Control of Microbial Growth by Antibiotics

Question 1

5 ways antibiotics inhibit or kill micro-organisms

Answer 1

1. interfering with the synthesis of the bacterial cell wall
2. inhibiting protein synthesis
3. inhibiting nucleic acid synthesis
4. disrupting the plasma membrane
5. affecting the synthesis if metabolites essential to the bacteria

Question 2

Mechanism of action used by the 3 different antibiotics used in the lab?

Answer 2

Penicillin G: inhibit synthesis of cell wall
Streptomycin: interferes with protein synthesis
Norfloxacin: interferes with nucleic acid synthesis

Question 3

Difference between disk diffusion and E-test methods to test antibiotic susceptibility?

Answer 3

Disk diffusion: tests whether bacteria are affected by antibiotics. The bigger the inhibition zone, the more effective the antibiotic will be.

E-test: determines what concentration of antibiotic is suitable.

Question 4

Explain the disk diffusion method?

Answer 4

placing commercially prepared antibiotic-impregnated discs of known concentration on bacteria plated on an agar medium. the plates are incubated. the antibiotic diffuses away from the disc. zone of inhibition is measured to test the susceptibility of the antibiotic.

Question 5

Explain the E-test method?

Answer 5

rectangular strip that has been impregnated with the drug to be studied is placed on an agar plate. bacteria is spread and grown on an agar plate, and the Etest strip is laid on top; the drug diffuses out into the agar, producing an exponential gradient of the drug to be tested. after incubation, an elliptical zone of inhibition is produced and the point at which the ellipse meets the strip gives a reading for the minimum inhibitory concentration (MIC) of the drug.

Question 6

What is the zone of inhibition?

Answer 6

area of no growth around the disc

Question 7

why should the zones of inhibition of antibiotic discs on the same plate not be compared but can be compared to the same disc used on a different species of bacteria?

Answer 7

Same plate: may only reflect the solubility or rate of diffusion of an antibiotic.

different species: given the same conditions, they will diffuse at the same rate

Question 8

What is a natural antibiotic?

Answer 8

made from microbial sources

Question 9

What is a semi-synthetic antibiotic?

Answer 9

made partially by chemicals

Question 10

What is a synthetic antibiotic?

Answer 10

made entirely from chemicals

Question 11

What is the minimum inhibitory concentration (MIC)?

Answer 11

lowest concentration of an antibiotic (mg/ml) that prevents the growth of the bacteria being tested