Triple sugar iron (TSI) agar slants are inoculated with two unknown gram-negative rods isolated from separate stool cultures. Organism X shows an alkaline slant, acid butt, gas bubbles and blackening of the butt. Organism Y shows an acid slant, acid butt, no gas and no blackening. Organism Y is most consistent with:
- A Shigella species ✓
- B Salmonella species other than S. Typhi
- C Vibrio cholerae
- D Klebsiella pneumoniae
Correct answer: A. Shigella species
Explanation
Shigella ferments glucose without gas production, does not produce hydrogen sulfide, and typically attacks lactose late or weakly enough that the slant remains acidic on routine reading, giving acid over acid with no gas and no H2S. The pattern in option B, alkaline slant with acid butt, gas and H2S, is the classic Salmonella reaction and is the intended distractor. Vibrio cholerae oxidase positive and Klebsiella lactose fermenting with copious gas do not fit this profile.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.