Explain that antibiotics can only be used to treat bacterial infections…

The idea

Antibiotics work by targeting structures or processes found in bacteria but absent from human cells, so they damage the pathogen while leaving host tissue unharmed. Many antibiotics, such as penicillins, interfere with bacterial cell wall synthesis; human cells have no cell wall, so the drug cannot act on them. Others block bacterial ribosomes or enzymes for folic acid synthesis, which differ from human versions. Because viruses are not cells and reproduce only inside host cells using host machinery, antibiotics have nothing bacterial to attack, so they cannot treat viral infections such as influenza. This selective toxicity explains why antibiotics treat bacterial infections only.

What earns the marks

• Antibiotics inhibit processes or structures present in bacteria, for example cell wall synthesis or bacterial ribosomes.
• Human host cells lack these bacterial targets, so the antibiotic does not inhibit host cell processes.
• Viruses are not bacteria and lack the bacterial targets, so antibiotics cannot treat viral infections.

Common mistakes and how to put them right

• Thinking antibiotics kill all pathogens including viruses.

  Put it right: Antibiotics target bacterial structures and processes that viruses do not possess.

• Assuming antibiotics harm human cells as well as bacteria.

  Put it right: The targets are absent from host cells, so host processes are not inhibited.

• Believing antibiotics boost the immune system directly.

  Put it right: They act on the bacterium's own cell processes, not on host immunity.

Exam tips

• Link each antibiotic to a specific bacterial target, such as the cell wall, and state why the host lacks it.
• Use the term selective toxicity to show why host cells are unaffected.
• Contrast bacteria with viruses explicitly when explaining why antibiotics fail against viral infections.