Antimicrobial Susceptibility Testing: Kirby-Bauer Method

Antimicrobial Susceptibility Testing

Antimicrobial drugs are derived from microorganisms capable of killing or inhibiting the growth of other microorganisms.

Selecting Antibiotics

It is necessary to maintain a routine panel of antibiotics, supplemented by additional agents when dealing with highly resistant bacterial strains. It is recommended to prioritize antibiotics with a narrower spectrum and lower potency, reserving broad-spectrum, high-potency agents for cases of significant bacterial resistance where treatment options are limited.

Antibiotic Susceptibility Techniques

• Agar diffusion method (Kirby-Bauer method)
• Broth dilution method
• Microdilution method

Kirby-Bauer Agar Diffusion Technique

This method involves inoculating a standardized amount of bacteria onto solid agar, where they are exposed to filter paper disks impregnated with specific antibiotics.

Nutrient Agar

Mueller-Hinton agar is the most commonly used medium, as it supports the growth of non-fastidious bacteria.

Antibiotic Disks

These are small filter paper circles impregnated with a specific antibiotic at therapeutic concentrations.

Temperature and Incubation Time

The suggested incubation temperature for inoculated plates is 36°C for a duration of 18 to 24 hours under aerobic conditions.

Zones of Inhibition

The growth inhibition zones surrounding the antibiotic-impregnated filter paper disks are measured in millimeters.

Interpretation of Results

There are three primary categories for reading results:

• Sensitive (S): The bacteria failed to grow in the area surrounding the disk, forming a "zone of inhibition." This indicates the bacteria cannot survive in the presence of the drug.
• Intermediate or Moderately Sensitive (I, MS): The bacteria grew, but the inhibition halo is small, making the drug's clinical efficacy questionable.
• Resistant (R): The bacteria grew around the disk, indicating the microorganism is not inhibited by the drug and suggesting it should not be used to treat the infection.

Limitations of the Kirby-Bauer Method

This technique is only reliable for fast-growing pathogens, such as enterobacteria, staphylococci, Pseudomonas, Acinetobacter, and certain streptococci.