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Antibiotics: Penicillins
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Antibiotics active against bacteria are bacteriostatic or bacteriocidal; that is, they either inhibit growth of susceptible organisms or destroy them. On the basis of their mechanism of action, antibiotics are classified as (1) those that affect bacterial cell-wall biosynthesis, causing loss of viability and often cell lysis (penicillins and cephalosporins, bacitracin, cycloserine, vancomycin); (2) those that act directly on the cell membrane, affecting its barrier function and leading to leakage of intracellular components (polymyxin); (3) those that interfere with protein biosynthesis (chloramphenicol, tetracyclines, erythromycin, spectinomycin, streptomycin, gentamycin); (4) those that affect nucleic acid biosynthesis (rifampicin, novobiocin, quinolones); and (5) those that block specific steps in intermediary metabolism (sulfonamides, trimethoprim). See ... Enzyme; Sulfonamide.
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The penicillins are the oldest class of antibiotics, and have a common chemical structure which they share with the cephalopsorins. The two groups are classed as the beta-lactam antibiotics, and are generally bacteriocidal-that is, they kill bacteria rather than inhibiting growth. The penicillins can be further subdivided. The natural pencillins are based on the original penicillin G structure; penicillinase-resistant penicillins, notably methicillin and oxacillin, are active even in the presence of the bacterial enzyme that inactivates most natural penicillins. Aminopenicillins such as ampicillin and amoxicillin have an extended spectrum of action compared with the natural penicillins; extended spectrum penicillins are effective against a wider range of bacteria. These generally include coverage for Pseudomonas aeruginaosa and may provide the penicillin in combination with a penicillinase inhibitor.
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The mass production of antibiotics began during World War II with streptomycin and penicillin. Now most antibiotics are produced by staged fermentations in which strains of microorganisms producing high yields are grown under optimum conditions in nutrient media in fermentation tanks holding several thousand gallons. The mold is strained out of the fermentation broth, and then the antibiotic is removed from the broth by filtration, precipitation, and other separation methods. In some cases new antibiotics are laboratory synthesized, while many antibiotics are produced by chemically modifying natural substances; many such derivatives are more effective than the natural substances against infecting organisms or are better absorbed by the body, e.g., some semisynthetic penicillins are effective against bacteria resistant to the parent substance.
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The non-synthetic antibiotics are fungal by-products called mycotoxins. Penicillin is perhaps the best example. In other words, mycotoxins kill off fungi’s competitors, allowing fungi to grab up all of the nutrients for themselves. Alexander Fleming himself observed this in action, and it later led him to develop penicillin. When a mold -- molds are fungi -- contaminated a bacteria colony upon which Fleming was performing an experiment, the invader cleared the area around it of all bacteria. When Fleming investigated, It turned out that the fungus had produced a substance he would later call penicillin, killing the bacteria in residence.
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Cephalosporins — Cephalosporins are a class of antibiotics closely related to penicillin. There are a number of cephalosporin medications, a few of which include cephalexin (Keflex®), cefaclor (Ceclor®), cefuroxime (Ceftin®), cefadroxil (Duricef®) , cephadrine (Velocef®). cefprozil (Cefzil®), loracarbef (Lorabid®), ceftibuten (Cedax®), cefdinir (Omnicef®), cefditoren (Spectracef®), cefpodoxime (Vantin®) and cefixime (Suprax®).
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The macrolide antibiotics are derived from Streptomyces bacteria, and got their name because they all have a macrocyclic lactone chemical structure. Erythromycin, the prototype of this class, has a spectrum and use similar to penicillin. Newer members of the group, azithromycin and clarithyromycin, are particularly useful for their high level of lung penetration. Clarithromycin has been widely used to treat Helicobacter pylori infections, the cause of stomach ulcers.
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Penicillins