Introduction to Antimicrobials and Drug Susceptibility

Introduction to Antimicrobial Medications

  • Terminology and Etymology of "Anti-" Agents:
    • The prefix "anti-" means "against."
    • Antiviral: Medications acting against viruses. Examples include drugs ending in the suffix "-vir," such as Ecyclovir or brand names like Valtrex.
    • Antibiotic: Medications acting against bacteria.
    • Antiparasitic: Medications used to treat parasitic infections, such as worms (e.g., roundworms) in the stomach or eyes.
    • Antifungal: Medications used against fungal infections such as Athlete's foot. Common drug store examples include Lotrimin and tough actin tenactin (which often end in the suffix "-in").
    • Antimicrobials (General Definition): A global term for agents used to prevent and treat infections caused by microorganisms.

Characteristics of the Ideal Antimicrobial Drug

  • Toxic and Non-toxic Targets: The drug should be toxic to the microbe but nontoxic to host cells (selective toxicity).
  • Impact on Human Microbiome:
    • Antibiotics can be problematic because they often kill both "good" and "bad" bacteria.
    • Example: E. coli in the gut synthesizes Vitamin K. Destroying this E. coli with antibiotics like amoxicillin, penicillin, or Cipro can lead to a Vitamin K deficiency.
    • Recovery often requires the use of probiotics and prebiotics.
  • Microbicidal vs. Microstatic:
    • Microbicidal: The drug kills the microorganisms.
    • Microstatic/Bacteriostatic: The drug inhibits the growth or reproduction of the microbes without necessarily killing them.
    • Metaphor: Placing meat in a refrigerator slows growth (static), placing it in a freezer might stop growth but does not kill existing bacteria. In medicine, the ideal is to kill (murder) the pathogens.
  • Solubility and Delivery:
    • Drugs should be highly soluble (can be dissolved in water easily) and remain active in tissues and body fluids for long periods.
    • Routes of Administration:
      • Intravenous (IV): Used for fast-acting delivery directly into the bloodstream. Vital for lethal infections like acute endocarditis (inflammation of the heart valves or endocardium) which can kill a patient in hours or days.
      • Intramuscular (IM): Used for fat-soluble drugs that need to be absorbed into fatty tissues.
      • Pill/Liquid Form: Common for non-emergency or pediatric use (e.g., liquid droplet forms for children like amoxicillin).
  • Additional Ideal Traits:
    • Fast-acting (works instantly or quickly).
    • Reasonably priced and easy to administer (e.g., small, compact tablets vs. large vitamins).
    • Minimal potential for allergic reactions or side effects (e.g., drowsiness or nausea).
    • Does not lead to the development of antimicrobial resistance.

Testing for Drug Susceptibility: The Kirby-Bauer Method

  • Agar Preparation:
    • Agar is a solid gelatinous substance derived from algae.
    • In the lab process, the agar is heated until liquid and then cooled to approximately 40–4540\text{--}45 degrees Celsius before adding bacteria. If the liquid is too hot, it will kill the bacterial strain (e.g., B serrheus or B. Cereus).
  • The Kirby-Bauer Disk Diffusion Test:
    • Bacteria are mixed into the agar or spread across it as it solidifies.
    • Special paper discs are soaked in various antimicrobial substances (e.g., ethyl alcohol, erythromycin, gentamicin).
    • The discs are placed on the agar plate and incubated.
  • Diffusion Mechanics: Particles move from an area of high concentration (the disc) to an area of low concentration (the agar surrounding it).
  • Zone of Inhibition:
    • The clear ring or area of "no growth" surrounding an antimicrobial disc.
    • Direct Correlation: The larger the zone of inhibition, the more effective the antimicrobial drug is against that specific strain of