BackPhenol Red Broth: Differential Medium for Lactose Fermentation
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Phenol Red Broth: Differential Medium
Overview of Differential Medium
Phenol Red Broth is a commonly used differential medium in microbiology to test for the ability of microorganisms to ferment specific carbohydrates, such as lactose. This medium allows for the observation of metabolic properties by indicating changes in color and gas production.
Differential medium: A type of growth medium that distinguishes between different groups of microorganisms based on their biological characteristics.
Application: Used to identify and differentiate bacteria based on their ability to ferment carbohydrates.
Metabolic Property Tested
The main metabolic property assessed by Phenol Red Broth is lactose fermentation.
Lactose fermentation: The process by which bacteria metabolize lactose to produce acid and sometimes gas.
Significance: Differentiates between lactose-fermenting and non-fermenting bacteria.
Substrates in the Medium
The medium contains specific substrates to test for the metabolic property:
Lactose: The carbohydrate source for fermentation.
Peptone: Provides nitrogen and other nutrients; can be degraded by bacteria, leading to alkaline byproducts.
Indicators Used in the Test
Indicators are essential for visualizing metabolic changes:
Phenol Red: pH indicator that detects acid production from fermentation.
Durham Tube: Small inverted tube placed inside the broth to capture gas (CO2) produced during fermentation.
Indicator Changes and Interpretation
The results are interpreted based on color changes and gas production:
Phenol Red:
Turns yellow if acid is produced (positive for lactose fermentation).
Turns pink if peptone degradation occurs (alkaline byproducts).
Durham Tube:
Presence of an air bubble indicates CO2 production (gas from fermentation).
Appearance of Positive and Negative Results
Result Type | Tube Color | Durham Tube |
|---|---|---|
Positive (Lactose Fermentation) | Yellow | Air bubble present (CO2 produced) |
Positive (Peptone Degradation) | Pink | No air bubble |
Negative | No color change | No air bubble |
Requirement for Additional Reagents
No additional reagents are required for this test; all necessary indicators are included in the medium.
Summary Table: Phenol Red Broth Test
Component | Purpose |
|---|---|
Lactose | Fermentation substrate |
Peptone | Alternative nutrient source |
Phenol Red | pH indicator |
Durham Tube | Detects gas production |
Key Equations
Fermentation of lactose by bacteria can be represented as:
Example Application
Example: Escherichia coli ferments lactose, producing acid (yellow color) and gas (bubble in Durham tube).
Non-fermenter: Pseudomonas aeruginosa does not ferment lactose; tube remains red with no bubble.
Additional info: The Phenol Red Broth test is widely used in clinical microbiology to differentiate members of the family Enterobacteriaceae and other Gram-negative rods based on their carbohydrate fermentation profiles.
