Selective Toxicity

Selective Toxicity, can be defined as, the capability of an antibiotic to harm one type of microorganism without harming other organism (humans) that is intimately associated with it.
This principle is used in agriculture, pharmacology and diagnostic microbiology. It is mostly important in the 'systemic chemotherapy of infectious diseases."

1. Selective action against microorganisms is based on the differences in the cell physiology of the parasite and the mammalian host.
2. Energy yielding processes do not offer possibilities for selective toxicity.
3. Chemicals that inhibit a particular step in a metabolic pathway that is important to the parasite, but that does not occur or is not accessible in the cells of the host, exhibit selective toxicity.
4. The treatment of bacterial infections is more successful than that of viral diseases. This is because viruses depend on many enzymes of the host cell for their replication.
5. A high degree of selective toxicity may be associated with a narrow antimicrobial spectrum and with the emergence of drug-resistant organisms.

Selective toxicity of an antibiotic depends on the therapeutic index i.e; the ratio of toxic dose to the therapeutic dose. The larger the index, more safe is the antibiotic for human use.

Example:

• Protein synthesis inhibitors such as, streptomycin or tetracycline  act only on the protein synthesis of the microorganism because, bacterial ribosomes are different from the human or eukaryotic ribosomes.
• Penicillin antibiotics act on the cell wall of the microorganisms. They are safe for the human beings as they do not have a cell wall.