Overview
This lecture introduces antimicrobial treatment, focusing on the history, goals, terminology, selection criteria, and testing methods for choosing effective drugs to combat microorganisms within the body.
History and Goals of Antimicrobial Treatment
- Antimicrobial treatment targets microorganisms inside the body, unlike previous methods focused on environmental control.
- The first usable antibiotic, penicillin, was discovered by Alexander Fleming in the 1930s.
- Antibiotics are chemicals produced by microorganisms that inhibit other microbes, stemming from microbial antagonism.
- Antimicrobial drugs reduce infection rates but cannot eliminate infectious diseases entirely due to resistance.
- Selective toxicity is the goal: destroy microbes without harming the host.
Characteristics of Ideal Antimicrobials
- Should be selectively toxic, harming microbes but not host cells.
- Preferably microbicidal (kills) rather than microbistatic (inhibits).
- Functions at low concentrations and remains active in the body.
- Minimizes development of resistance and works in tissues and fluids.
- Non-allergenic, cost-effective, and does not disrupt host health.
- No current drug meets all these criteria perfectly.
Key Terminology
- Chemotherapeutic drugs: any chemical used to treat or relieve disease symptoms.
- Prophylactic drugs: prevent illness before symptoms start.
- Antimicrobial chemotherapy: drugs targeting infectious agents.
- Antibiotics: natural metabolic products from some microorganisms that inhibit others.
- Semisynthetic drugs: chemically modified antibiotics.
- Synthetic drugs: entirely lab-made, no natural source.
- Narrow-spectrum antibiotics: effective against limited microbes.
- Broad-spectrum antibiotics: effective against many types of bacteria.
Antimicrobial Selection and Susceptibility Testing
- Before therapy: identify microbe, test drug susceptibility, assess patient’s medical condition.
- Susceptibility testing often skipped for common infections when likely cause is well-known.
- Kirby-Bauer method: bacteria streaked on plates, antibiotic discs added, zones of inhibition measured to determine effectiveness.
- Tube dilution test: determines minimum inhibitory concentration (MIC)—lowest dose that inhibits growth.
- Therapeutic index: ratio of toxic dose to MIC; higher index = safer drug.
Limitations and Patient Considerations
- Lab results (in vitro) may not match real-life body effects (in vivo).
- Treatment fails if drugs can't reach infection site, resistance arises, drugs interact negatively, or patients fail to follow prescriptions.
- Mixed (polymicrobial) infections complicate therapy.
- Patient history (allergies, liver/kidney issues, age, pregnancy, drug interactions) influences drug choice.
- Site of infection and administration route should match for best results.
- Cost is always a factor in drug selection.
Key Terms & Definitions
- Selective toxicity — ability of a drug to harm microbes without harming the host.
- Chemotherapeutic drug — chemical used to treat or relieve a disease.
- Prophylactic drug — drug used to prevent disease.
- Antibiotic — naturally-derived substance that inhibits other microbes.
- Semisynthetic drug — antibiotic chemically modified in a lab.
- Synthetic drug — entirely lab-made medication.
- Narrow-spectrum antibiotic — targets a small range of microbes.
- Broad-spectrum antibiotic — effective against a wide variety of bacteria.
- Kirby-Bauer test — lab test measuring zones of inhibition for antibiotics.
- Minimum inhibitory concentration (MIC) — lowest drug amount that prevents microbe growth.
- Therapeutic index — ratio of toxic dose to MIC; measures drug safety.
Action Items / Next Steps
- Review assigned readings on antimicrobial mechanisms of action.
- Prepare case study notes for patient drug choice scenarios.
- Study common examples of narrow vs. broad-spectrum antibiotics.