A positive result for esculin hydrolysis on bile esculin agar appears
a a halo around the growth.
b a darkening around the growth.
c no growth.
d a yellowing of the media.
The correct answer and explanation is:
Correct Answer: b. a darkening around the growth.
Explanation:
Bile esculin agar is a selective and differential medium primarily used to identify organisms capable of hydrolyzing esculin in the presence of bile salts. It is especially useful in distinguishing group D streptococci and enterococci from other gram-positive cocci.
When esculin is hydrolyzed by certain bacteria, it produces esculetin and glucose. The esculetin then reacts with ferric citrate present in the medium to form a dark brown or black complex. This blackening of the medium, especially around and beneath the bacterial growth, indicates a positive result.
The presence of bile salts in the medium inhibits the growth of gram-positive organisms that are not group D streptococci or enterococci. Organisms that can tolerate bile and hydrolyze esculin will cause the black coloration. The medium remains unchanged or light brown if the organism either cannot grow in the presence of bile or cannot hydrolyze esculin, indicating a negative result.
This test is clinically important because it helps identify Enterococcus faecalis, Enterococcus faecium, and some group D streptococci like Streptococcus bovis, all of which can grow in bile and hydrolyze esculin. These bacteria are often associated with infections of the gastrointestinal tract, urinary tract, or heart valves.
Other options are incorrect for the following reasons:
- A halo around the growth is more typical in hemolysis tests on blood agar, not esculin hydrolysis.
- No growth would suggest the organism is inhibited by bile, not an indicator of esculin hydrolysis.
- A yellowing of the media indicates acid production in other tests, such as sugar fermentation, not esculin breakdown.
Thus, a darkening around the growth is the correct visual cue for a positive esculin hydrolysis result.