Microbiology Lab Tests: Enzyme Activities, Biochemical Reactions, and Microbial Analysis

Biochemical Tests for Microbial Identification

Overview

Biochemical tests are essential tools in microbiology for identifying and characterizing microorganisms based on their metabolic properties. These tests detect the presence of specific enzymes, metabolic pathways, and the ability to utilize or degrade various substrates. The following notes summarize key laboratory tests, their principles, and interpretations.

Enzyme and Substrate-Based Tests

Indole Test (TEK)

• Purpose: Detects the ability of bacteria to produce indole from tryptophan via the enzyme tryptophanase.

• Substrate: Tryptophan

• Indicator: Kovac’s reagent; a red ring indicates a positive result.

• Example: Escherichia coli is indole positive.

Methyl Red (MR) Test (PEM)

• Purpose: Identifies bacteria that perform mixed acid fermentation of glucose, producing stable acid end-products.

• Substrate: Pyruvic acid (from glucose fermentation)

• Indicator:Methyl red, turns red at pH < 4.8 (positive).

• Example: E. coli is MR positive.

Voges-Proskauer (VP) Test (PABEB)

• Purpose: Detects organisms that ferment glucose via the butanediol pathway, producing acetoin.

• Substrate: Pyruvic acid

• Indicator: VP A and VP B reagents detect acetoin (red color is positive).

• Example: Enterobacter aerogenes is VP positive.

Citrate Utilization Test (CCBB)

• Purpose: Determines if an organism can use citrate as its sole carbon source via the enzyme citrase.

• Substrate: Citrate

• Indicator: Bromthymol blue; color change from green to royal blue indicates alkalinization (positive).

Hydrogen Sulfide (H2S) Production (ICBP)

• Purpose: Detects reduction of sulfur-containing compounds to H2S gas.

• Indicator: Iron compound in medium forms a black precipitate with H2S (positive).

• Example: Salmonella species are H2S positive.

Motility Test (MOB)

• Purpose: Determines if bacteria are motile.

• Method: Observe for diffuse growth radiating from the line of inoculation in semi-solid medium.

Lipid Hydrolysis Test (LODLB)

Purpose: Detects production of lipase that hydrolyzes triglycerides.

• Indicator: Dark blue to light blue color change and reduction of oil droplets indicate lipase activity.

Oxygen Requirements

• Purpose: Determines the oxygen needs of bacteria by observing growth patterns in anaerobic chambers.

• Interpretation: Obligate aerobes grow only in the presence of oxygen; obligate anaerobes grow only in its absence.

Oxidase Test (CPC)

• Purpose: Detects the presence of cytochrome oxidase enzyme.

• Indicator: Purple color on test strip within 30 seconds is positive.

• Example: Pseudomonas species are oxidase positive.

Casein Hydrolysis Test (CHCO)

• Purpose:Detects caseinase enzyme that hydrolyzes casein (milk protein).

• Indicator: Clearing of opaqueness around colonies indicates positive result.

Gelatin Hydrolysis Test (GLG)

• Purpose: Detects gelatinase enzyme that liquefies gelatin.

• Method: After 7 days incubation, refrigerate for 20 minutes; if medium remains liquid, gelatinase is present (positive).

Nitrate Reduction Test (NaNiN2)

• Purpose: Determines the ability to reduce nitrate to nitrite or further to nitrogen gas.

• Indicator: Add nitrate reagents; if no color, add zinc. No color after zinc indicates positive for nitrate reductase.

Litmus Milk Reactions (RPCF)

• Purpose: Tests for multiple enzymatic activities (caseinase, renin, proteases, fermentation enzymes) using milk as a substrate.

• Indicators and Results:

  • Pink: Acid fermentation (PAF)

  • Deep purple: Alkaline reaction (PUAR)

  • White precipitate: Reduced litmus (WRL)

  • Amber clear fluid: Peptonization (AP)

  • Curd or coagulation: Protein denaturation (CPD)

DNase Test (MGCC)

• Purpose: Detects DNase enzyme that hydrolyzes DNA.

• Indicator: Methyl green dye; clearing around colonies indicates DNase activity.

Starch Hydrolysis Test (ASIC)

• Purpose: Detects amylase enzyme that hydrolyzes starch.

• Indicator: Iodine; clear zone around growth after iodine addition indicates positive result.

Urease Test (UACP)

• Purpose: Detects urease enzyme that hydrolyzes urea to ammonia and carbon dioxide.

• Indicator: Phenol red; pink color indicates positive result.

Water Quality and Microbial Enumeration

Most Probable Number (MPN) Test for Water (CDAGP)

• Purpose: Estimates the number of coliform bacteria in water samples using statistical tables.

• Method: Serial dilutions of water are inoculated into lactose broths in replicates (usually 5 per dilution).

• Presumptive Test: Acid and gas production in lactose broth indicates possible coliforms.

• Confirmation Test: Positive tubes are further tested in bile broth and on EMB agar. Escherichia coli is an indicator of fecal contamination.

• Enumeration: Use MPN tables to estimate bacteria per 100 mL based on positive tubes at each dilution.

Microbial Contamination of Beverages

• Purpose: Quantifies bacteria in beverages such as milk using serial dilution and plating.

• Method: 1 mL of sample is added to 9 mL diluent (1:10 dilution), then plated on EMB and nutrient agar.

• Counting: Plates with 20–250 colonies are counted; multiply by dilution factor to determine original concentration.

Antibiotic Susceptibility Testing

Kirby-Bauer Disk Diffusion Method (EOAC)

• Purpose: Determines bacterial susceptibility to antibiotics.

• Method: Antibiotic-impregnated disks are placed on Mueller-Hinton agar inoculated with bacteria.

• Interpretation: Zone of inhibition around the disk indicates susceptibility; size is compared to standard charts.

Summary Table: Key Biochemical Tests

Test	Enzyme	Substrate	Indicator/Result
Indole	tryptophanase	tryptophan	Kovac’s reagent/red ring
Methyl Red	Fermentation enzymes	Pyruvic acid	Methyl red (red at pH < 4.8)
Voges-Proskauer	Fermentation enzymes	Pyruvic acid	VP A & B (red for acetoin)
Citrate Utilization	citrase	citrate	Bromthymol blue (green to blue)
H2S Production	Reduction enzymes	Sulfur compounds	Black precipitate
Motility	N/A	N/A	Growth from stab line
Lipid Hydrolysis	lipase	Triglycerides	Blue color change, oil reduction
Oxidase	cytochrome oxidase	O2	Purple color on strip
Casein Hydrolysis	caseinase	casein	Clearing of opaqueness
Gelatin Hydrolysis	gelatinase	gelatin	Liquefaction after refrigeration
Nitrate Reduction	nitrate reductase	nitrate	Color after reagents/Zn
DNase	DNase	DNA	Clearing with methyl green
Starch Hydrolysis	amylase	starch	Clear zone with iodine
Urease	urease	urea	Pink with phenol red

Additional info:

• Many of these tests are part of the IMViC series (Indole, Methyl Red, Voges-Proskauer, Citrate) used to differentiate Enterobacteriaceae.

• EMB (Eosin Methylene Blue) agar is selective and differential for Gram-negative enteric bacteria, especially coliforms.

• Serial dilution and plating are standard quantitative methods in microbiology for estimating viable cell counts.