Biology 5.16 - Antibiotics
Antibiotics inhibit processes in bacterial cells. Use differences between bacteria, human cells and viruses to explain selective treatment and its limits.
Selective antibiotic action
An antibiotic is a medicine that can be used to treat a bacterial infection. Some antibiotics kill bacteria; others stop the bacteria growing or reproducing, giving the body's defences a better chance to remove them. This does not mean that every bacterial infection needs an antibiotic, but an antibiotic's treatment target is bacterial rather than viral.
The action happens at the scale of cells. A bacterium is a cell with structures and chemical processes that it needs to survive and reproduce. An antibiotic inhibits one of these processes. If the bacteria die or stop increasing in number, the infection can be brought under control.
For treatment to be useful, the antibiotic must be selective: its target should be present in the bacterium but absent from, or sufficiently different in, the cells of the host organism. The host is the person or other organism carrying the infection.
One clear example is bacterial cell-wall production:
- A growing bacterium needs to build and maintain its cell wall.
- Some antibiotics inhibit enzymes involved in making that wall.
- The bacterium cannot maintain this essential structure, so its growth is stopped or it dies.
- Human cells do not have a bacterial cell wall, so that bacterial process is not available as the same target in the host.
Other antibiotics inhibit bacterial protein production by acting on bacterial ribosomes. Host cells also make proteins, but their ribosomes are structurally different, allowing an antibiotic to act much more strongly on the bacterial machinery. Selectivity is not the same as being harmless: medicines can still cause side effects, but their intended action is on the bacterium rather than the host's cell processes.
Antibiotic treatment depends on a difference between bacterial and host cells: bacterial target inhibited -> bacterial growth or survival reduced -> host cells are not the intended target.
A laboratory test of human cells only shows effects on those cells in those conditions. It cannot reveal every possible effect on other cell types, organs, doses or the whole person. A strong bacterial effect with high human-cell survival is evidence of selectivity, not proof that side effects are impossible.
Bacteria, hosts and viruses
Viruses are not bacterial cells. They do not have a bacterial cell wall or bacterial ribosomes, and they use the machinery inside infected host cells to make new virus particles. An antibiotic aimed at bacterial wall production or bacterial ribosomes therefore has no matching target in a virus.
| Biological system | Relevant feature | Consequence for an antibiotic |
|---|---|---|
| Bacterium | Has its own cellular structures and processes | A suitable bacterial process can be inhibited |
| Human host cell | Has no bacterial cell wall and has different ribosomes | The bacterial target is absent or different |
| Virus | Has no bacterial cell machinery and reproduces using host cells | An antibiotic has no bacterial target to inhibit |
This is why deciding whether an infection is bacterial or viral matters. A patient with influenza has a viral infection, so an antibiotic will not stop the influenza virus reproducing. If a person with a viral illness also developed a separate bacterial infection, an antibiotic could be used for that bacterial infection, not for the virus itself.