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Antibiotics STRUCTURE, SYNTHESIS AND BIOLOGICAL EFFECTS Slide 2 Definition and classification An antibiotics are the agent that either kills or inhibits the growth of a microorganism by biochemical process. The term antibiotic was first used in 1942 by Selman Waksman and his collaborators in journal articles to describe any substance produced by a microorganism that is antagonistic to the growth of other microorganisms in high dilution. Antibiotics that kill bacteria are called "bactericidal" Antibiotics that stop the growth of bacteria are called "bacteriostatic" Antibiotics are commonly classified based on their mechanism of action, chemical structure, spectrum of activity. Selman Abraham Waksman 1888 - 1973 Most antibiotics target bacterial functions or growth processes. Antibiotics that target the bacterial cell wall (penicillins, cephalosporins), or cell membrane (polymixins), or interfere with essential bacterial enzymes (quinolones, sulfonamides) are usually bactericidal in nature. Those that target protein synthesis, such as the aminoglycosides, macrolides, and tetracyclines, are usually bacteriostatic. Slide 3 Slide 4 Further categorization is based on their target specificity: narrow-spectrum antibiotics target particular types of bacteria, such as Gram-negative or Gram-positive bacteria, while broad-spectrum antibiotics affect a wide range of bacteria. Gram-staining: A test, resulting in the classification of bacteria, developed in the last century by Hans Christian Gram, a Danish microbiologist: - Gram positive bacteria will retain the original blue stain - Gram negative bacteria will lose the blue stain upon intermediate acetone treatment and will stain red Gram staining differentiates bacteria by the chemical and physical properties of their cell walls by detecting peptidoglycan, which is present in a thick layer in gram-positive bacteria. In a Gram stain test, gram-positive bacteria retain the crystal violet dye, while a counterstain (commonly safranin or fuchsin) added after the crystal violet gives all gram-negative bacteria a red or pink coloring. The Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique. Slide 5 Classes of antibiotics -Lactam antibiotics 1928: A. Fleming discovered that his cultures of staphylococci was contaminated with a fungus, and that the colonies of staphylococci immediately surrounding the fungus had been destroyed, whereas other staphylococci colonies farther away were normal. This fungi was the Penicillium notatum which produced a bactericidal agent. The penicillins are the oldest of the clinical antibiotics, but are still the most widely used. The first of the many penicillins to be employed on a significant scale was penicillin G (benzylpenicillin), obtained from the fungus Penicillium chrysogenum by fermentation in a medium containing corn-steep liquor. The - aminoadipyl side-chain of isopenicillin N is removed and replaced by another acid according to its availability from the fermentation medium. Several other penicillins are accessible by supplying different acids. Alexander Fleming 1881 - 1955 Penicillium notatum Slide 6 Synthesis of penicillins by fermentation Penicillin is a secondary metabolite of certain species of Penicillium and is produced when growth of the fungus is inhibited by stress. It is not produced during active growth. Penicillin is produced by the fungus Penicillium chrysogenum which requires lactose, other sugars, and a source of nitrogen (in this case a yeast extract) in the medium to grow well. F GXKOV Slide 7