Biochemical and Hydrolytic Tests in Microbiology: Study Guide

Biochemical / Hydrolytic Tests

Urase and Starch Tests

Biochemical and hydrolytic tests are essential for identifying and differentiating microorganisms based on their metabolic properties. The urase and starch tests are commonly used to detect the presence of specific enzymes and their activity on substrates.

• Purpose: To determine the ability of bacteria to produce specific enzymes that hydrolyze urea or starch.

• Enzyme Released: Urease (for urea hydrolysis), Amylase (for starch hydrolysis).

• Substrate: Urea (for urease test), Starch (for starch hydrolysis test).

• Positive Result: Urease test: pink color (ammonia production, pH increase); Starch test: clear zone around growth after iodine addition (starch hydrolysis).

• Negative Result: Urease test: no color change or yellow/orange; Starch test: blue-black color after iodine (starch present, no hydrolysis).

Example: Proteus vulgaris is urease positive, while Escherichia coli is urease negative.

Biochemical Test: Litmus Milk

Purpose and Interpretation

The litmus milk test is a differential medium used to distinguish bacteria based on their ability to metabolize lactose, reduce litmus, and digest casein. The test reveals various metabolic activities through color and physical changes in the medium.

• Acid Reaction (Pink color): Indicates lactose fermentation, acid production, and a drop in pH. Curd formation (coagulation) of milk proteins may occur. Organism example: Escherichia coli

• Alkaline Reaction (Blue or Purple color): Indicates proteolysis or peptone utilization, leading to ammonia formation and a rise in pH. These organisms do not ferment lactose but break down proteins, releasing alkaline products. Organism examples: Pseudomonas aeruginosa, Alcaligenes faecalis, Bacillus subtilis

• Reduction (White, colorless lower zone): Indicates reduction of litmus dye, resulting in loss of color. Organism examples: Clostridium species, Enterococcus faecalis

• Peptonization or Digestion (Clear or translucent medium): Indicates proteolytic digestion of casein, leaving a clear, watery medium. Organism examples: Bacillus subtilis, Pseudomonas aeruginosa

Example: Escherichia coli produces an acid reaction, while Pseudomonas aeruginosa produces an alkaline reaction and peptonization.

IMViC Test

Purpose and Components

The IMViC series (Indole, Methyl Red, Voges-Proskauer, Citrate) is a set of biochemical tests used to differentiate members of the family Enterobacteriaceae and other Gram-negative rods. These tests assess the ability of bacteria to ferment glucose via different pathways and utilize citrate as a carbon source.

• Purpose: To identify and differentiate Enterobacteriaceae based on metabolic characteristics.

1. Sulfide (H2S), Indole, and Motility (SIM Agar Deep)

• Medium: SIM (Sulfide-Indole-Motility) agar deep.

• Indole Test: Detects tryptophanase activity. Reagent: Kovac's reagent reveals a red color for a positive result.

2. Methyl Red Test (MR)

• Medium: MRVP (Methyl Red-Voges Proskauer) broth.

• Purpose: Detects stable acid production from glucose fermentation (mixed acid pathway).

• Reagent: Methyl Red. A red color indicates a positive result.

3. Voges-Proskauer Test (VP)

• Medium: MRVP broth.

• Purpose: Detects acetoin production from glucose fermentation (butanediol pathway).

• Reagents: VP A (alpha-naphthol) and VP B (potassium hydroxide). A red color after reagent addition indicates a positive result.

4. Citrate Test

• Medium: Simmons' Citrate Agar Slant.

• Purpose: Determines the ability of an organism to use citrate as its sole carbon source. A positive result is indicated by a color change from green to blue.

Example: Escherichia coli is typically MR positive and VP negative, while Enterobacter aerogenes is MR negative and VP positive.

Test	Medium	Positive Result	Negative Result	Key Reagent
Indole	SIM Agar Deep	Red ring (after Kovac's)	No color change	Kovac's reagent
Methyl Red	MRVP Broth	Red color	Yellow/orange	Methyl Red
Voges-Proskauer	MRVP Broth	Red color	No color change	VP A & VP B
Citrate	Simmons' Citrate Agar	Blue color	Green (no change)	None

Additional info: The IMViC tests are especially important in clinical microbiology for distinguishing between Escherichia coli and Enterobacter species, which have different implications for human health.