1. Isolate the Pathogen:
- The first step is to identify and isolate the infectious organism responsible for the infection. This may involve collecting a sample (e.g., a swab or blood culture) from the infected area and culturing it in a laboratory.
2. Select Antibiotics for Testing:
- Based on the type of infection and the suspected causative organism, the scientist selects a panel of antibiotics for testing.
3. Antimicrobial Susceptibility Testing:
- The most common method is the Kirby-Bauer disk diffusion test, also known as the disk diffusion method:
- Small disks containing different antibiotics are placed on an agar plate with the cultured bacteria.
- As the bacteria grow on the plate, they diffuse around the antibiotic disks.
- If the antibiotic is effective against the bacteria, a clear zone of inhibition (no bacterial growth) forms around the disk.
- Other methods include:
- Agar dilution method: Serial dilutions of antibiotics are incorporated into agar plates, and the lowest concentration that inhibits bacterial growth is determined.
- Broth microdilution: Antibiotics are diluted in broth, and the lowest concentration that prevents visible bacterial growth is recorded.
4. Interpretation of Results:
- The zones of inhibition or minimum inhibitory concentrations (MICs) obtained from the susceptibility tests are interpreted based on established guidelines and breakpoints.
- Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) provide standardized interpretive criteria.
- Antibiotics that show a sufficient zone of inhibition or low MIC (indicating susceptibility or effectiveness against the pathogen) are considered suitable for treating the infection.
By following this systematic approach, scientists can determine the most appropriate antibiotic for a specific infection. It helps ensure targeted treatment, minimizes the risk of antibiotic resistance, and improves patient outcomes.
