Different antibiotics attack bacteria through different chemical mechanisms:

• Penicillin and related drugs block the enzyme that builds bacterial cell walls. Without walls, bacteria burst from osmotic pressure. Human cells have no cell walls, so penicillin leaves us unharmed.
• Tetracycline binds to bacterial ribosomes and blocks protein synthesis. Bacterial ribosomes differ in structure from human ones.
• Ciprofloxacin blocks DNA gyrase, an enzyme bacteria need to unwind and copy their DNA.
• Sulfonamides block the enzyme bacteria use to make folic acid, which they need to build DNA.