In-depth Notes on Antimicrobial Drugs

Antimicrobial Drugs Notes

I. Introduction

• Antimicrobials: Drugs with the ability to kill or inhibit microorganisms, categorized as antibiotics, antifungals, antivirals, or antiparasitics.

A. Antibiotics

• Ideal Characteristics:

  • Spectrum of activity should include the pathogenic organism.
  • Ability to reach the infection site.
  • Patient tolerance at effective concentrations.

• Spectrum of Activity:

  • Broad Spectrum: Effective against a range of Gram (+) and Gram (-) bacteria, but not all.
  • Narrow Spectrum: Effective against specific Gram (+) or Gram (-) bacteria.

• Cidal vs Static:

  • Bactericidal: Actively kill bacteria.
  • Bacteriostatic: Inhibit bacterial growth, rely on the host's immune response.

• First line vs Second line drugs:

  • First Line: Historically safe and effective, preferred in treatment.
  • Second Line: Higher risk of adverse effects, more expensive, or unapproved; used when necessary.

• Breakpoint & MIC:

  • Breakpoint: Concentration of an antibiotic to determine a bacteria's resistance.
  • Minimum Inhibitory Concentration (MIC): Lowest concentration that inhibits bacterial growth.

B. Resistance

• Types of Resistance:

  • Natural Resistance: Drug targets not present in certain microorganisms.
  • Acquired Resistance: Microorganisms change through mutation or genetic transfer, losing sensitivity to antibiotics.

• Avoiding Resistance:

  • Use appropriate drugs as per diagnosis.
  • Follow label regimen.
  • Prefer local therapy over systemic where effective.
  • Use susceptibility testing to guide treatment.
  • Minimize treatment duration to achieve the clinical response.
  • Limit treatment to ill or at-risk animals only.
  • Account for withdrawal times in food-producing animals.

II. How Antibiotics Work

A. Time vs Concentration Dependent

• Time Dependent:
  • Require consistent drug levels to be effective; multiple doses needed.
• Concentration Dependent:
  • Achieve higher concentrations than MIC for efficacy.

B. Mechanisms of Action

1. Inhibition of cell wall synthesis (only in dividing bacteria).
2. Damage to cell membrane.
3. Inhibition of protein synthesis.
4. Interference with metabolism.
5. Interference with DNA or RNA processes.

III. Antibiotic Drug Classes

A. Beta Lactams (Cell Wall Inhibitors)

1. Action & Side Effects:

   • Bactericidal by disrupting cell wall synthesis, causing cell lysis.
   • Side effects include anorexia, vomiting, diarrhea, and hypersensitivity.

2. Types of Penicillins:

   • Natural Penicillins:
     • Penicillin G: Effective against anaerobes; not for Gram (-) bacteria.
     • Penicillin V: Similar to Pen G; orally effective.
   • Semisynthetic Penicillins:
     • Aminopenicillins (e.g., amoxicillin): Better Gram (-) coverage.
   • Beta-lactamase Resistant Penicillins: Target resistant Staph spp.
   • Potentiated Penicillins: Combine with beta-lactamase inhibitors.

3. Cephalosporins (Use restricted in food animals):

   • Semisynthetic, broad spectrum; categorized into generations (e.g., cefazolin, cefovecin).

B. Cell Membrane Disruptors

1. Polymyxins:
   • Highly nephrotoxic; used topically or systemically in low dosage.

C. Protein Synthesis Inhibitors

1. Aminoglycosides: Effective against Gram (-) bacteria; nephrotoxic and ototoxic properties.

2. Tetracyclines: Bacteriostatic; resistance common; contraindicated in juveniles due to effects on bone and teeth.

3. Macrolides: Primarily act on Gram (+) bacteria; examples include erythromycin and azithromycin.

4. Lincosamides: Good for anaerobic Gram (+) infections.

D. Metabolism Inhibitors

1. Sulfonamides: Bacteriostatic, often combined with trimethoprim.

E. Nucleic Acid Inhibitors

1. Fluoroquinolones: Broad spectrum; prohibited in food animals, can cause joint issues in juveniles.

IV. Antifungal Drugs

A. Overview

• Treatment can be fungicidal or fungistatic, affecting cell wall or membrane.

B. Classes of Antifungal Drugs

1. Polyenes: E.g., Nystatin, Amphotericin B; can be nephrotoxic.
2. Imidazoles: E.g., Ketoconazole, Itraconazole; potential for toxicity.
3. Antimetabolites: E.g., Flucytosine; used with Amphotericin B.
4. Superficial Antifungals: E.g., Griseofulvin for dermatophytosis.

V. Antiviral Drugs

A. Overview

• Target viral replication and attachment; challenging due to toxicity and resistance.

B. Examples of Antivirals

1. Acyclovir: Used for feline herpesvirus; can cause blood cell disorders.
2. Interferon: Used for various viral infections in cats.
3. Zidovudine (AZT): Improves clinical signs in FIV or FeLV cats.

C. Additional Medications

• Cidofovir and Trifluridine for specific viral keratitis treatments.