An isolate grows as black-centered colonies with a green metallic sheen on EMB agar. This appearance is characteristic of:

• A Salmonella typhi
• B Shigella flexneri
• C Proteus mirabilis
• D Escherichia coli ✓

Explanation

Eosin methylene blue agar is selective for gram-negative bacilli because the dyes inhibit gram-positive growth. Vigorous lactose fermenters such as E. coli produce large amounts of acid that precipitate the dyes, giving colonies with a green metallic sheen. Salmonella and Shigella are non-lactose fermenters and form colorless or pale colonies; Proteus forms colorless colonies with swarming suppressed on EMB.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.