Microbiology Questions and Answers – Microorganisms Microscopic Examination – Microbial Stains

This set of Microbiology Multiple Choice Questions & Answers (MCQs) focuses on “Microorganisms Microscopic Examination – Microbial Stains”.

1. The dye eosinate of methylene blue belongs to which group?
a) Acidic dye
b) Basic dye
c) Neutral dye
d) Oxazine dye
View Answer

Answer: c
Explanation: A neutral dye is a complex salt of a dye acid with a dye base so eosinate of methylene blue is an example of a neutral dye. An acidic dye is one in which the charge on the dye ion is negative and a basic dye is one in which the charge carried by the dye ion is positive.

2. What is the correct order of staining reagents in Gram-Staining?
a) Crystal violet, alcohol, iodine solution, safranin
b) Crystal violet, iodine solution, alcohol, safranin
c) Crystal violet, safranin, alcohol, iodine solution
d) Iodine solution, crystal violet, alcohol, safranin
View Answer

Answer: b
Explanation: Gram staining is a type of differential staining. In this process the fixed bacterial smear is subjected to the following staining reagents in the order listed: crystal violet, iodine solution, alcohol (decolorizing agent), and safranin.

3. Which bacteria appears purple-violet colour after staining?
a) Gram-positive
b) Gram-negative
c) Both Gram-positive and Gram-negative
d) Neither Gram-positive nor Gram-negative
View Answer

Answer: a
Explanation: Gram-positive bacteria retain the crystal violet and hence appear deep purple-violet colour as it has a higher content of the peptidoglycan layer. Gram-negative bacteria have lesser amount of peptidoglycan layer and on treatment with alcohol it loses the purple-violet colour, are counterstained by safranin and hence appears red.

4. Which of the following are true for Gram-negative bacteria?
a) upon alcohol treatment, the permeability of the cell wall increases
b) crystal violet-iodine (CV-I) complex is extracted
c) pore size decreases and the CV-I complex cannot be extracted
d) alcohol treatment increases the permeability of the cell wall and the CV-I complex can be extracted
View Answer

Answer: d
Explanation: Experimental evidence suggests that during staining of Gram-negative bacteria the alcohol treatment extracts the lipid, which results in increased permeability of the cell wall. Thus the crystal violet-iodine (CV-I) complex can be extracted and the Gram-negative organism is decolorized. These cells subsequently take on the color of the safranin counterstain.

5. Gram-positive bacteria are usually more susceptible to?
a) streptomycin
b) tetracyclin
c) penicillin
d) ampicillin
View Answer

Answer: c
Explanation: Gram-positive bacteria are usually more susceptible to penicillin and less susceptible to disintegration by mechanical treatment or exposure to some enzymes than Gram-negative bacteria. Gram-negative bacteria are more susceptible to other antibiotics like streptomycin.
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6. Which of the staining technique helps in demonstrating spore structure in bacteria as well as free spores?
a) Acid-fast stain
b) Endospore stain
c) Capsule stain
d) Flagella stain
View Answer

Answer: b
Explanation: There are numerous other staining techniques designed to identify some particular feature of cell structure or composition. Endospore staining demonstrates spore structure in bacteria as well as free spores.

7. In Gram-staining, iodine is used as a ______________
a) fixative
b) mordant
c) solublizer
d) stain
View Answer

Answer: b
Explanation: In Gram-staining, iodine acts as a mordant i.e. it combines with the dye or stain and thereby fixes it on the material. It increases the interaction between the stain solution and the bacterial cell.

8. Which of the following is the example of Gram-negative bacteria?
a) Lactobacillus
b) Eschericia coli
c) Staphylococcus aureus
d) Bacillus subtilis
View Answer

Answer: b
Explanation: Eschericia coli is an example of Gram-negative bacteria. Others are all examples of Gram-positive bacteria.

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