Microbiology: Principles of Antimicrobial Therapy

Principles of Antimicrobial Therapy

  • Historical Context:

    • The 1930s marked a medical revolution with the introduction of modern drugs to control infections.
    • Antimicrobial drugs have significantly reduced the incidence of several infections, but they have not eradicated infectious diseases and are unlikely to do so.
    • Medical professionals currently face the threat of a "post-antibiotic era" where existing drugs are no longer effective.

  • Main Goal of Antimicrobial Chemotherapy:

    • The primary objective is to administer a drug to an infected person that destroys the infective agent without harming the host's cells.

  • Criteria for an Effective Antimicrobial Drug:

    • Easy to administer and capable of reaching the infectious agent anywhere in the human body.
    • Absolutely toxic to the infectious agent while remaining absolutely nontoxic to the host (Selective Toxicity).
    • Remains in the body for the necessary duration to be effective.
    • Easily broken down and safely excreted from the body.

  • Characteristics of the Ideal Antimicrobial Drug (Table 10.1):

    • Selectivity: Toxic to the microbe but nontoxic to host cells.
    • Activity: Microbicidal (kills) rather than microbistatic (inhibits growth).
    • Solubility: Relatively soluble and functions even when highly diluted in body fluids.
    • Stability: Remains potent long enough to act; not excreted or broken down prematurely.
    • Resistance: Does not lead to the development of antimicrobial resistance.
    • Host Support: Complements or assists host defenses.
    • Tissue Activity: Remains active in tissues and body fluids.
    • Delivery: Readily delivered to the site of infection.
    • Economics: Reasonably priced.
    • Safety: Does not cause allergies or predispose the host to other infections (e.g., superinfections).

Terminology of Chemotherapy (Table 10.2)

  • Chemotherapeutic Drug: Any chemical used in the treatment, relief, or prophylaxis of a disease.
  • Prophylaxis: Use of a drug to prevent imminent infection of a person at risk.
  • Antimicrobial Chemotherapy: The use of chemotherapeutic drugs to control infection.
  • Antimicrobials: An all-inclusive term for any antimicrobial drug, regardless of its origin.
  • Antibiotics: Substances produced by natural metabolic processes of microorganisms (bacteria or fungi) that inhibit or destroy other microbes.
  • Semisynthetic Drugs: Drugs isolated from natural sources and then chemically modified in a laboratory.
  • Synthetic Drugs: Drugs produced entirely by chemical reactions within a laboratory.
  • Narrow-Spectrum (Limited Spectrum): Antimicrobials effective against a limited array of microbial types (e.g., primarily gram-positive bacteria).
  • Broad-Spectrum (Extended Spectrum): Antimicrobials effective against a wide variety of microbial types (e.g., both gram-positive and gram-negative bacteria).

Origins and Identification

  • Microbial Origins:

    • Antibiotics are common metabolic products of aerobic bacteria and fungi, produced to inhibit competitors in the same habitat.
    • Bacterial Genera: Streptomyces and Bacillus.
    • Mold Genera: Penicillium and Cephalosporium.

  • Three Factors for Starting Treatment:

    • 1. The nature of the microorganism causing the infection.
    • 2. The degree of the microorganism’s susceptibility (sensitivity) to various drugs.
    • 3. The overall medical condition of the patient.

  • Identifying the Agent:

    • Identification should begin immediately, ideally before antimicrobial drugs are administered to avoid reducing microbial numbers.
    • Direct Examination: Rapid method for detection.
    • Doctors often start therapy based on immediate findings, informed guesses, or epidemiological statistics.

