Sugar Fermentation Test

18 September, 2025

Introduction

The sugar fermentation test is a fundamental biochemical test in microbiology used to determine whether bacteria can ferment a given carbohydrate (sugar) with the production of acid, gas, or both. This test is commonly performed with sugars like glucose, lactose, sucrose, mannitol, maltose, etc., and helps in the identification and differentiation of bacteria.

Principle

Bacteria are inoculated into a broth medium containing:
- A specific sugar (e.g., glucose, lactose).
- Peptone (as nitrogen source).
- Phenol red (pH indicator).
- Durham’s tube (inverted tube to detect gas production).
If the bacteria ferment the sugar → acid is produced → pH drops → medium changes from red to yellow.
If gas is produced → bubbles appear in Durham’s tube.
If bacteria cannot ferment the sugar → medium remains red or turns deep pink (alkaline due to protein utilization).

Procedure

Prepare sugar fermentation broth (peptone water + sugar + phenol red indicator) and place a Durham’s tube inverted inside.
Inoculate the broth with a pure bacterial culture.
Incubate at 37°C for 24–48 hours.
Observe for color change and gas formation.

Interpretation

Result	Observation	Interpretation
Acid production	Medium turns yellow	Sugar fermented, acid produced
Acid + Gas production	Yellow medium + bubble in Durham tube	Sugar fermented with gas production
No fermentation	Medium remains red	Sugar not fermented
Alkaline reaction	Medium turns deep pink	Sugar not used, peptone utilized → ammonia produced

Examples of Bacterial Reactions

Glucose fermentation: E. coli, Klebsiella, Enterobacter.
Lactose fermentation: E. coli, Klebsiella (positive); Salmonella, Shigella (negative).
Sucrose fermentation: Vibrio cholerae (positive), Vibrio parahaemolyticus (negative).
Mannitol fermentation: Staphylococcus aureus (positive), Staphylococcus epidermidis (negative).

Uses of Sugar Fermentation Test

Differentiation of Enterobacteriaceae members (e.g., E. coli vs Salmonella).
Identification of non-fermenters (e.g., Pseudomonas).
Useful in differentiating Gram-negative bacilli in diagnostic microbiology.
Helps in Vibrio identification (e.g., sucrose fermentation on TCBS agar).
Differentiates Staphylococcus aureus (mannitol fermenter) from other staphylococci.

Conclusion

The sugar fermentation test is one of the most important biochemical tests in microbiology. By detecting acid and gas production, it provides valuable information for bacterial identification and differentiation, especially among Enterobacteriaceae, Vibrio, and Staphylococcus species.

Category: Uncategorized