Chapter 10: Controlling Microbial Growth in the Body – Antimicrobial Drugs

Antimicrobial Drugs: Mechanisms and Clinical Considerations

Drugs Affecting Cell Walls

Antimicrobial agents targeting cell walls disrupt the structural integrity of bacteria, leading to cell lysis and death. These drugs are especially effective against actively growing bacteria.

• Beta-lactams (e.g., penicillins, cephalosporins) inhibit the synthesis of peptidoglycan, a key component of bacterial cell walls.

• Vancomycin and bacitracin interfere with peptidoglycan cross-linking.

• Example: Penicillin is used to treat infections caused by Gram-positive bacteria.

Drugs Affecting Protein Synthesis

These drugs target bacterial ribosomes, which differ structurally from eukaryotic ribosomes, allowing selective toxicity.

• Aminoglycosides (e.g., streptomycin) cause misreading of mRNA.

• Tetracyclines block tRNA attachment to the ribosome.

• Macrolides (e.g., erythromycin) inhibit peptide chain elongation.

• Example: Tetracycline is used for broad-spectrum bacterial infections.

Drugs Disrupting Cytoplasmic Membranes

These agents compromise membrane integrity, causing leakage of cellular contents.

• Polymyxins disrupt the membranes of Gram-negative bacteria.

• Azoles and amphotericin B target fungal cell membranes by interfering with ergosterol.

• Example: Amphotericin B is used for systemic fungal infections.

Drugs Inhibiting Metabolic Pathways

These drugs block essential metabolic reactions unique to microbes.

• Sulfonamides inhibit folic acid synthesis in bacteria.

• Trimethoprim acts synergistically with sulfonamides.

• Example: Sulfamethoxazole is used for urinary tract infections.

Drugs Inhibiting Nucleic Acid Synthesis

These agents interfere with DNA or RNA synthesis, preventing replication and transcription.

• Quinolones (e.g., ciprofloxacin) inhibit DNA gyrase.

• Rifamycins block RNA polymerase.

• Example: Rifampin is used in tuberculosis treatment.

Broad vs. Narrow Spectrum Drugs

Antimicrobial drugs vary in their range of activity against different pathogens.

• Broad-spectrum drugs are effective against a wide variety of microorganisms (e.g., tetracycline).

• Narrow-spectrum drugs target specific types of bacteria (e.g., penicillin for Gram-positive bacteria).

• Benefit: Broad-spectrum drugs can treat mixed infections.

• Drawback: Broad-spectrum drugs may disrupt normal microbiota, leading to secondary infections.

Diffusion Susceptibility Test (Kirby-Bauer Test)

This test evaluates the effectiveness of antibiotics against specific bacteria.

• Bacterial lawn is inoculated on agar plate.

• Antibiotic disks are placed on the surface.

• Zones of inhibition are measured to determine susceptibility.

• Interpretation: Larger zones indicate greater sensitivity.

Minimum Inhibitory Concentration (MIC) Test

The MIC test determines the lowest concentration of an antimicrobial that prevents visible growth of a microorganism.

• Serial dilutions of drug are prepared.

• Bacteria are added to each dilution.

• The MIC is the lowest concentration with no visible growth.

• Equation: $\text{MIC} = \text{Lowest concentration inhibiting growth}$

Minimum Bactericidal Concentration (MBC) Test

The MBC test identifies the lowest concentration of an antimicrobial that kills the bacteria.

• Samples from MIC test are plated on drug-free media.

• MBC is the lowest concentration with no colony growth.

• Equation: $\text{MBC} = \text{Lowest concentration killing bacteria}$

Routes of Administration for Antimicrobial Drugs

Drugs can be administered by various routes, each with specific benefits and drawbacks.

Route	Benefits	Drawbacks
Oral	Convenient, self-administered	Variable absorption, patient compliance
Intravenous (IV)	Rapid, high concentration	Requires medical supervision, risk of toxicity
Intramuscular (IM)	Moderate absorption	Painful, limited volume
Topical	Localized effect	Limited to surface infections

Safety and Side Effects of Antimicrobial Agents

Safety is a critical consideration in antimicrobial therapy.

• Toxicity: Some drugs may harm host tissues (e.g., nephrotoxicity, ototoxicity).

• Allergic reactions: Hypersensitivity can occur (e.g., penicillin allergy).

• Disruption of normal microbiota: May lead to secondary infections (e.g., Clostridioides difficile).

Bacterial Resistance to Antibiotics

Bacteria can develop resistance through various mechanisms, reducing drug effectiveness.

• Enzymatic degradation: e.g., beta-lactamases break down penicillins.

• Altered targets: Mutation changes drug binding sites.

• Efflux pumps: Bacteria expel drugs from the cell.

• Reduced permeability: Changes in membrane prevent drug entry.

Limiting Antibiotic Resistance: Patient Actions

Patients play a vital role in preventing the spread of antibiotic resistance.

• Complete prescribed antibiotic courses.

• Avoid unnecessary antibiotic use.

• Practice good hygiene to prevent infection.

• Do not share or use leftover antibiotics.

Antibiotic Resistance in Healthcare: Causes, Impact, and Solutions

Increasing antibiotic resistance is a major public health concern.

• Causes: Overuse and misuse of antibiotics, poor infection control, agricultural use.

• Impact: Harder to treat infections, increased morbidity and mortality, higher healthcare costs.

• Solutions: Stewardship programs, development of new drugs, infection prevention, public education.