Bacterial Biochemical Identification

Introduction

Biochemical tests are essential tools in microbiology for identifying and differentiating bacterial species, especially among Gram-negative rods such as the Enterobacteriaceae. These tests exploit differences in metabolic pathways, enzyme production, and fermentation abilities to distinguish between closely related organisms. Below are the major biochemical tests, their principles, procedures, and clinical relevance.

MR-VP Tests (Methyl Red and Voges–Proskauer)

Principle and Purpose

The Methyl Red (MR) and Voges–Proskauer (VP) tests are used together to differentiate bacteria based on their glucose fermentation pathways. The MR test detects strong acid production (mixed acid fermentation), while the VP test detects neutral end-products (acetoin, 2,3-butanediol) from glucose fermentation.

• MR Test: Detects stable acid end-products. Positive result = red color (pH < 4.4); negative = yellow/orange (pH > 6).

• VP Test: Detects acetoin. Positive result = red color after adding Barritt’s reagents; negative = no color change or copper tone.

• Key organisms: Escherichia coli (MR+), Proteus vulgaris (MR+), Enterobacter and Klebsiella (VP+).

Procedure

1. Inoculate MR-VP broth with the test organism and incubate at 35–37°C for 48 hours.

2. Split the culture into two tubes.

3. For MR: Add methyl red indicator and observe color change.

4. For VP: Add Barritt’s reagents A and B, wait 15–30 minutes, and observe color change.

Interpretation Table

Indole Test

Principle and Purpose

The Indole test detects the ability of bacteria to degrade tryptophan to indole using the enzyme tryptophanase. Indole reacts with Kovac’s reagent to produce a red or pink color at the top of the medium.

• Positive: Red/pink layer (e.g., E. coli, Proteus vulgaris).

• Negative: No color change/yellow (e.g., Klebsiella, Enterobacter).

Procedure

1. Inoculate tryptone broth with the test organism and incubate at 35–37°C for 24–48 hours.

2. Add a few drops of Kovac’s reagent.

3. Observe for a red/pink layer (positive) or no color change (negative).

Oxidase Test

Principle and Purpose

The Oxidase test identifies bacteria that produce cytochrome c oxidase, an enzyme of the electron transport chain. The reagent (TMPD) turns purple/blue when oxidized by cytochrome oxidase.

• Positive: Dark purple/blue within 10–30 seconds (Pseudomonas aeruginosa, Neisseria spp.).

• Negative: No color change or delayed color change (>60 sec) (all Enterobacteriaceae).

Procedure

1. Grow bacteria on non-selective medium.

2. Transfer a colony to filter paper or oxidase test strip.

3. Add oxidase reagent and observe color change within 10–30 seconds.

Nitrate Reduction Test

Principle and Purpose

The Nitrate Reduction Test determines if bacteria can reduce nitrate (NO3−) to nitrite (NO2−) or further to nitrogen gas (N2), nitrous oxide (N2O), or ammonia (NH3). It is important for differentiating Gram-negative bacteria, especially Enterobacteriaceae.

• Positive: Red after reagents A & B (nitrate → nitrite) or no color after zinc (nitrate reduced beyond nitrite).

• Negative: Red only after zinc (nitrate not reduced).

Procedure

1. Inoculate nitrate broth and incubate at 35–37°C for 24–48 hours.

2. Add sulfanilic acid (A) and α-naphthylamine (B): red = positive.

3. If no color, add zinc powder: red = negative; no color = positive (reduced beyond nitrite).

Motility Test

Principle and Purpose

The Motility test determines whether bacteria can move through semi-solid agar, indicating the presence of flagella or other motility structures. Motile organisms diffuse away from the stab line, while non-motile organisms grow only along the stab line.

• Motile: Diffuse/hazy growth (E. coli, Salmonella, Proteus).

• Non-motile: Growth only on stab line (Klebsiella, Shigella).

Procedure

1. Inoculate semi-solid agar with a straight stab.

2. Incubate at 35–37°C for 24–48 hours.

3. Observe for diffuse or restricted growth.

Urease Hydrolysis Test

Principle and Purpose

The Urease test detects the enzyme urease, which hydrolyzes urea to ammonia and carbon dioxide. Ammonia raises the pH, turning the phenol red indicator pink/magenta in alkaline conditions.

• Positive: Pink/magenta (Proteus vulgaris, Proteus mirabilis, H. pylori).

• Negative: Yellow/orange (E. coli, Salmonella, Shigella).

Procedure

1. Inoculate urea broth and incubate at 35–37°C.

2. Observe for color change to pink (positive) or yellow (negative).

Motility-Indole-Urease (MIU) Test

Principle and Purpose

The MIU test is a single-tube test that simultaneously assesses motility, indole production, and urease activity. It is efficient for presumptive identification of enteric bacteria.

• Motility: Diffuse growth (positive) or growth only on stab line (negative).

• Indole: Red/pink layer after Kovac’s reagent (positive).

• Urease: Pink/magenta (positive), yellow/orange (negative).

Examples: Proteus vulgaris (M+, I+, U+); E. coli (M+, I+, U−); Klebsiella pneumoniae (M−, I−, U+).

Hydrogen Sulfide (H2S) Production Test

Principle and Purpose

The H2S production test detects the ability of bacteria to produce hydrogen sulfide gas by reducing sulfur-containing compounds. H2S reacts with iron salts in the medium to form a black precipitate (FeS).

• Positive: Black precipitate (Salmonella, Proteus).

• Negative: No blackening (E. coli, Shigella, Klebsiella).

Procedure

1. Inoculate SIM, TSI, or KIA medium and incubate at 35–37°C for 18–24 hours.

2. Observe for black precipitate (positive) or no color change (negative).

Summary Table of Biochemical Tests

Medical Terms of the Week: Immune & Lymphatic System

• Lympho-: Lymph or lymphatic system

• Lymphaden-: Lymph nodes

• Spleno-: Spleen

• Immuno-: Immune system or immunity

• Autoimmune: Immune system attacks the body’s own cells

• Pathogen: Disease-causing microorganism

• Antigen: Foreign substance triggering immune response

• Antibody: Protein produced to neutralize antigens

• Leukocyte: White blood cell

• Inflammation: Tissue response to injury or infection (redness, heat, swelling, pain)