Page 114 - MNUmicrobiology practical 2025

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

• Mechanism/reaction: Carbohydrate fermentation

results in acid and sometimes gas production causing a

pH change and possibly gas being trapped in the

Durham tube.

• Directions: Inoculate tubes and incubate at 35ºC for 24

– 48 hours.

• Interpretation: Observe for color change and gas
production.

• (+)= color change from re to yellow, pH < 7.0

• (-)= no color change
• Gas production (+)= bubble trapped in the inverted Durham tube
• No gas production (-)= no bubble trapped in the inverted Durham tube

6- Nitrate reduction

• Purpose: to determine the ability of an organism to reduce nitrate to nitrite.

Anaerobic metabolism requires an electron acceptor other than

atmospheric oxygen. Many Gram-negative bacteria use nitrate

as the final electron acceptor. Therefore, Nitrate reduction may

be coupled to anaerobic respiration in some species.

• Media used: Nitrate broth medium

• Reagents: Reagent A ( 0.8% Sulfanilic acid )

Reagent B (0.5% N,N-Dimethyl-alpha-
naphthylamine)

• Mechanism/reaction: If the bacteria capable of producing the nitrate reductase

enzyme reduce the nitrate, present in the broth, to nitrite. This test is based on the

detection of nitrite and its ability to form a red compound when it reacts with

General Microbiology & Immunology (PM 401) Practical note Page 103