Sulphur reduction test principle procedure Microbiology

The Sulphur Reduction Test is a microbiological test used to detect the ability of certain bacteria to reduce sulfur-containing compounds to hydrogen sulfide (H₂S). This test is typically performed using media such as Triple Sugar Iron (TSI) agar or SIM (Sulfide, Indole, Motility) medium.

The SRT test is primarily used for:

1. Identification of Gram-negative enteric bacteria (e.g., Salmonella, Proteus, Citrobacter).
2. Differentiating between bacteria based on their ability to produce hydrogen sulfide gas (H₂S).

Principle

Some bacteria can reduce sulfur-containing compounds (like thiosulfate or cysteine) to hydrogen sulfide gas (H₂S) during anaerobic respiration or protein degradation. This gas reacts with iron salts (e.g., ferric ammonium sulfate) present in the medium, forming an insoluble black precipitate of ferrous sulfide (FeS).

Sulphur reduction test procedure (Tube method)

1. Inoculate the bacterial on culture media such as Triple sugar iron medium or SIM with a sterile inoculating loop.
2. Incubate the inoculated medium at 35-37°C for 18-24 hours.
3. Observe the media, If the bacterium produces insoluble black precipitate of ferrous sulfide, test will be positive.

Result Interpretation

• Positive result: Formation of a black precipitate indicates H₂S production, suggesting the presence of sulfur-reducing bacteria.
• Negative result: No black precipitate indicates the absence of H₂S production.

Test Control

• Positive control for sulfur reduction:
  • Salmonella spp.,
  • Proteus spp.,
  • Citrobacter freundii.
• Negative control for sulfur reduction:
  • Escherichia coli,
  • Shigella spp.,
  • Klebsiella spp..

Sulphur reduction test applications

• It is used to diagnose gastrointestinal infections caused by enteric bacteria.
• This test is used to differentiate the closely related bacterial species in clinical microbiology labs.