In nature, there are moulds and bacteria that attack other organisms in order to obtain nourishment and to reproduce. Antibiotics are chemical substances produced by organisms (usually microorganisms) that are harmful to other bacteria and life forms. If you moisten a piece of stale bread and keep it in the open, there will be a greenish growth on it a little later. That is because of the spread of a mould called Penicillium Notatum. It produces penicillin. Penicillin is a highly effective antibiotic. Most bacteria double every half an hour. Just one bacterium (singular of bacteria) becomes thousands within a few hours. An antibiotic does not permit the multiplication of bacteria. In many cases, the antibiotic does not permit even the survival of bacteria.