Which organism is catalase-negative and capable of growth in bile-esculin and 6.5% NaCl?

Which organism is catalase-negative and capable of growth in bile-esculin and 6.5% NaCl?

• A. Enterococcus faecalis
• B. Staphylococcus aureus
• C. Streptococcus pyogenes
• D. Bacillus cereus

Answer: (A)

The main idea is to use a combination of metabolic tests that separate these Gram-positive cocci. Enterococcus species are catalase-negative, can grow in bile-esculin medium (and hydrolyze esculin in the presence of bile, turning the medium dark), and are able to grow in 6.5% NaCl. This specific combination—catalase-negative plus bile-esculin positive and 6.5% NaCl growth—is characteristic of Enterococcus faecalis, making it the best match. Staphylococcus aureus would be catalase-positive, so it would not fit the catalase-negative requirement. Streptococcus pyogenes is catalase-negative like Enterococcus but typically does not tolerate 6.5% NaCl, so it wouldn’t grow in that high-salt condition. Bacillus cereus is catalase-positive and not the typical bile-esculin–positive organism, so it wouldn’t match the profile either.

The main idea is to use a combination of metabolic tests that separate these Gram-positive cocci. Enterococcus species are catalase-negative, can grow in bile-esculin medium (and hydrolyze esculin in the presence of bile, turning the medium dark), and are able to grow in 6.5% NaCl. This specific combination—catalase-negative plus bile-esculin positive and 6.5% NaCl growth—is characteristic of Enterococcus faecalis, making it the best match.

Staphylococcus aureus would be catalase-positive, so it would not fit the catalase-negative requirement. Streptococcus pyogenes is catalase-negative like Enterococcus but typically does not tolerate 6.5% NaCl, so it wouldn’t grow in that high-salt condition. Bacillus cereus is catalase-positive and not the typical bile-esculin–positive organism, so it wouldn’t match the profile either.