Biochemical Tests in Microbiology – Types & Examples

29 August, 2025

Biochemical Tests in Microbiology –Types & Examples

Introduction

Biochemical tests are laboratory procedures used to identify bacteria based on their enzymatic activities and metabolic characteristics. They are crucial in clinical microbiology for differentiating organisms, confirming diagnosis, and guiding treatment.

Definition

A biochemical test is a laboratory diagnostic method that detects the presence or absence of specific enzymes, metabolites, or biochemical pathways in microorganisms.

Importance of Biochemical Tests

1. Identification of bacteria at genus and species level
2. Differentiation of closely related organisms
3. Confirmatory tests after Gram stain & culture
4. Selection of appropriate antibiotics
5. Rapid diagnosis of infectious diseases

Common Biochemical Tests in Microbiology

1. Carbohydrate Fermentation Tests

Detect ability to ferment sugars (glucose, lactose, sucrose, mannitol).
Indicator: Phenol red (acid = yellow, gas = Durham tube bubble).
Example: E. coli ferments lactose, Salmonella does not.

2. Catalase Test

Detects catalase enzyme that breaks down H₂O₂ into water + oxygen.
Positive: Bubbling (e.g., Staphylococcus).
Negative: No bubbling (e.g., Streptococcus).

3. Oxidase Test

Detects cytochrome c oxidase enzyme.

Positive: Purple color (e.g., Pseudomonas).

Negative: No color change (e.g., E. coli)

4. Coagulase Test

Detects coagulase enzyme that clots plasma.
Positive: Staphylococcus aureus.
Negative: Staphylococcus epidermidis.

5.Urease Test

Detects urease enzyme → breaks urea into ammonia + CO₂.
Positive: Pink (alkaline, e.g., Proteus spp.).
Negative: No color change.

6. Indole Test

Detects tryptophanase enzyme (produces indole).
Positive: Red ring after Kovac’s reagent (E. coli).
Negative: No color change (Klebsiella).

7. Methyl Red (MR) Test

Detects stable acid production from glucose fermentation.
Positive: Red (e.g., E. coli).
Negative: Yellow (Enterobacter).

8. Voges-Proskauer (VP) Test

Detects acetoin production.
Positive: Red (e.g., Enterobacter).
Negative: No color (E. coli).

9. Citrate Utilization Test

Detects ability to use citrate as sole carbon source.
Positive: Blue color (Klebsiella).
Negative: Green (E. coli).

10.Triple Sugar Iron (TSI) Agar Test

Differentiates bacteria based on sugar fermentation (glucose, lactose, sucrose) & H₂S production.
Example: Salmonella → red slant/yellow butt + H₂S.
E. coli → yellow slant/yellow butt.

Other Special Tests

Nitrate reduction test
Gelatin hydrolysis test
Lysine decarboxylase test
Phenylalanine deaminase test

Biochemical Tests and Organisms examples

Test	Positive Organism	Negative Organism
Catalase	Staphylococcus	Streptococcus
Oxidase	Pseudomonas	E. coli
Coagulase	Staph. aureus	Staph. epidermidis
Urease	Proteus	E. coli
Indole	E. coli	Klebsiella
MR	E. coli	Enterobacter
VP	Enterobacter	E. coli
Citrate	Klebsiella	E. coli

Applications in Medicine

1.Rapid bacterial identification in clinical labs
2.Differentiation of Enterobacteriaceae (IMViC tests)
3.Detection of pathogens (Salmonella, Shigella, Vibrio)
4.Guidance for antibiotic therapy

Conclusion

Biochemical tests are cornerstones of clinical microbiology, helping identify bacteria by their metabolic properties. For medical students, mastering these tests is essential for both theory and practical exams, as well as clinical practice.

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