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18.5: Microbial Resistance

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    A common problem in antimicrobial chemotherapy is the development of resistant strains of bacteria. Most bacteria become resistant to antimicrobial agents by one or more of the following mechanisms:

    1. Producing enzymes which inactivate the antibiotic, e.g., penicillinase and other beta-lactamases.

    2. Altering the target site in the bacterium to reduce or block binding of the antibiotic, e.g., producing a slightly altered ribosomal subunit that still functions but to which the drug can't bind.

    3. Altering the membranes and transport systems to prevent the entry of the antibiotic into the bacterium and/or using an efflux pump to transport the antibiotic out of the bacterium.

    4. Developing an alternate metabolic pathway to by-pass the metabolic step being blocked by the antimicrobial agent, e.g., overcoming drugs that resemble substrates and tie-up bacterial enzymes.

    5. Increasing the production of a certain bacterial enzyme, e.g., overcoming drugs that resemble substrates and tie-up bacterial enzymes.

    These changes in the bacterium that enable it to resist the antimicrobial agent occur naturally because of mutation or genetic recombination of the DNA in the nucleoid, or as a result of obtaining plasmids from other bacteria. Exposure to the antimicrobial agent then selects for these resistant strains of organism.

    The spread of antibiotic resistance in pathogenic bacteria is due to both direct selection and indirect selection. Direct selection refers to the selection of antibiotic resistant pathogens at the site of infection. Indirect selection is the selection of antibiotic-resistant normal floras within an individual anytime an antibiotic is given. At a later date, these resistant normal floras may transfer resistance genes to pathogens that enter the body. In addition, these resistant normal flora may be transmitted from person to person through such means as the fecal-oral route or through respiratory secretions.

    As an example, many Gram-negative bacteria possess R (resistance) plasmids which have genes coding for multiple antibiotic resistance through the mechanisms stated above, as well as transfer genes coding for a sex pilus. Such an organism can conjugate with other bacteria and transfer an R plasmid to them. Escherichia coli, Proteus, Serratia, Salmonella, Shigella, and Pseudomonas are examples of bacteria which frequently have R plasmids. Because of the problem of antibiotic resistance, antibiotic susceptibility testing is usually done in the clinical laboratory to determine which antimicrobial chemotherapeutic agents will most likely be effective on a particular strain of microorganism. This is discussed in the next section.

    To illustrate how plasmids carrying genes coding for antibiotic resistance can be picked up by antibiotic-sensitive bacteria, in today's lab we will use plasmid DNA to transform an Escherichia coli sensitive to the antibiotic ampicillin into one that is resistant to the drug.

    The E. coli will be rendered more "competent" to take up plasmid DNA (pAMP), which contains a gene coding for ampicillin resistance, by treating them with a solution of calcium chloride, cold incubation, and a brief heat shock. They will then be plated on 2 types of media: Lauria-Bertani agar (LB) and Lauria-Bertani agar with ampicillin (LB/amp). Only E. coli that have picked up a plasmid coding for ampicillin resistance will be able to form colonies on the LB/amp agar.

    Further Information

    For more information on microbial resistance to antibiotics, see the following CourseArc lesson:

    Contributors and Attributions


    This page titled 18.5: Microbial Resistance is shared under a not declared license and was authored, remixed, and/or curated by Gary Kaiser.

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