Tellurite agar is used as a selective and differential medium in the isolation of Corynebacterium diphtheriae. A throat swab from a suspected diphtheria case grows black colonies on tellurite agar. Albert's stain of the organism shows metachromatic granules (volutin granules). Which staining pattern is characteristic?

• A Uniform blue-staining rods with polar flagella
• B Gram-negative coccobacilli with bipolar staining (safety-pin appearance)
• C Acid-fast beaded rods in chains
• D Blue-green rods with reddish-purple polar granules (Babes-Ernst bodies) ✓

Explanation

Albert's stain differentially colours the cytoplasm bluish-green and the metachromatic granules (volutin/polyphosphate granules, also called Babes-Ernst bodies) reddish-purple. The granules stain metachromatically because polyphosphate polymers interact differently with the basic dye. This pattern of blue-green rods with reddish-purple polar granules is characteristic of C. diphtheriae and used for rapid identification. Safety-pin appearance with bipolar staining is characteristic of Yersinia and Burkholderia pseudomallei.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.