Microbiology Lab Review: Chapter 17

Lab Review

Chapter 17: Disk Diffusion Method

1. [CH-17-01] Explain the differences between antimicrobial drug and antibiotic –

Antimicrobial: Any compound that interferes with growth of microbes within host.
            Antibiotic: Substance produced by bacterial fungi that inhibit growth/kills bacteria.

2. [CH-17-02] What is the difference between a bactericidal and a bacteriostatic antibiotic? Give examples of both –

Bactericidal: Kills bacteria (Penicillin – Against Gram-positive, Streptomycin)
Bacteriostatic: Prevents more bacterial growth (Tetracycline)

3. [CH-17-03] How can you prove that antibiotic used in Kirby-Bauer (disk diffusion) test is bactericidal? –

Take a small sample of the microbe from the zone of inhibition and place it into a liquid medium. If growth = bacteriostatic. If NO growth = bactericidal.

4. [CH-17-04] How does the concentration of antibiotics vary within the zone of inhibition formed in the disk diffusion test? –

The concentration of the antibiotics is greatest at the center, decreasing the more distance away from the center.

5. [CH-17-05] If two different antibiotics produce zones of inhibition of the same size in disk diffusion test run with E. coli, can the assumption be made that these antibiotics would be equally effective in the treatment of the disease caused by the same bacterium in humans? Yes or No. Explain your answer –

No, the antibiotics could possibly have detrimental effects on human cells. The possible antibiotics must undergo additional tests before human testing can happen.

6. [CH-17-06] How would you explain the presence of isolated colonies within an otherwise clear zone of inhibition produced in Kirby-Bauer (disk diffusion) test? –

Specific cells of the bacteria being resistant to the antibiotic OR possible contamination with another species of bacteria.

7. [CH-17-07] What cellular processes the following antibiotics are affecting –
1. Ampicillin – Inhibits transpeptidase needed to make cell wall, Gram+/-
2. Bacitracin – Inhibits synthesis of bacterial cell wall, effective against Gram+
3. Ciprofloxacin – Inhibits DNA gyrase, type 2,4 topoisomerase = stops DNA replication, Gram+/-
4. Erythromycin – Reversibly inhibits protein synthesis, bacteriostatic, Gram+/-
5. Penicillin – Inhibits synthesis of bacterial cell wall, Gram+
6. Rifampin – Inhibits RNA synthesis (inhibits RNA polymerase), acid-fast (combo)
7. Streptomycin – Irreversibly inhibits protein synthesis, kills microbes. Gram +/-
8. Tetracycline – Reversibly inhibits protein synthesis, effective against Gram+/-

8. [CH-17-08] Explain the term “broad-spectrum antibiotic” –

Antibiotics that are effective against a wide variety of bacteria, Gram+/- (Ex: Streptomycin, Chloramphenicol, Tetracycline).

9. [CH-17-09] Why are Gram-negative bacteria generally less sensitive to antibiotic treatment than Gram-positive bacteria? –
   Gram-negative bacteria have 2 cell membranes with lipopolysaccharide on the outer cell membrane. VS. Gram-positive bacteria with only one cell membrane.

10. [CH-17-10] Tetracycline is effective against Gram-positive and Gram-negative bacteria. Why is this antibiotic not used in treatment of tuberculosis? –
    Tuberculosis is an acid-fast bacteria that is difficult to kill due to its highly hydrophobic cell wall with waxy-like material. Bacteriostatic drugs like tetracycline only reversibly inhibit protein synthesis. What is needed are bactericidal drugs like streptomycin with its irreversible inhibition of protein synthesis.