Testing for Drug Susceptibility

  • Kirby-Bauer Technique:

    • Method: Bacteria are spread on an agar plate; small discs with specific antibiotic concentrations are applied.
    • Measurement: The "Zone of Inhibition" is measured and compared to a standard.
    • Antibiogram: Provides a profile of drug sensitivity for selection.
    • Limitations: Less effective for anaerobic, fastidious, or slow-growing bacteria.
    • Examples of Evaluation:
      • Enrofloxacin (5 μg5\,\mu g): Resistant (R<17 mmR < 17\,mm); Sensitive (S>22 mmS > 22\,mm).
      • Ampicillin (10 μg10\,\mu g): Resistant (R<14 mmR < 14\,mm); Sensitive (S>22 mmS > 22\,mm).
      • Chloramphenicol (30 μg30\,\mu g): Resistant (R<21 mmR < 21\,mm); Sensitive (S>21 mmS > 21\,mm).

  • Tube Dilution Tests:

    • More sensitive and quantitative than Kirby-Bauer.
    • Drug is serially diluted in broth tubes and inoculated with a pure culture.
    • Minimum Inhibitory Concentration (MIC): The smallest concentration (highest dilution) of a drug that visibly inhibits growth.
    • Calculated in units like μg/ml\mu g/ml.

The Therapeutic Index and Clinical Choice

  • Therapeutic Index (TI): The ratio of the dose of the drug that is toxic to humans compared to its minimum effective (therapeutic) dose.

    • TI=Toxic DoseMIC (Therapeutic Dose)TI = \frac{\text{Toxic Dose}}{\text{MIC (Therapeutic Dose)}}.
    • A high index is preferable because it indicates a wider margin of safety (TI=10TI = 10 is safer than TI=1.1TI = 1.1).

  • Factors Leading to Treatment Failure:

    • Inability of the drug to diffuse into specific body compartments (e.g., brain, joints, skin).
    • Resistant microbes present in the infection but not in the tested sample.
    • Mixed infections where some pathogens are drug-resistant.

  • Patient History Considerations:

    • Preexisting conditions, drug allergies, and underlying liver or kidney disease.
    • Special precautions for infants, the elderly, and pregnant women.
    • Potential drug-drug interactions, genetic abnormalities, site of infection, and cost.

Mechanisms of Antimicrobial Action and Drug Targets

  • Specific Modes of Action (Table 10.4):
    • Cell Wall Inhibitors: Block synthesis and repair. Includes Penicillins, Cephalosporins, Carbapenems, Vancomycin, Bacitracin, Fosfomycin, and Isoniazid.
    • Protein Synthesis Inhibitors (Ribosomes):
      • 50S Subunit: Erythromycin, Clindamycin, Synercid, Pleuromutilins.
      • 30S Subunit: Aminoglycosides (Gentamicin, Streptomycin), Tetracyclines, Glycylcyclines.
      • Both subunits: Linezolid (blocks initiation).
    • Folic Acid Synthesis (Cytoplasm): Block pathways and inhibit metabolism. Includes Sulfonamides (sulfa drugs) and Trimethoprim.
    • DNA and RNA Inhibitors:
      • DNA Gyrase (unwinding): Quinolones (Fluoroquinolones like Ciprofloxacin).
      • RNA Polymerase: Rifampin.
    • Cytoplasmic Membrane: Cause loss of selective permeability. Includes Polymyxins and Daptomycin.

Drug Resistance Development

  • Timeline: Resistance to penicillin was observed as early as 1940; large-scale treatment failures emerged in the 1980s and 1990s.

  • Acquisition Methods:

    • Spontaneous Chromosomal Mutation: Random changes in genes; can lead to "persisters" (microbes that stop metabolism to survive antibiotics).
    • Horizontal Gene Transfer: Acquisition of Resistance (R) factors via plasmids.

  • Specific Mechanisms of Resistance (Table 10.9):

    • Enzyme Synthesis: Bacteria produce enzymes like beta-lactamases or penicillinases to hydrolyze the beta-lactam ring.
    • Decreased Permeability: Mutations prevent drug uptake.
    • Drug Elimination: Multidrug-resistant (MDR) pumps actively transport drugs out of the cell.
    • Altered Binding Sites: Ribosomal or receptor sites change shape (e.g., 50S site alteration for erythromycin).
    • Metabolic Bypass: Microbes use alternative pathways for folic acid synthesis to avoid sulfonamide inhibition.

