In which test is Gram's iodine used as the reagent to detect starch?

• A. Nitrate Reduction Test
• B. Starch Hydrolysis Test
• C. Motility Test
• D. Urease Test

Answer: (B)

The test that uses Gram's iodine to detect starch relies on the starch-iodine reaction. After inoculating a starch-containing medium and incubating, flooding the plate with Gram’s iodine causes a dark blue color wherever starch remains. If the organism produces amylase and hydrolyzes starch, the area around the colony will appear as a clear halo because the starch has been removed and cannot form the blue complex with iodine. This clear zone is evidence of starch hydrolysis.

Nitrate reduction, motility, and urease tests use different reagents and indicators to detect nitrite production, diffusion patterns in semi-solid agar, or pH changes from urea breakdown, respectively, and do not involve starch or the starch-iodine reaction.