Microbiology Media Types and Biochemical Tests: A Study Guide

Microbiology Media Types and Biochemical Tests

This study guide provides an overview of common microbiological media and biochemical tests used to identify and differentiate bacteria in the laboratory. Each section includes the purpose, procedure, and interpretation of results for each medium or test.

Carbohydrate Broths

Carbohydrate broths are used to determine a microorganism's ability to ferment specific sugars, producing acid and sometimes gas.

• Definition: Liquid media containing a single carbohydrate (e.g., glucose, lactose) and a pH indicator.

• Purpose: To test for fermentation of carbohydrates by bacteria.

• Procedure:

  1. Obtain a sterile inoculating loop.

  2. Collect a bacterial sample.

  3. Inoculate the carbohydrate broth.

  4. Incubate the tube.

  5. Observe the results.

• Results:

  • Yellow = positive (acid produced)

  • Red = negative (no acid produced)

• Example: Escherichia coli ferments glucose, turning the broth yellow.

SIM Agar (Sulfide, Indole, Motility)

SIM agar is a multipurpose medium used to test for hydrogen sulfide production, indole formation, and bacterial motility.

• Definition: Semi-solid medium containing peptones, sodium thiosulfate, and ferrous ammonium sulfate.

• Purpose: To detect sulfur reduction, indole production, and motility.

• Procedure:

  1. Obtain bacterial sample.

  2. Stab into the medium with a needle.

  3. Incubate the tube.

  4. Add Kovac's reagent for indole test.

  5. Observe results.

• Results:

  • Sulfur Reduction: Black = positive; No black = negative

  • Indole: Pink = positive; No pink = negative

  • Motility: Cloudy butt = positive; No cloudy butt = negative

• Example: Proteus vulgaris is positive for all three tests.

MR-VP Test (Methyl Red and Voges-Proskauer)

The MR-VP test differentiates bacteria based on their pattern of glucose metabolism.

• Definition: Two related tests using MRVP broth to detect mixed acid fermentation (MR) and acetoin production (VP).

• Purpose: To distinguish between mixed acid fermenters and butanediol fermenters.

• Procedure:

  1. Obtain 2 tubes of MRVP broth.

  2. Inoculate each with the bacterial sample.

  3. Incubate tubes.

  4. For MR: Add methyl red indicator.

  5. For VP: Add VP reagents A and B, swirl.

  6. Observe results.

• Results:

  • Methyl Red (MR): Red = positive; Brown/yellow = negative

  • Voges-Proskauer (VP): Red = positive; Brown/yellow = negative

• Example: Escherichia coli is MR positive, VP negative; Enterobacter aerogenes is MR negative, VP positive.

Triple Sugar Iron (TSI) Agar

TSI agar is used to differentiate bacteria based on their ability to ferment sugars and produce hydrogen sulfide.

• Definition: Slant medium containing glucose, lactose, sucrose, ferrous sulfate, and phenol red.

• Purpose: To test for fermentation of sugars and sulfur reduction.

• Procedure:

  1. Obtain a slant of TSI agar and a sterile needle.

  2. Stab the butt and streak the slant.

  3. Incubate the tube.

  4. Observe results.

• Results:

  • Red/red = K/K (no fermentation)

  • Red/yellow = K/A (glucose only)

  • Yellow/yellow = A/A (glucose + lactose/sucrose)

  • Black = sulfur reduction

  • Cracks/movement = gas production (G)

• Example: Salmonella species often produce black precipitate due to H2S production.

Citrate Agar

Citrate agar tests an organism's ability to use citrate as its sole carbon source.

• Definition: Slant medium containing sodium citrate and bromothymol blue indicator.

• Purpose: To identify bacteria that can utilize citrate.

• Procedure:

  1. Obtain a slant of citrate agar.

  2. Stab and streak with inoculation needle.

  3. Incubate tube.

  4. Observe results.

• Results:

  • Blue = positive (citrate used)

  • Green = negative (citrate not used)

• Example: Enterobacter aerogenes is citrate positive.

Gelatin Broth

Gelatin broth is used to detect gelatinase enzyme activity, which liquefies gelatin.

• Definition: Deep tube of nutrient gelatin.

• Purpose: To test for gelatin hydrolysis.

