Methods for determining antibiotic susceptibility

Qualitative assessment (agar diffusion method)

Qualitative methods are designed to give a clear answer to a basic clinical question: whether the isolated microorganism is susceptible to the selected drug or resistant to it. The main representative of this approach is the agar diffusion method.

Technically, the procedure consists of several sequential steps:

• Thorough inoculation of the studied microbe onto a solid nutrient medium (agar) is performed.
• The antibiotic is introduced. This is achieved either by placing the drug into specially prepared wells within the agar or by applying special paper carriers to the surface of the medium (the "disk method").

The results of this study are evaluated 24 hours after inoculation. A specialist visually assesses the state of the nutrient medium around the drug source. The key criterion for antibiotic efficacy is the presence of growth inhibition zones of bacteria around the disk or well. If such zones are absent, the microorganism is considered resistant to the given substance.

Quantitative concentration determination methods

Quantitative approaches aim to identify specific values, bringing the assessment to a more precise level. They determine two fundamentally important parameters of drug-microbe interaction:

1. Minimum Inhibitory Concentration (MIC) is the lowest amount of an antibiotic that successfully prevents visible growth of microorganisms under laboratory conditions (in vitro).
2. Minimum Bactericidal Concentration (MBC) is the concentration of the drug whose exposure leads to complete sterilization of the nutrient medium, i.e., complete eradication of the pathogen.

The clinical significance of these parameters is extremely high. Quantitative methods allow the physician to calculate the exact and safest dose of the drug for the patient.

Antibacterial therapy is based on a strict dosing rule: the achieved concentration of the antibiotic in the patient's blood must significantly exceed the MIC for this specific pathogen. Strict adherence to this rule is absolutely essential to ensure effective infection treatment and prevent the emergence of new resistant microbial strains.

Process automation

In modern laboratory practice, the automation of microbiological analysis processes is becoming increasingly important. Classical manual methods, requiring at least a 24-hour wait, do not always meet the needs of clinicians. To solve this problem, rapid susceptibility testing methods based on automated analyzers exist. These devices significantly accelerate the reporting of results while maintaining high accuracy of qualitative and quantitative assessment.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of growth inhibition zone formation in agar.
• Algorithms for calculating the therapeutic dose based on MIC values.
• Operating principles of modern automated microbiological analyzers.
• The process of disk standardization for the diffusion method.
• Factors contributing to the emergence of resistant strains when underdosing.

Other topics in the section "Chemotherapeutic Antimicrobial Drugs"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.