Question

Why is it important for the Kirby-Bauer disc diffusion test to use a standard concentration of each of the bacterial strains being tested?

1) If you were to use one strain that was stationary phase (high concentration, replicating very slowly or not at all), and another strain that was just beginning log phase (low concentration but replicating quickly), you could see different results in the test, affecting your interpretation.
2) Growth on the Mueller-Hinton agar plates utilized is very sensitive to the phase of the growth curve the bacteria are in when they are placed on the plate. If they are not in the log phase when they are placed on the plate, they will not grow and the test will be worthless.
3) Antibiotic resistance is usually only manifested by bacteria that have achieved a very high concentration. It's important to use bacteria specifically at this particular point for disc diffusion testing.
4) Antibiotics only work within a narrow range of cell concentrations. If you use a concentration that is too low or too high, you will get inaccurate measurements of the zone of inhibition.
5) Bacteria only develop resistance when there are more than 1012 cells/ mL. If resistance is to be detected, the test must use at least this concentration of cells. If fewer cells are used, no zone of inhibition will develop.

Answer 1

Standardizing the bacterial concentration in the Kirby-Bauer disc diffusion test is crucial to obtain consistent and accurate results, ensuring reliable interpretation of antimicrobial susceptibility across different laboratories.

Step-by-step explanation:

It is important to use a standard concentration of bacterial strains in the Kirby-Bauer disc diffusion test because varying concentrations can lead to inconsistent results that do not accurately reflect the antimicrobial susceptibility of the organism. Specifically:

• Using bacteria at different growth phases, such as the stationary phase and the log phase, could lead to differences in growth and susceptibility to antibiotics.
• A standardized concentration ensures that the antibiotic's effect is not influenced by varying bacterial densities, which could alter the size of the zone of inhibition.
• Antibiotic resistance and effectiveness need to be assessed at concentrations representative of those encountered in clinical infections.

Overall, standardization is crucial for reproducibility and accurate interpretation of the Kirby-Bauer test across different laboratories.