• Procedure:

  1. Obtain a deep of gelatin agar.

  2. Stab with inoculation needle.

  3. Incubate tube.

  4. Chill tube.

  5. Observe results.

• Results:

  • Liquid after refrigeration = positive

  • Solid after refrigeration = negative

• Example: Bacillus subtilis is gelatinase positive.

Nitrate Broth

Nitrate broth tests for the reduction of nitrate to nitrite or other nitrogenous compounds.

• Definition: Broth containing potassium nitrate.

• Purpose: To detect nitrate reduction by bacteria.

• Procedure:

  1. Inoculate broth with bacteria.

  2. Incubate tube.

  3. Add reagents (sulfanilic acid and α-naphthylamine).

  4. If no color, add zinc.

  5. Observe results.

• Results:

  • Red after reagents = positive (nitrate reduced to nitrite)

  • Colorless after reagents, red after zinc = negative (nitrate not reduced)

  • Colorless after zinc = positive (nitrate reduced beyond nitrite)

• Example: Pseudomonas species can reduce nitrate to nitrogen gas.

Urea Broth

Urea broth is used to detect urease enzyme activity, which hydrolyzes urea to ammonia and carbon dioxide.

• Definition: Broth containing urea and phenol red indicator.

• Purpose: To test for urease production.

• Procedure:

  1. Inoculate tube of urea broth with bacteria.

  2. Incubate tube.

  3. Observe results.

• Results:

  • Magenta = positive (ammonia raises pH)

  • Light orange = negative

• Example: Proteus species are urease positive.

Eosin Methylene Blue (EMB) Agar

EMB agar is a selective and differential medium for Gram-negative bacteria, especially coliforms.

• Definition: Agar containing eosin Y and methylene blue dyes.

• Purpose: To isolate and differentiate lactose fermenters.

• Procedure:

  1. Streak EMB plate with bacterial sample.

  2. Incubate plate.

  3. Observe results.

• Results:

  • Green metallic sheen = strong lactose fermenter (e.g., E. coli)

  • Pink/brown, not metallic = weak fermenter

  • Colorless = Gram-positive or non-fermenter

Mannitol Salt Agar (MSA)

MSA is a selective and differential medium for Staphylococcus species.

• Definition: Agar with high salt concentration and mannitol sugar.

• Purpose: To select for salt-tolerant bacteria and differentiate mannitol fermenters.

• Procedure:

  1. Streak MSA plate with bacterial sample.

  2. Incubate plate.

  3. Observe results.

• Results:

  • Yellow = positive (mannitol fermentation)

  • Red/pink = negative

• Example: Staphylococcus aureus ferments mannitol (yellow).

Blood Agar

Blood agar is an enriched medium used to detect hemolytic activity of bacteria.

• Definition: Agar supplemented with sheep or horse blood.

• Purpose: To observe hemolysis patterns.

• Procedure:

  1. Streak blood agar plate with bacterial sample.

  2. Incubate plate.

  3. Observe results.

• Results:

  • Green = alpha-hemolysis (partial)

  • Clear zone = beta-hemolysis (complete)

  • No change = gamma-hemolysis (none)

• Example: Streptococcus pyogenes is beta-hemolytic.

Nutrient Agar

Nutrient agar is a general-purpose medium supporting the growth of a wide range of non-fastidious organisms.

• Definition: Simple agar containing peptone, beef extract, and agar.

• Purpose: To cultivate and observe colony morphology.

• Procedure:

  1. Streak nutrient agar plate with bacterial sample.

  2. Incubate plate.

  3. Observe colony shape, margin, and elevation.

• Results: Colonies may be described by form (e.g., circular, irregular), margin (e.g., entire, undulate), and elevation (e.g., flat, raised).

Skim Milk Agar

Skim milk agar is used to detect casein hydrolysis by proteolytic bacteria.

• Definition: Agar containing skim milk as a protein source.

• Purpose: To test for caseinase enzyme activity.

• Procedure:

  1. Streak SM agar plate with bacterial sample.

  2. Incubate plate.

  3. Observe results.

• Results:

  • Clear zone = positive (casein hydrolysis)

  • No clear zone = negative

Starch Agar

Starch agar is used to detect amylase enzyme activity, which hydrolyzes starch.

• Definition: Agar containing soluble starch.

• Purpose: To test for starch hydrolysis.

