Summary of biochemical tests in microbiology
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Summary of Biochemical Tests in Microbiology. “Better living through chemistry”. Catalase Test. Summary Tests for presence of catalase. Background Catalase is an enzyme that breaks down hydrogen peroxide into water and oxygen gas. Evolution of oxygen gas bubbles is positive.

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Summary of biochemical tests in microbiology

Summary of Biochemical Testsin Microbiology

“Better living through chemistry”


Catalase test
Catalase Test

Summary

Tests for presence of catalase.

Background

Catalase is an enzyme that breaks down hydrogen peroxide into water and oxygen gas. Evolution of oxygen gas bubbles is positive.

All + except EF –

Methods

Loop of bacteria added to a drop of Hydrogen Peroxide H2O2


Blood agar test
Blood Agar Test

Summary

Tests for presence of hemolysin.

Background

Hemolysin is an enzyme that destroys red blood cells.

Methods

Bacteria streak and loop stabs to blood agar.


3 types of blood agar results
3 Types of Blood Agar Results

Summary

Alpha – Partial hemolysis

Beta – Complete hemolysis

Gamma – No hemolysis

Background

Alpha hemolysis partially oxidizes hemoglobin changing it to green.

All B+ except SE and EF

Beta hemolysis lyses RBC completely making a clear zone

Gamma hemolysis – no activity


Mannitol salts agar msa test
Mannitol Salts Agar (MSA) Test

Summary

Selective for gram +

Differential for fermentation of mannitol sugar.

Background

Gram + bacteria tolerate sodium chloride (NaCL) salt.

SA + EF + SE - and others –

Methods

Streak surface of MSA Agar. Watch for change from pink to yellow as a +


Mcconkeys test
McConkeys Test

Summary

Tests for fermentation of lactose.

Background

Selects for gram - , Crystal violet inhibits gram +

Lactose and neutral red produce a dark pink differentiating lactose +

EC, CLO, KP are +

Methods

Loop of bacteria streaked on Mac agar


Emb agar test
EMB Agar Test

Summary

Tests for fermentation of lactose.

Background

Selects for gram - , Eosin and methylene blue inhibit gram +

Lactose and dyes produce a dark pink differentiating lactose + in CLO and a green sheen in EC

EC, CLO, KP are +

Methods

Loop of bacteria streaked on EMB agar


Nitrate reduction test
Nitrate Reduction Test

Summary

Tests for nitrate reductase and nitrite reductase.

Background

Potassium Nitrate KNO3 as substrate. Nitrate  Nitrite  N2

Methods

. Add Nitrate A & B and cherry red means + for nitrate reductase

All +

Follow up with Zinc = grey = + for nitrite reductase

PA +


Triple sugar iron tsi test
Triple Sugar Iron (TSI) Test

Summary

Differentiates gram – entericsby fermentation patterns.

Background

Lactose and glucose are substrates. Phenol red is the indicator. pH of 6.8 or lower turns yellow.

Methods

Stab and streak TSI slant.

Red slant & yellow but = glucose fermentation. ST SF PA

Yellow slant and yellow butt = lactose and glucose fermentation. SA SE EF BS EC CLO KP PV(false)

Red slant & red butt = no fermentation.

Black butt = Hydrogen sulfide production


Urease test
Urease Test

Summary

Differentiates organisms using urease to break down urea into ammonia

Background

Substrate is urea. pH indicator is phenol red (pH 6= yellow, 6.8 topaz, 8 = fuchsia)

Methods

Loop inoculation of urea broth.

SA SE KP PV are +


Imvic indole test
IMViCIndole Test

Summary and Background

Differentiates enteric organisms such as EC and PV which are able to metabolize the amino acid tryptophan into indole and pyruvic acid. This test also detects motility of bacteria and H2S2

production.

Methods

Needle inoculation of Sufide-Indole-Motility (SIM) media. Substrate is tryptophan. Add Kovac’sindole reagent. A red color is + for indole production.

Black = + for H2S2

Motility is observed as turbidity radiating out from the inoculation line.


Imvic methyl red
IMViC Methyl Red

Summary and Background

Differentiates enteric organisms such as EC from CLO using the timing of glucose fermentation products. Both produce acids but CLO further converts to non acidic ethanol and then elevates the pH.

Methods

Loop inoculation of MR VP broth. Add methyl red indicator.

Red = + for acid conditions. SA SE BS EC KP PV ST SF

Yellow is – indicating more basic conditions. EF CLO PA


Citrate test
Citrate Test

Summary

Differentiates organisms using citrate as a carbon source.

Background

Organisms using citrate must also have nitrogen found in the medium as ammonium dihydrogen phosphate. Citrate fermentation produces ammonia causing the indicator (bromothymol blue) to turn deep blue.

Methods

Loop inoculation of citrate slant.

Prussian blue = + for citrate fermentation and alkaline conditions.

Green or yellow = - (low pH)


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