Final answer:
An antimicrobial drug is selectively toxic because it specifically targets the unique features of microbial cells that differ from human cells, such as peptidoglycan in bacterial cell walls, minimizing harm to the human host.
Step-by-step explanation:
An antimicrobial drug is selectively toxic because it targets specific features of microbial cells that are different from those of human cells. For example, antibacterial compounds can target the bacterial cell wall synthesis process which involves peptidoglycan—a component not found in human cells—via substances like β-lactams, glycopeptides, and bacitracin. This focus on unique microbial structures or enzymes allows these drugs to be effective against specific microbes while minimizing harm to the human host.
The reason why antimicrobial drugs are easier to develop against bacteria rather than eukaryotic organisms, such as fungi, protozoa, and helminths, or viruses, is because bacteria are prokaryotic and have a greater number of distinctive targets for selective toxicity. Moreover, viruses present additional challenges as they replicate within human host cells, making it difficult to target the virus without affecting the host cell.
It is also important to understand that antimicrobial drugs can be bacteriostatic or bactericidal. Bacteriostatic drugs inhibit the growth of bacteria, and their effects are reversed when the drug is removed, while bactericidal drugs kill bacteria outright. The choice between bacteriostatic and bactericidal drugs depends on the type of infection and the patient's immune status.