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These are drugs used to treat or cure infections and to be effective they must kill or disable the pathogen, leaving host cells unharmed. Most antibiotics are used to treat bacterial and fungal infections, there are very few that are effective against viruses. A few antibiotics are synthetic but most are derived from living organisms. They work by either interfering with the growth or metabolism of the bacteria or fungi. They may inhibit the synthesis of the cell wall, translation or transcription of proteins, interfere with membrane function or enzyme action.

Antibiotics need to be carefully chosen. This is done by screening them against the strain of bacterium or fungus obtained from the sufferer. The samples obtained are grown on agar plates and antibiotic discs placed on to the plate. The disc with the greatest diameter of inhibition zone, is the most effective. Broad spectrum antibiotics are effective against a wide range of bacteria, while narrow spectrum antibiotics affect only a few.

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Penicillins are well known antibiotics, which work by preventing the synthesis of peptidoglycan polymer cross links in the cell walls of bacteria. They are only effective when the organism is making new cell walls, i.e. growing. Many bacteria are now resistant to penicillin as they have penicillinases (enzymes which destroy penicillin). Resistance to antibiotics, is coded for by small rings of DNA found in bacteria, called plasmids.

Some bacteria may contain up to five plasmids, each conferring resistance to a different antibiotic. DNA and therefore plasmids can be passed between members of the same species of bacteria during conjugation, or sexual reproduction. Specialised tubes or pili join one bacterium to another during conjugation.

Resistance to antibiotics is increasing and has a great impact on the treatment of a disease because it prolongs illness and increases mortality. Hospitals try and keep some antibiotics as a last resort and drug companies are continually looking for new ones.