General Microbiology — SCT I [Vol. 3]

Question 1

Describe the rules of collecting native urine for culturing bacteria! Describe also the conditions for keeping the specimen before sending it to the laboratory!

Answer 1

A midstream specimen, taken preferably in the morning, after thorough cleaning of the external genitals

Keep it up to 1 hour at room temp. or up to 24 hours at 4 °C

Question 2

Describe the rules of collecting hemoculture specimens!

Answer 2

Recommended to obtain 3 specimens

Preferably before fever spikes

From fresh peripheral venipuncture sites

Both aerobic and anaerobic bottles should be used

Venipuncture and plug of the bottle (containing the medium) must be properly disinfected

Amount of blood should be about 10% of the liquid medium

Specimen should be taken before any antibiotics given to patient

Question 3

Mention two clinical specimens that are heavily contaminated with bacteria belonging to the normal body flora!

Answer 3

1. Faeces
2. Throat/nasal secretions
3. Vaginal secretions

Question 4

Explain the purpose of using beta-lactamase inhibitors in antibacterial therapy!

Answer 4

Clavulanic acid + Ampicillin is an example of a combination that can overcome resistance mediated by many but not all beta-lactamases

Question 5

Mention a group of antimicrobial drugs that acts on the bacterial ribosome and usually has bactericidal effect!

Answer 5

Aminoglycosides

Question 6

Mention 2 broad-spectrium (effective against both Gram positive and Gram negative bacteria) pencillin derivatives.

Answer 6

1. Ampicillin
2. Amoxicillin
3. Piperacillin

Question 7

Mention an antifungal and an antibacterial drug that alter the function of the cell membrane!

Answer 7

Antifungal: amphotericin B

Antibacterial: polymyxins

Question 8

What are the possible mechanisms of acquired penicillin resistance of bacteria? Mention at least 3 mechanisms!

Answer 8

1. Active efflux
2. Reduced permeability
3. Beta-lactamase production
4. PBP alteration

Question 9

Mention a drug belonging to macrolides!

Answer 9

Erythromycin

Question 10

Mention 2 drugs belonging to aminoglycosides!

Answer 10

1. streptomycin
2. gentamicin

Question 11

Mention 2 groups of antimicrobial drugs that act on the 30 S subunit of the bacterial ribosome!

Answer 11

1. Aminoglycosides
2. Tetracyclines

Question 12

Mention 3 antimicrobial drugs that act on the 50 S subunit of the bacterial ribosome!

Answer 12

1. Erythromycin
2. Clindamycin
3. Chloramphenicol

Question 13

Mention 3 groups of antibacterial drugs inhibiting bacterial cell wall synthesis!

Answer 13

1. Pencillins
2. Glycopeptides
3. Cephalosporins

Question 14

Mention 4 groups of antibiotics which have bactericidal effects!

Answer 14

1. Penicillins
2. Aminoglycosides
3. Cephalosporins

Question 15

Which antibiotics inhibit bacterial DNA gyrase/topoisomerase enzymes?

Answer 15

1. Naldixic acids
2. Fluoroquinolons

Question 16

What is the mode of action of the antibiotics sulfonamides and trimethoprim, respectively?

Answer 16

They inhibit the synthesis of folic acids:

Sulfonamides: inhibit the synthesis of dihidrofolate

Trimethoprim: inhibition of dihidrofolate-reductase

Question 17

What is the mode of action of the antibiotic vancomycin?

Answer 17

Inhibits cell wall synthesis in Gram positive bacteria by blocking transglycosylation/transpeptidation

Question 18

Mention 4 groups of antibiotics inhibiting protein synthesis of bacteria!

Answer 18

1. Aminoglycosides
2. Chloramphenicol
3. Macrolides
4. Tetracyclines

Question 19

Give an example of antagonistic interaction between 2 antibiotics!

Answer 19

1. Pencillin + gentamicin
2. sulfonamide + trimethoprim

Question 20

Give an example of antagonistic interaction between 2 antibiotics!

Answer 20

Pencillin + tetracycline