  • Natural Selection: Exposure to drugs gives resistant mutants a selective advantage, allowing them to eventually replace the sensitive population.

Specialized Treatments for Eukaryotes and Viruses

  • Fungal Infections: Special challenge because fungi are eukaryotic. Drugs targeting bacteria are ineffective. Similarities to human cells increase toxicity risks.

    • Amphotericin B: Binds to fungal membranes; used for skin lesions (Candida albicans) and meningitis.
    • Azoles (Ketoconazole, Fluoconazole): Interfere with sterol synthesis.
    • Echinocandins: Inhibit cell wall synthesis.

  • Protozoal Infections:

    • Quinine: Traditional malaria treatment; replaced by synthetic quinolines (Chloroquine, Primaquine).
    • Metronidazole (Flagyl): Used for Entamoeba histolytica, Giardia lamblia, and Trichomonas vaginalis. Requires reduction of its nitro group to cause DNA strand breakage.

  • Helminthic Infections: Challenging because physiology is similar to humans. Drugs target all life stages.

    • Mebendazole/Albendazole: Inhibit microtubules.
    • Pyrantel: Paralyzes muscles.
    • Praziquantel: Treatment for tapeworms/flukes.

  • Viral Infections: Viruses rely on host metabolism. Vaccines are used for measles, mumps, and hepatitis.

    • HIV Drugs: Protease inhibitors (Indinavir), Reverse Transcriptase inhibitors (AZT), and Entry blockers (Fuzeon).
    • Herpes Drugs: Acyclovir (DNA termination).
    • Influenza Drugs: Tamiflu and Relenza (stop entry/release and neuraminidase action).
    • DRACO: A breakthrough double-stranded RNA-activated caspase oligomerizer being tested.

New Approaches and Ecological Balance

  • Novel Therapies:

    • Iron Scavenging: Targeting bacterial iron intake.
    • RNA Interference (RNAi): Small RNA pieces to shut down pathogenic gene expression (tested for Hepatitis C).
    • Defense Peptides: Mimicking defensins, magainins, or bacteriocins.
    • Bacteriophages: Used in Eastern Europe; Biophage-PA treats Pseudomonas biofilms.

  • Helping Nature (Biota Restoration):

    • Probiotics: Live microbes (e.g., Bifidobacterium) to replace lost biota.
    • Prebiotics: Nutrients like fructans that encourage beneficial growth.
    • Fecal Transplants: Used for recurrent Clostridium difficile infections.

  • Superinfection: Overgrowth of one microbe (e.g., Candida albicans or C. difficile) following the destruction of normal biota by broad-spectrum antibiotics.

Questions & Discussion

  • Q: Which drug is the safest: Drug A (Zone 30 mm, TI 1.2) or Drug B (Zone 20 mm, TI 12)?

    • A: Drug B is the safest choice because its TI (12) is much higher, indicating a larger safety margin, despite Drug A having a larger zone of inhibition.

  • Q: Which mode of action has the least toxic effect on humans: A (DNA replication), B (Cell membrane), or C (Peptidoglycan cell wall)?

    • A: Antibiotic C is the least toxic because human cells do not possess peptidoglycan cell walls.

  • Q: Categorize these drugs: Ribavirin, Oxacillin, Metronidazole, Mebendazole, Amphotericin B.

    • A: Ribavirin (Antiviral), Oxacillin (Antibacterial), Metronidazole (Antiprotozoal/Amoebicide), Mebendazole (Antihelminthic), Amphotericin B (Antifungal).

  • Q: Which resistance mechanism specifically affects penicillins and cephalosporins?

    • A: Enzymes being synthesized to inactivate the drug (e.g., beta-lactamases).

  • Q: Overgrowth of Candida albicans after antibacterial treatment for a UTI is an example of what?

    • A: Superinfection.