• Procedure:

  1. Streak starch agar plate with bacterial sample.

  2. Incubate plate.

  3. Flood plate with iodine.

  4. Observe results.

• Results:

  • Zone of clearing = positive (starch hydrolysis)

  • No zone = negative

MacConkey Agar

MacConkey agar is a selective and differential medium for Gram-negative, lactose-fermenting bacteria.

• Definition: Agar containing bile salts, crystal violet, lactose, and neutral red.

• Purpose: To isolate and differentiate enteric bacteria.

• Procedure:

  1. Streak MacConkey plate with bacterial sample.

  2. Incubate plate.

  3. Observe results.

• Results:

  • Pink = positive (lactose fermentation)

  • Colorless = negative

• Example: Escherichia coli forms pink colonies.

Catalase Test

The catalase test detects the presence of the catalase enzyme, which breaks down hydrogen peroxide.

• Definition: Biochemical test using hydrogen peroxide.

• Purpose: To distinguish catalase-positive from catalase-negative bacteria.

• Procedure:

  1. Transfer bacteria to slide.

  2. Add hydrogen peroxide.

  3. Observe for bubbles.

• Results:

  • Bubbles = positive

  • No bubbles = negative

• Example: Staphylococcus species are catalase positive.

Oxidase Test

The oxidase test identifies bacteria that produce cytochrome c oxidase.

• Definition: Biochemical test using oxidase reagent on a strip.

• Purpose: To detect presence of cytochrome c oxidase.

• Procedure:

  1. Apply bacteria to test strip.

  2. Wait 30 seconds.

  3. Observe color change.

• Results:

  • Blue/purple = positive

  • No color change = negative

• Example: Pseudomonas species are oxidase positive.

Coagulase Test (Bound and Free)

The coagulase test detects the enzyme coagulase, which causes clotting of plasma.

• Definition: Test for the presence of coagulase enzyme (bound or free).

• Purpose: To differentiate Staphylococcus aureus (coagulase positive) from other staphylococci.

• Procedure (Bound):

  1. Mix bacteria with saline and plasma on a slide.

  2. Observe for clumping (clotting).

• Procedure (Free):

  1. Mix bacteria with plasma in a tube.

  2. Incubate.

  3. Observe for clotting.

• Results:

  • Clotting = positive

  • No clotting = negative

CAMP Test

The CAMP test is used to identify Streptococcus agalactiae based on enhanced hemolysis.

• Definition: Test for synergistic hemolysis with S. aureus and S. agalactiae.

• Purpose: To identify Group B streptococci.

• Procedure:

  1. Streak S. agalactiae on blood agar.

  2. Streak test organism perpendicular, not touching.

  3. Incubate plate.

  4. Observe for arrowhead hemolysis.

• Results:

  • Arrowhead pattern = positive

  • No arrowhead = negative

Summary Table: Key Media and Test Results

Medium/Test	Positive Result	Negative Result	Purpose
Carbohydrate Broth	Yellow	Red	Fermentation
SIM Agar (Sulfur)	Black	No black	Sulfur reduction
SIM Agar (Indole)	Pink	No pink	Indole production
SIM Agar (Motility)	Cloudy	Not cloudy	Motility
MR Test	Red	Brown/yellow	Mixed acid fermentation
VP Test	Red	Brown/yellow	Acetoin production
TSI Agar	Yellow/black/cracks	Red	Sugar fermentation, H2S, gas
Citrate Agar	Blue	Green	Citrate utilization
Gelatin Broth	Liquid	Solid	Gelatinase
Nitrate Broth	Red/colorless after zinc	Red after zinc	Nitrate reduction
Urea Broth	Magenta	Light orange	Urease
EMB Agar	Green/pink	Colorless	Lactose fermentation
MSA	Yellow	Red/pink	Mannitol fermentation
Blood Agar	Green/clear	No change	Hemolysis
Skim Milk Agar	Clear zone	No clear zone	Caseinase
Starch Agar	Zone of clearing	No zone	Amylase
MacConkey Agar	Pink	Colorless	Lactose fermentation
Catalase Test	Bubbles	No bubbles	Catalase
Oxidase Test	Blue/purple	No color	Cytochrome c oxidase
Coagulase Test	Clotting	No clotting	Coagulase
CAMP Test	Arrowhead	No arrowhead	Group B strep

Additional info: This guide covers the practical laboratory identification of bacteria using differential and selective media, as well as key biochemical tests, which are essential for understanding microbial metabolism, physiology, and clinical diagnostics.