Page 116 - MNUmicrobiology practical 2025

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

• Directions: Deep stab inoculation, ⅔ of the way down the center of the tube.

After incubation, refrigerate for 1 hour before reading.

• Interpretation:

(+) = Liquefaction (after refrigeration)
(-) = Gels when refrigerated, no liquefaction.

9- Hydrogen sulfide (H2S) test
• Purpose: to detect the ability of bacteria to produce H2S gas

• Media: A wide variety of culture media are available for the detection of

hydrogen sulfide production. SIM (Sulfide Indole Motility) medium, KIA

(Kligler’s Iron Agar), TSI (Triple Sugar Iron) Agar Medium

• Mechanism/reactions: H2S-producing bacteria can metabolize the sulfur-

containing compounds (mostly sodium thiosulfate) present in the culture
media and reduce it releasing the H2S gas. The produced H2S gas then reacts

with the ferric ions forming water-insoluble black-colored ferrous sulfide.
This insoluble black-colored compound turns the culture media black

indicating a positive result for the H2S production test

• Interpretation:

(+) = Blackening of the culture media. (Whole

media may turn black or the junction of slant and

butt or any part of butt or slant may turn black.
A black patch at any inoculated area is
considered positive.)

(-)=No blackening of media in any area.

General Microbiology & Immunology (PM 401) Practical note Page 105