Antimicrobial Pharmacology: Susceptibility Testing and Therapeutic Index

As the drug diffuses from the disc into the agar, it creates a zone of inhibition (ZOI)—an area where the bacteria are unable to grow due to the presence of the drug. The diameter of this zone is measured and compared against standard values to determine if the organism is susceptible or resistant to that specific drug.

Tube Dilution Test

The tube dilution test is more sensitive and quantitative than the Kirby-Bauer method. In this procedure, the antimicrobial agent is serially diluted in tubes of broth. Each tube is then inoculated with a uniform sample of the pure culture and incubated.

This method allows researchers to find the Minimum Inhibitory Concentration (MIC), which is the smallest concentration of a drug that visibly inhibits the growth of the microbe. The MIC is vital for determining the smallest effective dosage and comparing the potency of different antimicrobials.

Evaluating Drug Safety: The Therapeutic Index

When choosing between two drugs that both show effectiveness in a lab, clinicians must look at the therapeutic index (TI). The therapeutic index is the ratio of the dose of the drug that is toxic to humans compared to its minimum effective (therapeutic) dose.

The relationship can be expressed as: $\text{Therapeutic Index} = \frac{\text{Toxic Dose}}{\text{Therapeutic Dose}}$

• Low TI: A small ratio (e.g., 1.1) indicates a high risk of toxicity, as the effective dose is very close to the toxic dose.
• High TI: A larger ratio (e.g., 10) indicates a wider margin of safety.

Worked Example: Imagine you are choosing between two drugs for a patient with no exceptional medical history:

• Drug A: Zone of inhibition = 30 mm; TI = 1.2
• Drug B: Zone of inhibition = 20 mm; TI = 12

While Drug A has a larger zone of inhibition (suggesting high potency in vitro), Drug B is the safer choice because it has a much higher therapeutic index, providing a much wider margin of safety for the patient.

Antibiotic Prescribing Considerations

Prescribing is both an art and a science. A clinician must take a careful history to avoid adverse reactions. Key considerations include:

• Preexisting conditions: Such as liver or kidney disease, which can affect how drugs are processed.
• Allergies: A history of allergy to specific drug classes (e.g., penicillins).
• Patient demographics: Infants, the elderly, and pregnant women require special precautions.
• Drug interactions: Other medications may increase toxicity or cause a drug to fail. Conversely, some combinations may have synergistic effects, allowing for reduced dosages.

Mechanisms of Action and Spectrum

Antimicrobials work by targeting specific structures or processes. The more unique the target is to the microbe, the better the selective toxicity.

Drugs are also categorized by their spectrum of activity:

• Narrow-spectrum: Effective against a limited array of microbes (e.g., Polymyxin, which is highly specific).
• Broad-spectrum: Effective against a wide variety of microbes, such as both Gram-positive and Gram-negative bacteria (e.g., Tetracycline).