TSI slant

TSI Slant

A Triple Sugar Iron (TSI) slant is a differential medium used in microbiology to identify gram-negative enteric bacteria based on their ability to ferment carbohydrates and produce hydrogen sulfide. It is a crucial tool in the identification and differentiation of members of the family Enterobacteriaceae.

Composition

The TSI slant contains three sugars: glucose, lactose, and sucrose, as well as peptone, sodium thiosulfate, and ferrous sulfate. The medium is prepared as a slant in a test tube, providing both an aerobic environment on the slant and an anaerobic environment in the butt.

• Glucose: Present at a concentration of 0.1%.
• Lactose and Sucrose: Each present at a concentration of 1%.
• Peptone: Serves as a source of nitrogen and amino acids.
• Sodium Thiosulfate: A source of sulfur for hydrogen sulfide production.
• Ferrous Sulfate: Reacts with hydrogen sulfide to form a black precipitate.
• Phenol Red: A pH indicator that turns yellow in acidic conditions and red in alkaline conditions.

Procedure

1. Inoculation: The TSI slant is inoculated by stabbing the butt with a needle and then streaking the slant surface. 2. Incubation: The tube is incubated at 35-37°C for 18-24 hours. 3. Observation: After incubation, the tube is observed for color changes and gas production.

Interpretation

The results of a TSI slant test are interpreted based on the color changes in the slant and butt, as well as the presence of gas or black precipitate:

• Yellow Slant/Yellow Butt (A/A): Indicates fermentation of glucose, lactose, and/or sucrose.
• Red Slant/Yellow Butt (K/A): Indicates fermentation of glucose only.
• Red Slant/Red Butt (K/K): No fermentation of any sugars; peptone catabolized.
• Black Precipitate: Indicates hydrogen sulfide production.
• Gas Production: Cracks or bubbles in the agar indicate gas production.

Applications

TSI slants are primarily used to differentiate members of the Enterobacteriaceae family, such as:

• Escherichia coli: Typically shows A/A with gas production.
• Salmonella spp.: Typically shows K/A with hydrogen sulfide production.
• Shigella spp.: Typically shows K/A without hydrogen sulfide production.

Limitations

While TSI slants are useful for differentiating enteric bacteria, they have limitations:

• Non-specific: Some non-enteric bacteria may produce similar results.
• Limited to Enterobacteriaceae: Not suitable for identifying non-enteric bacteria.

Also see

• MacConkey Agar
• Eosin Methylene Blue Agar
• Bacterial Fermentation
• Hydrogen Sulfide Production Test