Page 120 - MNUmicrobiology practical 2025

General Microbiology & Immunology (PM 401) second level Semester 4 2024/2025

• Reagent/indicators: - Barritt’s reagents A (alpha-napthol)

• Barritt's B (potassium hydroxide).

• Mechanism/reactions:

Glucose + diacetyl + KOHKOH + O2O2 + arginine → pink color (Acetoin is oxidized to

diacetyl in the presence of KOH)

• Directions: Inoculate broth and incubate 48

h. After incubation add 20 drops of Barritt’s

Reagent A (napthol) and 20 drops Barritt’s

Reagent B (KOH). Shake well at frequent

intervals and allow reaction to develop up

to 1 – 2 hours if necessary.

• Interpretation:
(+) = red layer at top in 10 minutes.
(Earliest detection), progressing
downward.
(-) = no red color

Note

• E. coli does not produce acetyl methyl carbinol, but Enterobacter and Klebsiella do.

Methyl Red-Voges Proskauer Tests

• E. coli is (MR+/VP-)

Enterobacter aerogenes is (MR-/VP+)

D-Citrate Test

• Purpose: To determine an organism’s ability to use citrate as the sole source of carbon.

Part of the IMViC tests.

General Microbiology & Immunology (PM 401) Practical note Page 109