A common medium is the Blood Agar Plate (BAP), which is enriched with sheep’s blood to reveal a bacterium’s ability to break down red blood cells, a process called hemolysis.
Most species of Enterococcus are non-hemolytic, causing no change to the agar; this is termed gamma-hemolysis. Some strains may exhibit alpha-hemolysis, a partial breakdown causing a greenish discoloration, or more rarely, beta-hemolysis, a complete clearing of the blood cells around the colony.
Differentiating from Similar Bacteria
Distinguishing Enterococcus from other morphologically similar bacteria is a common laboratory task. The most common point of confusion is with Streptococcus species, as both are Gram-positive cocci that can appear in pairs or chains. This visual overlap makes identification by microscopy alone unreliable, necessitating further biochemical testing.
To differentiate between these two genera, specific laboratory tests are employed. One definitive test assesses the organism’s ability to grow in a broth containing a 6.5% sodium chloride (NaCl) concentration, a condition that inhibits the growth of most streptococci.
Another widely used differential test is the Bile Esculin Agar (BEA). Enterococci are able to grow in the presence of bile and can hydrolyze esculin into esculetin, which reacts with ferric citrate in the medium to produce a dark brown or black precipitate. Additionally, the rapid pyrrolidonyl arylamidase (PYR) test can be used, where enterococci produce an enzyme that yields a positive, bright red color change.