What is the biochemical mechanism behind the formation of black-centered Salmonella colonies on XLD (Xylose Lysine Deoxycholate Agar) agar?
- Sulfer reduction-> Iron reaction-> Black precipitation
- Iron reaction -> Sulfer reduction -> Black precipitation
- Sulfate accumulation -> Sulfate consumption -> Black precipitation
- Sucrose fermentation -> pH changing -> Black precipitation
Answer. A Sulfer reduction-> Iron reaction-> Black precipitation
Reason: Salmonella breaks down the sodium thiosulfate in the agar, releasing colorless hydrogen sulfide (H_2S) gas. The H_2S gas reacts directly with the iron indicator (ferric ammonium citrate) embedded in the medium.This reaction creates ferrous sulfide, a jet-black insoluble solid that traps itself in the center of the colony.