5 Antibacterial agents- Part I

Antimicrobial Chemotherapy

Overview

• Focus on determination of bacterial susceptibility to antimicrobial agents.

• Presented by Chief Assist. Prof. Denitsa Tsaneva-Damyanova, MD, PhD.

Development of Chemotherapy

• Paul Ehrlich:

  • In 1904, discovered Trypan Red effective against trypanosome (causes African sleeping sickness).

  • Introduced arsenical compounds in 1910 for syphilis (later called Salvarsan).

• Gerhard Domagk:

  • Discovered sulfonamides (red streptocide, 1935) effective for streptococcal and staphylococcal infections; received Nobel Prize in 1939.

• Alexander Fleming:

  • Discovered penicillin in 1929.

  • Penicillin produced and purified by H. Flory and E. Chain in 1939; received Nobel Prize in 1945.

• Selman Waksman:

  • Found streptomycin in 1944; Nobel Prize in 1952.

Types of Antibacterial Agents

• Natural Antibiotics:

  • Actinomyces: Streptomyces (Streptomycin).

  • Fungi: Penicillium (Penicillin), Cephalosporium.

  • Bacteria: Chromobacterium, Pseudomonas.

• Chemotherapeutic Agents:

  • Laboratory synthesized.

  • Chemotherapeutic Index:

    • Calculated as maximum tolerated dose/minimum therapeutic dose; should be greater than 3.

Characteristics of Antimicrobial Drugs

• Must have:

  • Selective toxicity to microbes, non-toxic to host.

  • Microbicidal rather than microbistatic.

  • Relative solubility; effective in dilute body fluids.

  • Prolonged potency; resistant to premature breakdown/excretion.

  • No promotion of antimicrobial resistance.

  • Support host defense activities.

  • Activity in tissues and body fluids.

  • Ease of delivery to infection site.

  • Reasonable cost and safety (no allergies or increased infection susceptibility).

Introduction of Antibiotics into Clinical Practice

• 1940s: Penicillin, Tetracycline.

• 1950s: Quinolones.

• 1960s: Chloramphenicol, Glycopeptides.

• 1970s: Oxazolidinones.

• 1980s: Trimethoprim, Lipopeptides.

• 1990s-Onwards: Macrolides, Aminoglycosides.

Major Targets of Common Antimicrobial Agents

• DNA:

  • Fluoroquinolones, Novobiocin, Nitroimidazoles, Nitrofurans.

• Ribosomes:

  • Tetracyclines, Aminoglycosides, Macrolides.

• Cell Wall:

  • Beta-lactams (Penicillins, Cephalosporins), Glycopeptides.

Mechanisms of Antibiotic Action

• Inhibition of Cell Wall Synthesis:

  • Results in cell lysis (e.g., Penicillins, Cephalosporins).

• Disruption of Protein Synthesis:

  • Essential enzyme synthesis disrupted (e.g., Aminoglycosides, Macrolides).

• Inhibition of Nucleic Acid Synthesis:

  • Prevents DNA replication and transcription (e.g., Fluoroquinolones, Rifampin).

• Interaction with Plasma Membrane:

  • Disrupts membrane permeability (e.g., Polymyxins).

• Inhibition of Cell Metabolism:

  • Antimetabolites disrupting microbial metabolism without affecting host (e.g., Sulfonamides).

Mechanisms of Bacterial Resistance

1. Decreased permeability to antibiotics.

2. Efflux pumps expelling antibiotics.

3. Antibiotic degradation and inactivation.

4. Modification of antibiotic molecules.

Kirby-Bauer Disk Diffusion Susceptibility Test

• Purpose:

  • Determine sensitivity or resistance of bacteria to antimicrobial compounds.

• Procedure:

  • Culture bacteria on Mueller-Hinton agar with antibiotic-impregnated disks.

  • Observe growth inhibition zones.

• Optimal Medium:

  • Mueller-Hinton agar preferred for reproducibility and growth support.

• Bacterial Suspension:

  • Prepare 0.5 McFarland standard.

• Disk Placement and Incubation:

  • Place disks ≥ 24 mm apart, incubate at 35°C ± 2°C for 18 hours.

• Zone Measurement:

  • Measure inhibition zones in mm using a ruler, including disk diameter.

Antibiotic Susceptibility Testing Methods

• Quantitative Methods:

  • Minimum Inhibitory Concentration (MIC) determined through dilutions of antibiotics.

• Broth Dilution Method:

  • Serial two-fold dilutions in liquid culture; detects minimal inhibitory concentration.

• Agar Dilution Method:

  • Serial dilutions prepared in agar; incubate to observe growth inhibition.

• E-test:

  • Plastic strips with decreasing antibiotic concentrations to read MIC from inhibition zones.

• Automated Susceptibility Methods:

  • Quick bacterial growth determination using advanced techniques (e.g., VITEK, AutoSCAN).