Biochemical Testing in Microbiology

Introduction

Biochemical tests are essential tools in microbiology for identifying and differentiating microorganisms based on their metabolic properties. These tests often assess the ability of bacteria to utilize specific nutrients or to carry out particular redox reactions, providing critical information for classification and diagnosis.

Reduction Potential Tests

Oxidase Test

The oxidase test detects the presence of cytochrome c oxidase, an enzyme involved in the electron transport chain. This test is particularly useful for distinguishing between members of the Enterobacteriaceae (oxidase-negative) and Pseudomonadaceae (oxidase-positive) families.

• Principle: Cytochrome c oxidase catalyzes the transfer of electrons from cytochrome c to oxygen. The test uses a chromogenic reducing agent, TMPD (tetramethyl-p-phenylenediamine), which turns purple when oxidized.

• Procedure: Bacterial colonies are placed on paper saturated with TMPD. A color change to purple within 20 seconds indicates a positive result.

• Interpretation: A rapid color change signifies the presence of cytochrome c oxidase.

• Application: Differentiates Pseudomonas (positive) from Escherichia (negative).

Nitrate Reduction Test

Principle and Purpose

The nitrate reduction test determines the ability of bacteria to reduce nitrate (NO3-) to nitrite (NO2-) or other nitrogenous compounds via anaerobic respiration. This test is commonly used to differentiate Gram-negative bacteria.

• Pathways: Bacteria may reduce nitrate to nitrite, nitrogen gas (N2), or ammonia (NH4+).

• Detection: The presence of nitrite is detected by adding reagents A (sulfanilic acid) and B (α-naphthylamine), which react with nitrite to produce a red color.

• No Color Change: If no red color appears, nitrate may not have been reduced, or it may have been reduced beyond nitrite. Zinc powder is added to confirm: if red appears after zinc, nitrate was not reduced by the organism; if no color, nitrate was reduced to other products.

Test Interpretation

• Red after reagents A & B: Positive for nitrate reduction to nitrite.

• No color after reagents, red after zinc: Negative for nitrate reduction.

• No color after zinc: Positive for reduction beyond nitrite (e.g., to N2 or NH4+).

Citrate Utilization Test

Principle and Application

The citrate test determines the ability of an organism to use citrate as its sole carbon source. This is part of the IMViC series of tests for differentiating Enterobacteriaceae.

• Mechanism: Bacteria with citrate permease can transport and metabolize citrate, producing alkaline byproducts (ammonia) that raise the pH.

• Indicator: Bromophenol blue turns from green (pH < 6.9) to blue (pH > 7.6) as pH increases.

• Interpretation: Growth with blue color indicates positive citrate utilization.

Decarboxylase Tests

Principle and Application

Decarboxylase tests detect the ability of bacteria to decarboxylate specific amino acids, producing alkaline amines. These tests are useful for differentiating Enterobacteriaceae.

• Mechanism: Decarboxylation is induced in acidic, anaerobic conditions (mineral oil overlay). The reaction increases pH, detected by bromocresol purple (yellow at pH < 5.2, purple at pH > 6.8).

• Interpretation: Purple color indicates positive decarboxylation; yellow indicates negative.

Phenylalanine Deaminase Test

Principle and Application

This test identifies bacteria capable of deaminating phenylalanine to phenylpyruvic acid and ammonia, a trait found in genera such as Morganella, Proteus, and Providencia.

• Mechanism: Phenylalanine deaminase removes the amino group from phenylalanine, forming phenylpyruvic acid.

• Detection: Addition of ferric chloride (FeCl3) reacts with phenylpyruvic acid to produce a green color.

• Interpretation: Green color indicates a positive result.

Summary Table: Key Biochemical Tests

Additional Info

• These tests are foundational for microbial identification in clinical, environmental, and industrial microbiology.

• Proper controls (uninoculated tubes) are essential for accurate interpretation.

• Incubation times and temperatures must be strictly followed for reliable results.