Presence of Red Colonies with Black Centers on Xylose-Lysine-Decarboxylase Medium

Question

Presence of red colonies with black centers on xylose-lysine-decarboxylase medium is indicative of
A) lactose fermentation and utilization of citrate.
B) lactose fermentation and H2S positive.
C) nonlactose fermentation and tryptophan positive.
D) nonlactose fermenter and H2S positive.

Accepted Answer

The presence of red colonies with black centers on xylose-lysine-decarboxylase medium is indicative of nonlactose fermentation and H₂S positive. This is because the medium contains lysine and xylose, which are both substrates that can be used by nonlactose fermenters, and the black center is produced by the breakdown of lysine via the enzyme lysine decarboxylase, which is encoded by the H₂S gene.