A catalase-negative gram-positive coccus isolated from a urine sample forms a black pigment on bile-esculin agar and ferments glucose in 6.5% NaCl. Which organism is most likely?

A catalase-negative gram-positive coccus isolated from a urine sample forms a black pigment on bile-esculin agar and ferments glucose in 6.5% NaCl. Which organism is most likely?

• A. Group B streptococci
• B. Abiotrophia sp.
• C. Group D streptococci
• D. Enterococcus faecalis

Answer: (D)

Two traits drive the identification: bile esulin hydrolysis in the presence of bile and the ability to grow in high-salt conditions. The black pigment on bile-esculin agar shows the organism hydrolyzes esculin when bile is present, producing esculetin that reacts with ferric citrate to yield the dark color. Being catalase-negative places the organism among the streptococci/enterococci rather than staphylococci. The ability to ferment glucose in 6.5% NaCl demonstrates salt tolerance that is characteristic of Enterococcus species. Among Gram-positive cocci, only enterococci thrive in this high-salt environment, whereas non-enterococcal Group D streptococci generally do not. Taken together, bile esculin positivity with salt tolerance points to an Enterococcus species, most classically Enterococcus faecalis, a common urinary pathogen.

Two traits drive the identification: bile esulin hydrolysis in the presence of bile and the ability to grow in high-salt conditions. The black pigment on bile-esculin agar shows the organism hydrolyzes esculin when bile is present, producing esculetin that reacts with ferric citrate to yield the dark color. Being catalase-negative places the organism among the streptococci/enterococci rather than staphylococci.

The ability to ferment glucose in 6.5% NaCl demonstrates salt tolerance that is characteristic of Enterococcus species. Among Gram-positive cocci, only enterococci thrive in this high-salt environment, whereas non-enterococcal Group D streptococci generally do not. Taken together, bile esculin positivity with salt tolerance points to an Enterococcus species, most classically Enterococcus faecalis, a common urinary pathogen.