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Misc Antibiotics

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    419
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    Antibiotics are specific chemical substances derived from or produced by living organisms that are capable of inhibiting the life processes of other organisms. The first antibiotics were isolated from microorganisms but some are now obtained from higher plants and animals. Over 3,000 antibiotics have been identified but only a few dozen are used in medicine. Antibiotics are the most widely prescribed class of drugs comprising 12% of the prescriptions in the United States.

    Macrolides

    Macrolides are products of actinomycetes (soil bacteria) or semi-synthetic derivatives of them. Erythromycin is an orally effective antibiotic discovered in 1952 in the metabolic products of a strain of Streptocyces erythreus, originally obtained from a soil sample.

    652erythromycin.gif

    Erythromycin and other macrolide antibiotics inhibit protein synthesis by binding to the 23S rRNA molecule (in the 50S subunit) of the bacterial ribosome blocking the exit of the growing peptide chain. of sensitive microorganisms. (Humans do not have 50 S ribosomal subunits, but have ribosomes composed of 40 S and 60 S subunits). Certain resistant microorganisms with mutational changes in components of this subunit of the ribosome fail to bind the drug. The association between erythromycin and the ribosome is reversible and takes place only when the 50 S subunit is free from tRNA molecules bearing nascent peptide chains. Gram-positive bacteria accumulate about 100 times more erythromycin than do gram-negative microorganisms. The non ionized from of the drug is considerably more permeable to cells, and this probably explains the increased antimicrobial activity that is observed in alkaline pH.

    Tetracyclines

    Tetracyclines have the broadest spectrum of antimicrobial activity. These may include: Aureomycin, Terramycin, and Panmycin. Four fused 6-membered rings, as shown in the figure below, form the basic structure from which the various tetracyclines are made. The various derivatives are different at one or more of four sites on the rigid, planar ring structure. The classical tetracyclines were derived from Streptomyces spp., but the newer derivatives are semisynthetic as is generally true for newer members of other drug groups.

    652tetrcycline.gif

    Tetracyclines inhibit bacterial protein synthesis by blocking the attachment of the transfer RNA-amino acid to the ribosome. More precisely they are inhibitors of the codon-anticodon interaction. Tetracyclines can also inhibit protein synthesis in the host, but are less likely to reach the concentration required because eukaryotic cells do not have a tetracycline uptake mechanism.

    Streptomycin

    Streptomycin is effective against gram-negative bacteria, although it is also used in the treatment of tuberculosis. Streptomycin binds to the 30S ribosome and changes its shape so that it and inhibits protein synthesis by causing a misreading of messenger RNA information.

    Chloramphenicol

    Chloromycetin is also a broad spectrum antibiotic that possesses activity similar to the tetracylines. At present, it is the only antibiotic prepared synthetically. It is reserved for treatment of serious infections because it is potentially highly toxic to bone marrow cells. It inhibits protein synthesis by attaching to the ribosome and interferes with the formation of peptide bonds between amino acids. It behaves as an antimetabolite for the essential amino acid phenylalanine at ribosomal binding sites.

    652antibiotic.gif

    Table 1: Some clinically important antibiotics
    Antibiotic Producer organism Activity Site or mode of action
    Penicillin Penicillium chrysogenum Gram-positive bacteria Wall synthesis
    Cephalosporin Cephalosporium acremonium Broad spectrum Wall synthesis
    Griseofulvin Penicillium griseofulvum Dermatophytic fungi Microtubules
    Bacitracin Bacillus subtilis Gram-positive bacteria Wall synthesis
    Polymyxin B Bacillus polymyxa Gram-negative bacteria Cell membrane
    Amphotericin B Streptomyces nodosus Fungi Cell membrane
    Erythromycin Streptomyces erythreus Gram-positive bacteria Protein synthesis
    Neomycin Streptomyces fradiae Broad spectrum Protein synthesis
    Streptomycin Streptomyces griseus Gram-negative bacteria Protein synthesis
    Tetracycline Streptomyces rimosus Broad spectrum Protein synthesis
    Vancomycin Streptomyces orientalis Gram-positive bacteria Protein synthesis
    Gentamicin Micromonospora purpurea Broad spectrum Protein synthesis
    Rifamycin Streptomyces mediterranei Tuberculosis Protein synthesis

    Contributors

    Charles Ophardt (Professor Emeritus, Elmhurst College); Virtual Chembook


    Misc Antibiotics is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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