Antibiotics and Penicillin.

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Laura Wing                Biology PAC

Antibiotics: Penicillin

Introduction

Antibiotics belong to a group of chemical substances referred to as secondary metabolites. Antibiotics inhibit the growth of, or kill, bacteria. The first naturally-occurring antibiotic and the first to be used therapeutically was penicillin. It was first discovered by Alexander Fleming in 1929 almost by accident. He was growing plates of pathogenic bacteria and noticed that, when they were contaminated with the mould Penicillium notatum, growth of the bacteria was inhibited. Penicillin was isolated from the medium and found to be responsible for the effect. Since the 1940’s, Penicillin and other antibiotics of fungal origin have produced a revolution in the history of medicine to the extent that many major infectious diseases have largely been brought under control.

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As previously mentioned in the introduction, antibiotics belong to a group of chemical substances referred to as secondary metabolites. Secondary metabolites are substances (e.g. penicillin) that are by-products of processes that are not essential to life, but which have a useful role. Penicillin is an antibiotic made to kill off competitors when nutrients are scarce. The rate of production of a secondary metabolite is fastest during stationary phase when nutrients are beginning to run out. The concentration of a secondary metabolite rises throughout the stationary phase of production.

Penicillin works by preventing the cell wall of a bacteria cell from forming. The cell wall of all bacteria is critical to maintenance of the integrity of the cell, if the cell wall cannot be made for any reason, the cell will rupture and die. Bacteria reproduce to make two daughter cells. During this cell division process, the individual cell will enlarge to about twice the usual size – cytoplasmic substances will be made and the single chromosome of DNA will be replicated. The two chromosomes will separate and move apart from one another as the cell enlarges, the cell wall will be synthesised and finally the two new cells will separate. Penicillin works at this particular stage – the synthesis of the new cell wall. Penicillin will not harm any cell wall already made.

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Penicillin blocks the action of the enzyme transpeptidase and forms an irreversible connection to the active site (covalently connects). As the bacterial cell is growing (making a new cell wall) and dividing, penicillin prevents the cross-linking of small peptide chains in the cell wall of the bacteria. Pre-existing walls are unaffected, but all newly produced cells grow abnormally, unable to maintain their wall rigidity, and are susceptible to osmotic lysis.

The production of penicillin can be used to illustrate the principle of an industrial fermenter. Huge fermentation vats filled with liquid and nutrients necessary for moulds to grow ...

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