Why can an antimicrobial drug harm a microorganism without seriously harming human cells?

Antimicrobial drugs are able to harm microorganisms without causing serious damage to human cells primarily through the principle of selective toxicity. This means the drugs are designed to target specific structures or metabolic processes that are unique to bacteria or significantly different from those found in human cells.12

Key mechanisms include:

• Targeting Unique Structures: Many antibiotics, such as $\beta$-lactams, inhibit the synthesis of the bacterial cell wall, a structure that human cells do not possess.1
• Exploiting Biochemical Differences: While both bacteria and human cells synthesize proteins, the underlying machinery differs. Selectively toxic antibiotics bind to or inhibit bacterial ribosomes or enzymes without interfering with the corresponding human cellular components.13
• Differential Affinity: Some agents, like rifampin, target enzymes essential for RNA synthesis. These drugs have a much higher affinity for the bacterial version of the enzyme than for the human version, leaving human cells unaffected at therapeutic doses.1

While these drugs are highly effective, they are used because they possess a favorable chemotherapeutic index, meaning the therapeutic dose is lower than the amount required to cause significant toxicity to the patient.4