ELI5: How do antibiotics kill bacteria without harming our cells?

Antibiotics can kill bacteria without harming our cells because bacteria are built completely differently from human cells, allowing the medicine to target things the germs have that we do not.

Think of your body as a city made of normal houses, while bacteria are like invaders driving heavily armored tanks. If you want to stop the invaders without damaging the neighborhood, you use a special tool that only destroys tank armor, leaving the houses completely untouched.

In everyday life, this works just like using a weed killer on your lawn. The spray attacks weeds because their leaves and roots work differently than grass; it destroys the dandelion while your green grass stays healthy.

Antibiotics act like that weed killer by attacking three main differences:

• Cell Walls: Bacteria protect themselves with a tough outer shell called a cell wall, like a crusty suit of armor. Human cells only have a soft outer skin called a cell membrane. Antibiotics like penicillin punch holes in this armor. Without their walls, the bacteria pop and die, but our soft cells are ignored because they never had that armor in the first place.
• Protein Factories: Both human cells and bacteria need tiny machines called ribosomes to make food and grow. However, bacterial machines are built like miniature puzzles that fit together differently than ours. Certain antibiotics act like chewing gum tossed into those specific machines, jamming them up so the bacteria starve, while our own machines keep running smoothly.
• Copying DNA: To multiply, bacteria have to copy their instructions using special tools that twist and untangle their DNA. Some antibiotics lock up those specific bacterial tools, stopping the germs from having babies, while leaving human DNA alone.