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Antimicrobial Treatment and Resistance in Microbiology

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  • Main goal of antimicrobial therapy

    Destroy the infective agent without harming the host's cells.

  • Ideal characteristics of an antimicrobial drug

    Toxic to microbes, nontoxic to host, microbicidal, soluble, potent, resistant to premature breakdown, does not induce resistance, complements host defenses, and does not disrupt host health.

  • Primary sources of antimicrobial drugs

    Bacteria: Streptomyces, Bacillus; Molds: Penicillium, Cephalosporium.

  • Disc Diffusion (Kirby-Bauer) Test principle

    Measures zones of inhibition around antibiotic discs on agar to assess bacterial susceptibility.

  • Zone of inhibition meaning

    Area around antibiotic disc where bacterial growth is prevented; larger zones indicate greater sensitivity.

  • Tube Dilution Test and MIC

    Determines the Minimum Inhibitory Concentration (MIC), the lowest drug concentration that visibly inhibits bacterial growth.

  • Five major targets of antimicrobial drugs

    Cell wall synthesis, protein synthesis, nucleic acid synthesis, cytoplasmic membrane, and folic acid synthesis.

  • Penicillin core structure

    Beta-lactam ring fused to a thiazolidine ring; beta-lactam ring is essential for activity.

  • Mechanism: Enzymatic inactivation

    Microbial enzymes like penicillinase destroy the drug, rendering it inactive.

  • Mechanism: Altered target sites

    Modification of drug binding sites prevents drug action, e.g., modified ribosome prevents macrolide binding.

  • Mechanism: Efflux pumps

    Active transport pumps expel drugs from the microbial cell, reducing drug concentration inside.

  • Mechanism: Reduced permeability

    Changes in membrane porins block drug entry into the microbe.

  • Mechanism: Alternative metabolic pathways

    Microbes bypass blocked pathways, e.g., sulfonamide resistance by using different folic acid synthesis routes.

  • How antibiotic resistance develops

    Drug-resistant bacteria survive treatment, proliferate, and transfer resistance genes; misuse accelerates this process.

  • Therapeutic Index (TI) formula

    TI = \(\frac{Toxic\ Dose}{Therapeutic\ Dose}\); higher TI means safer drug.

  • Interpretation of Therapeutic Index values

    TI > 10 indicates safer drugs; TI close to 1 indicates higher risk of toxicity.

  • Common adverse effects of antimicrobials

    Can affect liver, kidneys, GI tract, cardiovascular system, nervous system, skin, bones, and teeth.

  • Allergic responses to antimicrobials

    Drugs may act as antigens; penicillin allergies are most common and can cause severe reactions upon re-exposure.

  • Superinfections caused by antibiotics

    Broad-spectrum antibiotics disrupt normal microbiota, allowing opportunistic pathogens to overgrow.

  • Purpose of antimicrobial susceptibility testing

    To guide drug choice and dosage by determining bacterial sensitivity to antibiotics.