Question

Which of the following is a non-leucoform dye used in the rapid test utilizing starter cultures for the detection of antibiotic residues in milk?

• A. Methylene blue
• B. Resazurin
• C. Tetrazolium
• D. Nigrosine

Hint:

To distinguish dye types in microbial assays:
- Leucoform = Colored (Oxidized) $\rightarrow$ Colorless (Reduced) (e.g., Methylene Blue, Resazurin).
- Non-leucoform = Colorless (Oxidized) $\rightarrow$ Deeply Colored (Reduced) (e.g., Tetrazolium $\rightarrow$ Red Formazan).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Detection of antibiotic residues in milk often utilizes microbial inhibition tests.
In these assays, a sensitive starter culture (such as Streptococcus thermophilus or Bacillus stearothermophilus) is inoculated into milk containing a redox dye indicator.
If the milk is free of antibiotics, the bacteria grow normally, metabolize, and reduce the dye.
The classification of the dye as "leucoform" or "non-leucoform" depends on whether its reduced state is colorless or colored.
Detailed Explanation:
Redox dyes like methylene blue and resazurin are classic "leucoform" indicators.
When reduced by bacterial metabolic processes, they gain electrons and convert into colorless compounds (leuco-methylene blue and dihydroresorufin, respectively).
Thus, active bacterial growth turns these dyes from colored to colorless.
In contrast, Tetrazolium salts (such as triphenyltetrazolium chloride, or TTC) are colorless or pale yellow in their oxidized state.
During active bacterial respiration, microbial dehydrogenase enzymes reduce the tetrazolium salt by transferring electrons to it.
This reduction breaks the tetrazole ring, forming an intensely colored, red, water-insoluble compound called triphenylformazan.
Because the reduced form of tetrazolium is highly colored (red) rather than colorless, it is classified as a non-leucoform dye.
If antibiotic residues are present in the milk, the starter culture is inhibited, no reduction occurs, and the milk remains its original color (white/pale).
Nigrosine is an acidic negative stain used primarily for background staining in microscopy and is not a metabolic redox indicator.

Step 2: Final Answer:
Tetrazolium is the non-leucoform dye used in rapid microbial inhibition tests for antibiotic residues.