Antibiotics

Antibiotics are an extremely important group of drugs isolated from natural sources. Penicillin was originally produced by growing the Penicillium notatum mold in small containers; the mycelium, or vegetative portion, formed a mat on the surface of the medium that contained the penicillin in solution. The process was difficult to operate and required a large labour force to inoculate and harvest the containers.

Penicillin production was revolutionized by the discovery that a strain of Penicilium chrysogenum mold (isolated from an overripe cantaloupe) would produce high yields when grown in deep culture. The growth of a mold is aerobic (it requires air), and in surface culture the growth and consequent production of antibiotic is limited by the rate at which air can diffuse into the medium. In submerged or deep culture, the mold is grown in large tanks supplied with a continnous flow of sterile air.

Though the medium used in fermentation varies for the particular antibiotic being produced, all contain a source of carbon (which may be lactose or glucose); nitrogen (in the from of ammonium salts); and trace elements (such as phosphorus, sulfur, magnesium, iron, zinc, and copper). If phenylacetic acid is added, the mold will utilize it and produce benzylpenicillin (penicillin G), whereas, if phenoxyacetic acid is added, phenoxymethylpenicillin (penicillin V) is obtained. In addition, corn-steep liquor may be added for penicillin production, and soybean meal and dried distiller’s residues for streptomycin; these help increase the yield of product. (Corn-steep liquor is prepared by steeping cleaned corn grain in water for about 40 hours at about 48 º C [118 º F]; the liquor is drawn off and evaporated at reduced pressure to a suitable concentration - about 55 percent solids. Corn-steep liquor stimulates penicillin formation due to certain amino acids, minerals, and precursors that it contains.) Before use the medium - as well as the fermenter and associated equipment - is steam sterilized, as bacterial contamination can destroy the antibiotic.

A large volume of concentrated, actively growing fungal suspension is required for the main fermenting tanks, to keep the fermentation time to a minimum. This is obtained in three stages. First, the selected culture is transferred from cold storage to a culture medium (agar) to produce an initial inoculum. This inoculum is then cultured in shake flasks to give a suspension. Finally, the suspension is grown in seed tanks in the plant for 24-28 hours to the desired volume and concentration before transfer to the main fermenters. Fermentation is continued for three to five days, during which the vessel is cooled - to keep the temperature between 23-27 º C (73-81 º F) - and stirred and aerated with sterilized air. The introduction of large volumes of air causes frothing, whish is controlled by the addition of antifoams such as lard oil, octadecanol, or silicones. When fermentation is complete, the mycelium is removed on a rotary filter and the penicillin extracted into an organic solvent (such as butyl acetate or methyl isobutyl ketone), after acidification. The free acids of penicillins are generally unstable and are converted into a metal salt by extraction with alkali, followed by freeze-drying of the extract, or by addition of a concentrated solution of a metal salt such as potassium acetate, whereby the potassium salt of the penicillin is precipitated. All products that are to be administered by injection are sterilized by passage through a sterilizing filter, followed by freeze-drying, precipitation, or crystallization under sterile conditions.