SAARS: Enterococcus spp. (microbiology)

The enterococci are facultative anaerobes which produce a small gray colony after 24 hour incubation at 35°C on sheep blood agar. Unlike the staphylococci (but like the streptococci), enterococcal colonies grown under anaerobic conditions are larger than those grown aerobically. Also, the enterococci are catalase-negative or (more commonly) weak positive, while the staphylococci are strongly catalase-positive (a small gray colony that is slightly alpha-hemolytic and weakly catalase-positive is a typical presentation for Enterococcus; streptococci do not demonstrate even weak catalase-positivity). Microscopically, Gram-positive cocci occurring in chains or pairs with individual cells being somewhat elongated can be presumed to be streptococci or enterococci.

The enterococci do bear the Lancefield Group D antigen, but Enterococcus is a genus separate from Streptococcus. Like Group D streptococci, Enterococcus is able to grow in the presence of 40% bile and to hydrolyze esculin, while other streptococci are not; unlike Group D streptococci, Enterococcus produces a positive PYR test (red color produced after addition of N,N methyl aminocynnamaldehyde reagent after exposure to L-pyrrolidonyl-beta-naphthylamide (PYR) substrate), and can be identified by these tests. The streptococci are most commonly encountered in respiratory specimens (although they also occur in wound infections and in the GI tract), while the enterococci are GI commensals which most often occur elsewhere (at least initially) as opportunists; e.g., in a mixed wound infection or as the causative agent of a UTI. Clinical microbiologists tend to think of the enterococci as enteric organisms which happen to be Gram-positive cocci rather than Gram-negative rods.

Three hemolytic patterns are commonly reported:

• Alpha-hemolytic enterococci cause an incomplete destruction of the red cells within the sheep blood agar, producing a typical dark green discoloration of the media surrounding or (more commonly) directly underneath the colony, reflecting the presence of biliverdin and other heme compounds. Very many clinical isolates of Enterococcus species, including E. faecium, are alpha-hemolytic.

• Beta-hemolytic enterococci are rarely encountered in the clinical microbiology laboratory. They completely destroy the red cells in the sheep blood agar, resulting in transparency of the agar.

• Non-hemolytic enteroococci including E. faecalis are often encountered in the clinical microbiology laboratory.

Approximately 90% of enterococcal infections are caused by this species, some small percent of which produce beta-lactamase.

Approximately 10% of enterococcal infections are caused by this species, but most highly-resistant strains including those resistant to vancomycin are E. faecium.

These include such uncommon clinical isolates as E. durans, E. gallinarum, and E. casseliflavus.