Q.47 A bacterial culture (200 μl containing
1.8 × 109 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 × 109 cells. This experiment showed that the
antibiotic Z is
Unraveling Antibiotic Action: Bacteriostatic vs Bactericidal in a Key Microbiology Experiment
This article dives into a classic microbiology experiment testing antibiotic Z on bacterial cells, revealing whether it’s bacteriostatic or bactericidal. Perfect for biotech students and researchers exploring antibiotic mechanisms.
Experiment Overview
Bacterial cultures and antibiotics form the backbone of microbiology and biotechnology research. Imagine treating a dense bacterial population—1.8 × 109 cells in 200 μl—with antibiotic Z at 50 μg/ml for 4 hours at 37°C. The culture splits into two 100 μl aliquots: one plated directly (Set A), the other centrifuged, washed, and plated (Set B). After 24 hours at 37°C, Set A shows zero colonies, but Set B reveals 0.9 × 109 cells. What does this reveal about antibiotic Z?
Correct Answer: (A) bacteriostatic
Why Bacteriostatic? Step-by-Step Experiment Breakdown
Initially, the 200 μl culture holds 1.8 × 109 cells, so each 100 μl aliquot starts with 0.9 × 109 cells.
- Set A (direct plating): No colonies form. Antibiotic Z remains in the media on Luria agar, inhibiting visible growth.
- Set B (washed cells): Centrifugation pellets the cells, washing removes Z, and plating yields 0.9 × 109 colonies—matching the original aliquot count.
The near-perfect recovery in Set B proves all cells survive; they just couldn’t grow while exposed to Z. This defines bacteriostatic action: antibiotics like tetracycline halt bacterial growth (e.g., by blocking protein synthesis) without killing cells. Once removed, bacteria resume dividing, forming colonies.
No cell death occurs—colonies match pre-treatment numbers, ruling out killing. This mirrors real-world tests distinguishing static from cidal effects in antimicrobial susceptibility.
Explaining All Options: Bacteriostatic vs Bactericidal Differences
Understanding options clarifies antibiotic classification in biotech and clinical settings. Here’s a breakdown:
- (A) Bacteriostatic: Inhibits growth reversibly. Matches the experiment—cells viable post-wash, no loss in count. Key phrase: antibiotic Z bacteriostatic experiment highlights recovery after removal.
- (B) Bactericidal: Kills bacteria outright (e.g., penicillin disrupts cell walls). If true, Set B would show far fewer (<0.9 × 109) or zero colonies, as dead cells can’t recover. Experiment disproves this—no killing evident.
- (C) Bacteriolytic: A bactericidal subset causing cell lysis (rupture), like beta-lactams. Dead, lysed cells wouldn’t form colonies in Set B. The full 0.9 × 109 recovery eliminates this.
- (D) Apoptotic: Refers to programmed cell death in eukaryotes (e.g., via caspases). Bacteria lack apoptosis machinery; they use different stress responses. Irrelevant here—purely a distractor for bioengineering students.
| Option | Mechanism | Set B Outcome if True | Matches Experiment? |
|---|---|---|---|
| (A) Bacteriostatic | Stops growth, reversible | Full colony recovery | Yes |
| (B) Bactericidal | Kills cells | Reduced/no colonies | No |
| (C) Bacteriolytic | Lysed cells | No colonies | No |
| (D) Apoptotic | Eukaryote-specific death | Reduced/no colonies | No |
Implications for Biotechnology and Research
This setup tests antibiotic persistence. Bacteriostatic drugs suit low-virulence infections but risk resistance if host immunity falters. In fermentation or microbial kinetics (your biotech wheelhouse), distinguishing these guides strain engineering.
For enzyme kinetics fans, think Michaelis-Menten: Z likely binds reversibly, like a competitive inhibitor, freeing cells post-wash.
Replicate in lab: Use OD600 for growth curves pre/post-exposure to quantify static vs cidal effects mathematically.
