Penicillin

Penicillin, antibiotic derived from the mould Penicillium notatum. The action of this antibiotic was first observed in 1928 by the British bacteriologist Sir Alexander Fleming, but it was another ten years before penicillin was concentrated and studied by the British biochemist Ernst Chain, the Australian pathologist Sir Howard Florey, and other scientists.

Penicillin acts both by killing bacteria and by inhibiting their growth. It does not kill organisms in the resting stage but only those that are growing and reproducing. It is effective against a wide range of disease-bearing micro-organisms, including pneumococci, streptococci, gonococci, meningococci, the clostridium of tetanus, and the syphilis spirochaete. The drug has been successfully employed to treat such deadly diseases as subacute bacterial endocarditis, septicaemia, gas gangrene, gonorrhoea, and scarlet fever. Toxic symptoms produced by penicillin are limited largely to allergic reactions which can be determined by scratch tests before administration of the drug.

Despite the effectiveness displayed by penicillin in curing a wide range of diseases, infections caused by certain strains of staphylococci could not be cured by the antibiotic as a result of the ability of the organism to produce an enzyme, penicillinase, capable of destroying the antibiotic. In addition, enterococci and many gram-negative bacilli known to cause respiratory and urinary-tract infections were found to be intrinsically resistant to the action of penicillin. Appropriate chemical treatment of a biological precursor to penicillin, isolated from bacterial cultures, resulted in the formation of a number of so-called semi-synthetic penicillins. The most important of these are Methicillin and Ampicillin. Methicillin is remarkably effective against penicillinase-producing staphylococci and Ampicillin is not only active against all organisms normally killed by penicillin, but also inhibits enterococci and most gram-negative bacilli.

The strength and dosage of penicillin are measured in terms of international units. Each of these units is equal to 0.0006 g of the crystalline fraction of penicillin called penicillin G. In the early days of penicillin therapy, the drug was administered every three hours in small doses. More recently a preparation called benzathine penicillin G has been produced that provides detectable levels of antibiotic for as long as four weeks after a single intramuscular injection. It is useful for treatment of syphilis and strep throat. Bacterial resistance to penicillin has increased over the years, causing a need for alternative drugs and increases in penicillin dosage.