This set of Microbiology Multiple Choice Questions & Answers (MCQs) focuses on “Antibiotics and their Mode of Action”.
1. Bacterial cells grown in a medium exposed to high osmotic pressure, changes shape from rod-shaped to __________ shaped.
a) spherical
b) rod shaped
c) irregular
d) elongated
View Answer
Explanation: The high osmotic pressure prevents the cells from bursting. Rod-shaped cells become spherical because they lack the cell structure which imparts shape.
2. Cell-wall biosynthesis is inhibited by antibiotics by inhibiting the biosynthesis of which of the following?
a) lipopolysaccharide
b) cellulose
c) peptidoglycan
d) proteins
View Answer
Explanation: Antibiotics exert their microbial effect by inhibiting biosynthesis of the peptidoglycan polymer, resulting in the inhibition of cell-wall formation. This results in the inability of the bacterium to survive because of the absence of a protective covering.
3. Structurally all penicillins have only beta-lactam present in them.
a) True
b) False
View Answer
Explanation: All penicillins have a common basic nucleus, a fused beta-lactam-thiazolidine ring with different side chains that give each its unique properties.
4. The crystalline sodium or potassium salts are slightly soluble in ___________
a) ether
b) dioxane
c) water
d) chloroform
View Answer
Explanation: Natural penicillins can be prepared as salts of sodium, potassium, procaine, and other bases. The crystalline sodium or potassium salts are freely soluble in water, ester, ethers, ethyl alcohol, and dioxane but only slightly soluble in chloroform and benzene.
5. Which of the following does not affect the activity of penicillin?
a) bile
b) hydrochloric acid
c) cysteine
d) sodium hydroxide
View Answer
Explanation: The natural penicillins are inactivated by heat, cysteine, sodium hydroxide, penicillinase, and hydrochloric acid. They are not affected by the action of saliva or bile.
6. Benzylpenicillin is the chemical name for which of the following penicillin?
a) Penicillin G
b) Penicillin V
c) Penicillin F
d) Phenethicilin
View Answer
Explanation: Peniciilin G is also known as Benzylpenicillin which is natural penicillin and has a basic core of 6-aminopenicillanic acid different side chain.
7. Ampicillin is a bactericidal antibiotic.
a) True
b) False
View Answer
Explanation: Ampicillin is another semisynthetic penicillin which is strongly bactericidal and lacks toxicity, but is not resistant to penicillinases.
8. Streptomyces orientalis produces which of the following antibiotics?
a) Cephalosporins
b) Cycloserine
c) Bacitracin
d) Vancomycin
View Answer
Explanation: Vancomycin is an antibiotic produced by Streptomyces orientalis. It is a complex chemical entity consisting of amino acids and sugars.
9. Which of the following interferes with the regeneration of the monophosphate form of bactoprenol from the pyrophosphate form?
a) Vancomycin
b) Ampicillin
c) Bacitracin
d) Cephalosporins
View Answer
Explanation: Bacitracin interferes with regeneration of the monophosphate form of bactoprenol from the pyrophosphate form. It is a polypeptide chemically.
10. Polymyxins inhibits the growth of the microbes by carrying out which of the following actions?
a) inhibition of cell-wall synthesis
b) damage to cytoplasmic membrane
c) inhibition of nucleic acid and protein synthesis
d) inhibition of specific enzyme systems
View Answer
Explanation: Polymyxins, gramicidins, and tyrocidines cause damage to cytoplasmic membrane. They adversely affect the normal permeability characters of the cell membrane.
11. Streptomycin is produced by which of the following organisms?
a) Stretomyces noursei
b) Streptomyces nodosus
c) Streptomyces fradiae
d) Streptomyces griseus
View Answer
Explanation: Streptomycin is produced by Streptomyces griseus, a soil organism isolated by Schatz, Bugie and Waksman, who reported on its antibiotic activities in 1944.
12. Antibiotic produced by Streptomyces rimosus is _______________
a) chlortetracycline
b) oxytetracycline
c) tetracycline
d) doxycycline
View Answer
Explanation: Antibiotic produced by Streptomyces rimosus is oxytetracycline whereas Streptomyces aureofaciens produces chlortetracycline. They are broad spectrum antibiotics with similar antimicrobial spectra, and cross resistance of bacteria to them is common.
13. Which of the following inhibits protein synthesis by combining with the 50S subunit ribosome?
a) Streptomycin
b) Tetracycline
c) Chloramphenicol
d) Penicillin
View Answer
Explanation: Chloramphenicol inhibits protein synthesis by combining with the 50S subunit ribosome. The transpeptidation and translocation functions associated with this site are blocked.
14. Tyrocidines are more effective against ___________________
a) Gram-positive organisms
b) Gram-negative organisms
c) Mycoplasmas
d) Spirochetes
View Answer
Explanation: Tyrocidines and gramicidines are more effective against Gram-positive organisms whereas polymyxins are particularly effective against Gram-negative bacteria.
Sanfoundry Global Education & Learning Series – Microbiology.
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