Potassium tellurite medium produces what color colonies of C. diphtheriae?

Potassium tellurite medium is specifically designed for the isolation of Corynebacterium diphtheriae, the bacterium that causes diphtheria. When C. diphtheriae grows on this medium, it reduces the tellurite present in the medium, resulting in the formation of gray-black colonies. This contrasting color is a key characteristic used in laboratory identification of the organism. The formation of gray-black colonies is due to the production of tellurium granules, which indicate the presence of C. diphtheriae and are an important diagnostic feature.

The other colors mentioned in the choices do not correspond with the growth characteristics of C. diphtheriae in potassium tellurite medium. Yellow, white, or red colonies would typically suggest the presence of other microbial species or suggest that the medium is not selective for C. diphtheriae. Thus, recognizing the distinct gray-black coloration is vital for microbiological diagnostics involving this pathogen.