Madical brain Notes 10

  Antimicrobial Agents The term  chemotherapy  is most closely associated in the minds of most people with the treatment of cancer. In fact the term was first used by Paul Ehrlich to describe any use of a drug or other chemical substance for the treatment of disease; thus, it has much wider terms of reference. In our present discussion, we shall confine ourselves to chemotherapy as it relates to the treatment of infectious diseases. It was Ehrlich who, 100 years ago, observed how certain dyes would stain bacteria but not the surrounding tissues, leading him to formulate the idea of  selective toxicity , whereby a substance would selectively target harmful microorganisms but leave human tissues undamaged. He tested hundreds of synthetic compounds in the search for his ‘magic bullet’ before finding, in 1910, an arsenic-containing drug, Salvarsan, which was effective against  Treponema pallidum , the causative agent of syphilis (Figure 14.1). It was another 20 year...

  Antibiotics The other major breakthrough in the treatment of infectious diseases was of course the discovery of naturally occurring antimicrobial agents, or  antibiotics . These are metabolites produced by certain microorganisms, which inhibit the growth of certain other microorganisms. As we shall see, the definition has been extended to in-clude semisynthetic derivatives of these naturally occur-ring molecules. Table 14.1 lists some commonly used antibiotics. One of the best known of all stories of scientific discovery is that of how Sir Alexander Fleming discovered penicillin in 1928. Before we consider that, however, it is worth noting that a number of treatments for infectious diseases practised over the preceding   centuries can, with the benefit of hindsight, be regarded as a form of antibiotic therapy. Many hundreds of years ago for example, the Chinese used mouldy soybean curd in the treatment of boils and South American Indians controlled foot infections by we...

  What other properties should an antibiotic have?   Selective toxicity is the most important single attribute of an antibiotic, but ideally it should also have as many of the following properties as possible: ·                antibiotics, like other chemotherapeutic agents, need to be  soluble in body fluids , in order to exert their effect by penetrating the body tissues. The compound must not be metabolised so quickly that it is excreted from the body before having a chance to act. ·   if administered orally, it  must not be inactivated  by the acid environment of the stomach, and must be capable of being absorbed by the small intestine. ·                an antibiotic should not have any significant effecton the  resident microflora  of the host. ·          ...

  How do antibiotics work? All antibiotics have the common property of interfering in some way with a normal, critical function of the target bacterial cell. The most commonly used antibiotics exert their effect by one of the following methods: 1 Inhibition of cell wall synthesis (group I)   2   Disruption of cell membranes (group II)   3   Interference with protein synthesis (group III)   4   Interference with nucleic acid synthesis (group IV) Table 14.2 lists examples of each group. Those antibiotics belonging to groups I and III are better able to discriminate between procaryotic and eucaryotic cells, and  consequently show more selective toxicity and a higher therapeutic index.   Group I: Inhibitors of cell wall synthesis The main group which work in this way are the β-lactam antibiotics, so-called because they contain a β-lactam ring in their structure. Included among this group are the penicillins and the cephalosporins.   You...