What is Antibiotics?

An antibiotic is a molecule that kills bacteria or stops their growth at concentrations harmless to the host, by acting on a target the host lacks or has in a different form — selective toxicity. The targets are few. The cell wall: $\beta$-lactams (penicillins, cephalosporins, carbapenems) mimic the terminal D-Ala-D-Ala of the peptidoglycan peptide and acylate the enzymes that cross-link it, so that the growing wall is weak and the cell bursts under its own turgor; vancomycin binds the D-Ala-D-Ala itself. The ribosome, whose bacterial form differs from the eukaryotic: aminoglycosides (the small subunit, causing misreading), tetracyclines (blocking tRNA entry), macrolides and chloramphenicol (the large subunit’s exit tunnel and peptidyl transferase). DNA gyrase and topoisomerase IV: the quinolones. RNA polymerase: rifampicin. Folate synthesis, which animals do not do: sulfonamides and trimethoprim. The membrane: the polymyxins, a last resort. The minimum inhibitory concentration (MIC) is the lowest concentration of a drug that prevents visible growth of a strain; a strain is called resistant when its MIC exceeds the concentration the drug reaches in the patient.