Controlling Microbial Growth in the Body: Antimicrobial Drugs

Selective Toxicity

Selective toxicity is a fundamental principle in antimicrobial therapy, ensuring that drugs target pathogens more than the host.

• Definition: An effective antimicrobial agent is more toxic to a pathogen than to its host.

• Importance: Minimizes harm to human cells while effectively eliminating infectious microbes.

• Example: Penicillins target bacterial cell walls, which human cells lack.

Mechanisms of Antimicrobial Action

Antimicrobial drugs act by targeting specific structures or processes in microbes.

• Cell Wall Disruption: Some drugs weaken the bacterial cell wall, causing the cytoplasmic membrane to bulge and the cell to lyse as water enters.

• Example: Beta-lactam antibiotics (e.g., penicillins) inhibit peptidoglycan synthesis.

• Mycobacterium Treatment: Mycobacterium species (e.g., leprosy, tuberculosis) divide slowly (every 12–24 hours), requiring prolonged treatment.

Major Classes of Antimicrobial Drugs

• Macrolides: Inhibit protein synthesis by binding to the 50S ribosomal subunit. Example: Erythromycin.

• Quinolones and Fluoroquinolones: Inhibit DNA gyrase, an enzyme necessary for correct coiling and uncoiling of bacterial DNA. These drugs have little to no effect on eukaryotes or viruses.

• Broad-Spectrum Antibiotics: Effective against a wide range of bacteria, including Gram-positive, Gram-negative, chlamydias, and rickettsias. Example: Tetracycline.

Microbial Antagonism

Microbial antagonism refers to the competition between normal microbiota and pathogens for nutrients and space, which helps prevent infection by pathogens.

• Definition: The competition between normal microbes and pathogens for nutrients and space.

• Significance: Disruption of normal microbiota (e.g., by broad-spectrum antibiotics) can lead to secondary infections.

Testing Antimicrobial Effectiveness

Several laboratory methods are used to determine the susceptibility of microbes to antimicrobial agents.

• Diffusion Susceptibility Test (Kirby-Bauer Test): A Petri plate is uniformly inoculated with a pathogen. Paper disks with drugs are placed on the surface. After incubation, a clear area (zone of inhibition) indicates where bacteria do not grow due to the drug's effect.

• Zone of Inhibition: The clear area around a disk where bacterial growth is prevented; its size indicates the drug's effectiveness.

Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC)

• MIC: The smallest amount of a drug that will inhibit the growth and reproduction of a pathogen, usually measured in μg/mL.

• Broth Dilution Test: Serial dilutions of antimicrobial agents are inoculated with a standardized amount of bacteria. The lowest concentration with no visible growth (no turbidity) is the MIC.

• MBC: The minimum concentration of a drug required to kill the bacteria, determined by subculturing from the MIC tubes onto drug-free media.

Routes of Administration

The method of drug administration affects the concentration and effectiveness of antimicrobial agents in the body.

• Intramuscular (IM): Drugs diffuse slowly into the bloodstream; peak concentration is lower than with intravenous (IV) administration.

• Intravenous (IV): Delivers drugs directly into the bloodstream, achieving higher concentrations more rapidly.

Development of Drug Resistance

Resistance to antimicrobial agents is a significant concern in clinical microbiology.

• Selection, Not Induction: The presence of an antimicrobial agent does not create resistance but selects for the replication of resistant microbes already present in the population.

• Implication: Overuse or misuse of antibiotics accelerates the selection of resistant strains.

Summary Table: Key Terms and Concepts

Key Equations

• MIC Determination (Broth Dilution):

• Zone of Inhibition Measurement:

Additional info: Expanded explanations and context were added for clarity and completeness, including definitions, examples, and summary tables.