A bacterial culture (200 µl containing $1.8 \times 10^9$ cells) was treated with an antibiotic Z (50 µg per ml) for 4 h at 37°C. After this treatment, the culture was divided into two equal aliquots. Set A: 100 µl was plated on Luria agar. Set B: 100 µl was centrifuged, the cell pellet washed and plated on Luria agar. After incubating these two plates for 24 h at 37°C, Set A plate showed no colonies, whereas the Set B plate showed $0.9 \times 10^9$ cells. This experiment showed that the antibiotic Z is
Antibiotic Z Bacteriostatic Action Explained
The experiment involved treating a bacterial culture (initially $1.8 \times 10^9$ cells in 200 µl) with antibiotic Z ($50$ µg/ml) for $4$ hours at $37$°C.
Experimental Setup and Results
The treated culture was divided into two sets:
- Set A: $100$ µl plated directly on Luria agar. Result: No colonies observed after $24$ h incubation.
- Set B: $100$ µl centrifuged, cells washed, then plated on Luria agar. Result: $0.9 \times 10^9$ cells observed after $24$ h incubation.
Interpreting the Results
The key difference lies in the washing step for Set B. Washing removes the antibiotic Z from the vicinity of the cells. The observation of $0.9 \times 10^9$ cells in Set B indicates that the bacteria were still viable (alive) after the antibiotic treatment. If the antibiotic were bacteriocidal (killing) or bacteriolytic (bursting cells), washing would not restore their ability to be present or form colonies.
Understanding Antibiotic Effects
- Bacteriostatic: Inhibits bacterial growth and reproduction but does not kill the cells.
- Bacteriocidal: Kills bacteria.
- Bacteriolytic: Causes bacterial cells to lyse or burst.
Conclusion
Since the bacteria remained viable after washing (Set B) and showed no growth when plated directly with the antibiotic (Set A), antibiotic Z likely prevented the bacteria from growing or reproducing, rather than killing them. This defines a bacteriostatic effect.
Important Questions from Antibiotics and Antimicrobial Resistance
- Which of the following statement(s) is(are) TRUE about fluoroquinolone drugs?
- Which of the following antimicrobial agent(s) is/are growth factor analog(s)?
- A list of pathogens (Group I) and a list of anti-microbial agents (Group II) used to treat their infections are given below. Match the pathogens with the corresponding anti-microbial agents.
Group I Group II [P] Influenza A virus 1. Isoniazid [Q] Fungus 2. Amantadine [R] Plasmodium 3. Fluconazole [S] Mycobacterium 4. Artemisinin 5. Iodoquinol Match the antibiotics in Group I with their modes of action in Group II.
Group I Group II P) Chloramphenicol 1) Inhibits protein synthesis by acting on 30S ribosomal subunit Q) Rifampicin 2) Interferes with DNA replication by inhibiting DNA gyrase R) Tetracycline 3) Inhibits protein synthesis by acting on 50S ribosomal subunit S) Quinolone 4) Interferes with RNA polymerase activity 5) Inhibits $\beta$-lactamase activity - Tetracycline inhibits the
