An antibiotic is a chemical that kills bacteria or inhibits their growth, without harming the cells of the host. Many antibiotics were first found as substances made by fungi or bacteria to suppress competing microorganisms. The first, penicillin, was discovered by Alexander Fleming in 1928, when he noticed that the mould Penicillium had stopped bacteria growing on an agar plate. Many modern antibiotics are made synthetically or are chemically modified versions of natural ones.Antibiotics work because of selective toxicity: they target structures or processes that bacteria have but human cells do not, or that are different in bacteria. The main targets are:
• the peptidoglycan cell wall (human cells have no cell wall);
• 70S ribosomes (human cytoplasm has 80S ribosomes);
• enzymes of metabolic pathways found only in bacteria, such as the pathway for making folic acid;
• the enzyme DNA gyrase, used in bacterial DNA replication;
• the bacterial cell membrane.Antibiotics have no effect on viruses. Viruses have no cell wall, ribosomes or metabolism of their own; they use the host cell's machinery. Prescribing antibiotics for viral infections such as colds and flu does not help and increases the chance of resistance.The effectiveness of an antibiotic can be tested by placing paper discs soaked in it on an agar plate spread with bacteria. After incubation, a clear zone of inhibition forms around each disc where bacteria have not grown. For discs of equal concentration, the larger the zone, the more effective the antibiotic is against those bacteria.
• the peptidoglycan cell wall (human cells have no cell wall);
• 70S ribosomes (human cytoplasm has 80S ribosomes);
• enzymes of metabolic pathways found only in bacteria, such as the pathway for making folic acid;
• the enzyme DNA gyrase, used in bacterial DNA replication;
• the bacterial cell membrane.Antibiotics have no effect on viruses. Viruses have no cell wall, ribosomes or metabolism of their own; they use the host cell's machinery. Prescribing antibiotics for viral infections such as colds and flu does not help and increases the chance of resistance.The effectiveness of an antibiotic can be tested by placing paper discs soaked in it on an agar plate spread with bacteria. After incubation, a clear zone of inhibition forms around each disc where bacteria have not grown. For discs of equal concentration, the larger the zone, the more effective the antibiotic is against those bacteria.