Final answer:
Metronidazole is selectively toxic to microbes because it targets cellular processes unique to microbes or those that differ significantly from human cellular processes, such as cell wall synthesis and specific metabolic pathways. Its selective activation in anaerobic environments typical of certain bacteria avoids damage to human cells. The challenge of finding such selective targets is greater for drugs against eukaryotic pathogens like fungi and protozoa, due to the similarity of their cells to human cells.
Step-by-step explanation:
The characteristic of metronidazole that makes it selective for harming microbes instead of humans lies in its ability to exploit the differences between prokaryotic and eukaryotic cells. In bacterial cells, metronidazole becomes activated in the anaerobic environment and causes damage to DNA. The selective toxicity of metronidazole and similar antibacterial drugs is largely due to their exploitation of these unique targets present in microbial cells but absent or significantly different in human cells, such as cell wall synthesis, protein synthesis, and metabolic pathways.
For example, antimicrobial drugs that inhibit cell wall synthesis target the bacterial peptidoglycan layer, which is not part of human cell structure. Anti-protozoan and antiviral drugs must also operate selectively within eukaryotic cells or virus-infected cells to avoid harming the host. This often involves targeting specific life cycle stages or enzymes that are unique to the pathogens and not found in human cells. In contrast, the treatment for eukaryotic pathogens such as fungi, protozoa, and helminths is more challenging due to the similarity of their cells to human cells, thus complicating the development of drugs with selective toxicity.
With a limited number of antibiotics available for human treatment that meet the criteria of selective toxicity, only those that are effective against pathogens without being toxic to the host make the cut for clinical use. This principle is paramount in ensuring effective treatment of infections with minimal side effects to patients.