Bacteriostatic and bactericidal antibioticsSpec 6.13
In short
Bactericidal antibiotics kill bacteria, for example penicillin, which prevents cell wall formation so the bacteria burst by osmosis. Bacteriostatic antibiotics stop bacteria growing and dividing, for example tetracyclines, which block protein synthesis at the ribosome. The immune system then removes the bacteria.
| Feature | Bacteriostatic | Bactericidal |
|---|---|---|
| Effect on bacteria | Prevents growth and reproduction; bacteria stay alive | Kills bacteria |
| Typical mechanism | Inhibits protein synthesis by binding to bacterial (70S) ribosomes, or blocks folic acid synthesis (sulfonamides) | Prevents formation of cross-links in the peptidoglycan cell wall, so the wall is weak and the bacterium bursts by osmosis; or damages the cell membrane; or blocks DNA or RNA synthesis (rifampicin inhibits bacterial RNA polymerase) |
| Examples | Tetracycline, chloramphenicol | Penicillin, rifampicin |
| Depends on host immune system? | Yes: the immune system must remove the bacteria once they stop multiplying | Less so: important when the immune system is weak |
Antibiotics show selective toxicity: they target structures or processes in bacteria that human cells do not have (a peptidoglycan cell wall, 70S ribosomes). This is why they do little harm to human cells, and why they have no effect on viruses, which have no cell wall or ribosomes of their own and replicate inside human cells.
Overuse and misuse of antibiotics exerts selection pressure: bacteria with a mutation that gives resistance survive, reproduce and pass the allele on, producing antibiotic-resistant strains such as MRSA.
Do not say 'penicillin kills bacteria by bursting the cell'. Say it stops the cell wall being formed or repaired, so water enters by osmosis and the weakened wall cannot resist, and the bacterium bursts.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026