Potassium tellurite medium produces what color colonies of Corynebacterium diphtheriae?

Potassium tellurite medium is a selective and differential medium used to isolate and identify Corynebacterium diphtheriae. This organism is known for its ability to reduce tellurite, leading to a distinct change in colony appearance on this medium. When C. diphtheriae grows in potassium tellurite medium, it reduces potassium tellurite to tellurium, which results in the formation of gray-black colonies. This color change is a crucial identifying characteristic of C. diphtheriae and is an important aspect of its identification in laboratory settings.

The gray-black coloration occurs because of the dark precipitate formed from reduced tellurium, which is distinguishable from other organisms that may grow on different media. Thus, when examining colony characteristics on potassium tellurite medium, the presence of gray-black colonies is a key indicator of the presence of Corynebacterium diphtheriae.