In OF medium, what modification helps reveal oxidative metabolism by reducing interference from peptone-derived alkaline products?

• A. Increase carbohydrate to 2.0%
• B. Increase peptone to 1.0%
• C. Decrease peptone to 0.2%
• D. Decrease carbohydrate to 0.5%

Answer: (C)

In OF medium, you’re looking to distinguish whether a organism oxidizes carbohydrate using oxygen or ferments it, by watching where acid is produced. The pH indicator shifts color as acid is formed, so you want a clear acid signal. A problem comes from peptone in the medium: when the organism metabolizes peptone, it can generate alkaline byproducts (like ammonia) that raise the pH and can mask the acid color, especially in the open tube where oxygen is present and oxidative metabolism would show color change.

Lowering the peptone concentration reduces these alkaline byproducts, so any acid produced from carbohydrate oxidation stands out more clearly. That’s why decreasing peptone to a small amount (0.2%) helps reveal oxidative metabolism—the open tube will show the expected acidic change without being obscured by peptone-derived alkalinity. The other adjustments either raise alkalinity (more peptone) or diminish acid production (less carbohydrate), which wouldn’t aid in clearly detecting oxidative metabolism.