Antibiotics

An antibiotic is a chemical that kills bacteria or stops them growing, without harming the cells of the host. Many antibiotics were first found as substances made by microorganisms (penicillin comes from the fungus Penicillium), although many are now made or modified in the laboratory.Antibiotics are described by their effect:Bactericidal antibiotics kill bacteria. Penicillin is bactericidal: it inhibits the enzymes that form cross-links in peptidoglycan. As the bacterium grows, its wall becomes weak, water enters by osmosis and the cell bursts (osmotic lysis).Bacteriostatic antibiotics stop bacteria multiplying without killing them, giving the immune system time to destroy them. Tetracycline is bacteriostatic: it binds to the 30S subunit of 70S ribosomes and stops tRNA binding, so protein synthesis stops.Antibiotics work because they target features that bacteria have but human cells do not, such as the peptidoglycan cell wall and 70S ribosomes. This is selective toxicity. Other targets include bacterial DNA replication and bacterial metabolic pathways.Antibiotics have no effect on viruses. Viruses have no cell wall, no ribosomes and no metabolism of their own, so there is nothing for an antibiotic to act on.Bacteria can become resistant to antibiotics. A random mutation may give resistance; when the antibiotic is used, resistant bacteria survive and reproduce (natural selection), and the resistance gene can also spread on plasmids. Overuse and not finishing a course make this more likely.