Antibiotics

Antibiotics are chemicals that kill bacteria or slow their growth without harming the cells of the person taking them. Many were first found in fungi and other bacteria; for example, penicillin was discovered from the mould Penicillium.Antibiotics work because bacterial (prokaryotic) cells have structures and enzymes that human (eukaryotic) cells do not, so the drug can target these differences. Common targets are:1. Cell wall synthesis. Penicillin stops bacteria forming the cross-links in their peptidoglycan cell wall. As the bacterium grows, the weakened wall cannot resist the pressure of water entering by osmosis, so the cell bursts (lysis). Human cells have no cell wall, so they are unaffected.2. Protein synthesis. Antibiotics such as tetracycline bind to bacterial 70S ribosomes and stop translation. Human cytoplasmic ribosomes are 80S, so they are not affected in the same way.3. Nucleic acid synthesis or enzyme pathways found only in bacteria.Antibiotics are grouped by their effect:Bactericidal antibiotics kill bacteria, for example penicillin.Bacteriostatic antibiotics stop bacteria growing and dividing, for example tetracycline. The body's immune system then destroys the bacteria.Antibiotics have no effect on viruses. Viruses have no cell wall, no ribosomes of their own and no metabolism; they reproduce inside host cells using the host's machinery. This is why antibiotics are not prescribed for colds or flu.