Question

Are there any tests that distinguish Enterococcus organisms from other Gram positive organisms? And what does such a test use that distinguishes them.

Answer 1

Bile-esculin test is widely used to differentiate Enterococci and other gram positive organisms. Enterococci are bile tolerant and can hydrolyze esculin to esculetin and it grows well, but non- enterococci grows poorly on bile. It is a low cost, rapid test with good sensitivity and specificity (>90%).

Principle :

Bile-esculin test is based on the ability of certain bacteria, notably the Enterococcus species, to hydrolyze esculin in the presence of bile. Bacteria that are bile-esculin positive are, first of all, able to grow in the presence of bile salts. Hydrolysis of the esculin in the medium results in the formation of glucose and a compound called esculetin.

Esculetin, in turn, reacts with ferric ions (supplied by the inorganic medium component ferric citrate) to form a black diffusible complex which indicates the postive result. i.e the presence of Enterococcus faecalis, Enterococcus faecium, and Streptococcus bovis.