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MCB 3020 Lab Practical Exam Questions and Answers
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Milk Agar Plate Differential media Tests for the presence of the enzyme caseinase (Hydrolyzes casein (a milk protein) --> amino acid products) (+) = Clearing (-) = no clearing - Starch Agar Plate Differential media --> amylase Hydrolyses starch --> simple sugars that can be tra...

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  • October 9, 2024
  • 8
  • 2024/2025
  • Exam (elaborations)
  • Questions & answers

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  • mcb 3020 lab practical exam questions and answers
  • milk agar plate differential media

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  • MCB 3020
  • MCB 3020

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MCB 3020 Lab Practical Exam Questions
and Answers
Milk Agar Plate
Differential media
Tests for the presence of the enzyme caseinase
(Hydrolyzes casein (a milk protein) --> amino acid products)

(+) = Clearing
(-) = no clearing -

Starch Agar Plate
Differential media --> amylase
Hydrolyses starch --> simple sugars that can be transported into the cell
Indicator= grams iodine
Iodine + starch = purple color

(+) = No color change/ colorless
(-) = purple around the colony

Lipase Plate
Differential media --> lipase
Hydrolyzes fats --> glycerol and fatty acids
Indicator: spirit blue dye

(+) = blue color
(-) = no color

Oxidase Test
Differential test -->cytochrome c oxidase
(oxidize aromatic amines)
Indicator: dimethyl-p-phenylenediamine hydrochloride DMPD (an aromatic amine)
In the presence of cytochrome c oxidase, the reagent will be oxidized and turn dark blue
to black

(+) = dark blue/black
(-) = colorless

Catalase Test
Differential test --> catalase
Converts hydrogen peroxide (H2O2) --> water and oxygen
Anaerobic (no O2) growth produces hydrogen peroxide which is toxic to the cell
Cell produces catalase to get rid of hydrogen peroxide
Add 3% hydrogen peroxide to the edge of the colony

, Oxygen product will be seen as bubbles coming from the sample

(+) = bubbles
(-) = no bubbles

Litmus Milk
Litmus pH indicator (turns pink at acidic pH)
Acid production
Alkaline production
Curd
Reduction
Proteolysis

Results
(+) Moderate Fermenter= pink
(+) Heavy Fermenter= Curd reaction
(-) Alkaline = blue

Clearing= Casinase +

Labeled Litmus results --

PR Sugar Broth (test for specific sugar fermentation - either glucose, lactose or
mannitol)
Reagent: phenol red and a Durham tube

(+) yellow color = acid
(+) gas + yellow color = acid + gas
(-) Red
(-) Cerise = alkaline -

OF Glucose Media
Tests for Glucose fermentation (Obligate anaerobic fermentation
vs Facultative anaerobic fermentation vs Strict aerobes)
Indicators: Bromthymol blue pH indicator

(closed- adding oil to make anaerobic environment)
Both tubes blue/green = no result
Both tubes yellow = fermentative (F)
Open yellow/closed green = oxidative (O) - Answer-

Vogues Proskauer (VP)
tests for 2,3-butanediol fermenters
Glucose -->AMC-->2,3-butanediol
2,3-butanediol fermenters produce the acetylmethylcarbinol (AMC)/acetoin intermediate
Reagents:

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