Lab 13 -1: Study guide - part 1 -Biochemical Reactions in Microbial Identification

Introduction

Biochemical tests are essential tools in microbiology for distinguishing bacterial species based on their metabolic activities. These tests exploit differences in enzyme production and substrate utilization, providing a biochemical fingerprint for microbial identification. Specialized media and indicators are used to detect specific reactions, aiding in the classification and identification of unknown organisms.

Carbohydrate Fermentation Reactions

Biological Basis

Carbohydrate fermentation tests assess a microbe's ability to metabolize specific sugars for energy. Bacteria may utilize carbohydrates via aerobic respiration or fermentation, depending on their oxygen requirements and genetic capabilities.

• Strict aerobes oxidize sugars to CO2 and H2O using oxygen as the final electron acceptor.

• Facultative anaerobes and strict anaerobes perform fermentation, using organic intermediates as electron acceptors.

• Fermentation produces organic acids, alcohols, aldehydes, and gases (e.g., CO2), yielding less energy than aerobic respiration.

• The ability to ferment specific carbohydrates and the nature of end products are genetically determined.

Phenol Red Carbohydrate Broth

This test medium contains a single sugar, non-fermentable nitrogen sources, phenol red pH indicator, and a Durham tube to collect gas.

• Phenol red color changes: Yellow (< pH 6.8) = acid production; Red (pH 6.8–7.4) = no reaction; Pink (> pH 7.4) = alkaline production.

• Durham tube collects gas (CO2).

• Growth is indicated by turbidity.

• Positive fermentation: phenol red turns yellow due to acid production.

• Negative fermentation: intense pink color from protein degradation and ammonia production.

• Gas production: bubble in Durham tube.

Example: Escherichia coli ferments glucose, producing acid and gas (yellow color, bubble in Durham tube).

Interpretation and Applications

• Patterns of fermentation across sugars (e.g., glucose, lactose, sucrose) help identify bacterial species.

• Acid production lowers pH, turning phenol red yellow; gas production is indicated by a bubble.

• Alkaline reaction (pink) indicates protein utilization rather than sugar fermentation.

Gelatin Hydrolysis

Biological Basis

Gelatin hydrolysis tests for the presence of gelatinase, an extracellular proteolytic enzyme that degrades gelatin into peptides and amino acids. This activity is associated with protein digestion and can be linked to tissue invasion in pathogenic bacteria.

• Gelatin is a protein derived from collagen.

• Bacteria secrete enzymes (e.g., amylases, lipases, gelatinases) to break down macromolecules for nutrient uptake.

• Positive result: liquefaction of the gelatin medium.

• Negative result: medium remains solid.

Test Procedure

• Inoculate nutrient gelatin deep with an inoculating needle.

• Incubate at room temperature to prevent melting of gelatin.

• Inspect for liquefaction by tilting the tube; results may take up to one week.

Example: Bacillus species are commonly positive for gelatin hydrolysis; Escherichia coli and Micrococcus luteus are often negative.

Interpretation and Applications

• Gelatinase activity distinguishes species and may indicate pathogenic potential.

• Incubation temperature is kept below 37°C to avoid melting gelatin; chilling can help confirm true hydrolysis.

Nitrate Reduction Test

Biological Basis

The nitrate reduction test identifies bacteria capable of using nitrate (NO3−) as a terminal electron acceptor in anaerobic respiration. Reduction may yield nitrite (NO2−), nitrogen gas (N2), or other products.

• Nitrate-reducing organisms possess nitrate reductase and may further reduce nitrite to nitrogenous gases.

• Test medium contains nitrate broth and a Durham tube for gas collection.

Test Procedure and Interpretation

• Inoculate nitrate broth and incubate at 37°C.

• Observe for gas in Durham vial (indicates reduction to N2).

• If no gas, add sulfanilic acid and dimethyl-naphthylamine to test for nitrite.

• Red color after reagents: positive for nitrate reduction to nitrite.

• No color: add zinc powder.

• Red after zinc: negative (zinc reduces remaining nitrate to nitrite).

• No color after zinc: positive (nitrate reduced beyond nitrite).

Example: Escherichia coli may reduce nitrate to nitrite; Pseudomonas aeruginosa may reduce nitrate to nitrogen gas.

Applications

• Distinguishes bacteria based on anaerobic respiration pathways.

• Helps identify species with similar morphology but different metabolic capabilities.

Summary Table: Biochemical Tests and Their Interpretations

Key Equations

• Fermentation:

• Nitrate Reduction:

Conclusion

Biochemical tests such as carbohydrate fermentation, gelatin hydrolysis, and nitrate reduction are fundamental for microbial identification. They reveal metabolic capabilities, enzyme production, and respiration pathways, providing critical information for distinguishing bacterial species in clinical and environmental microbiology.