Stool culture of a patient with bacillary dysentery grows a gram-negative rod that is a non-lactose fermenter on MacConkey agar. Which combination of biochemical features reliably separates this genus from Salmonella?

• A Shigella ferments glucose with gas production and citrate positive
• B Shigella is oxidase positive and produces indole
• C Shigella is non-motile and does not produce H2S ✓
• D Shigella produces urease and grows on MacConkey agar

Explanation

All Shigella species are non-motile because they lack flagellar (H) antigens, and they do not produce hydrogen sulfide on TSI or Wilson and Blair media. Salmonella is motile with peritrichous flagella and typically produces abundant H2S. Both are glucose fermenters, oxidase negative, urease negative, and non-lactose fermenters except late lactose fermentation by S. sonnei. The absence of motility and H2S is the standard laboratory discrimination taught in diagnostic bacteriology.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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