The Kirby-Bauer test, officially known as the Disk Diffusion Susceptibility Test, is a foundational method in clinical microbiology laboratories worldwide. This standardized procedure determines if a specific bacterial isolate is inhibited by a selection of antimicrobial drugs. The test provides medical professionals with the necessary information to select an effective antibiotic treatment for a patient’s infection, aiding in successful patient care and infection management.
Determining Bacterial Susceptibility
The primary use of the Kirby-Bauer test is to perform Antimicrobial Susceptibility Testing (AST) on a bacterium isolated from a patient. This testing is necessary because the same species of bacteria can have different levels of resistance to various drugs. For instance, a Staphylococcus aureus isolate from one patient may be susceptible to penicillin, while an isolate from another patient may be fully resistant to the same drug.
Without AST, physicians would have to rely on empirical therapy, which is an educated guess based on the most common pathogens and known resistance patterns. While this approach is used when time is short, the Kirby-Bauer results reduce the risk of prescribing an ineffective drug. Using an ineffective drug can lead to treatment failure, prolonged illness, and potentially the selection of even more resistant bacteria within the patient.
The test is also used to monitor and combat the rise of antimicrobial resistance (AMR). By tracking the results from many individual tests, laboratories and health authorities can detect emerging resistance trends in hospitals and communities. This surveillance informs treatment guidelines and helps prevent the widespread use of drugs that are no longer effective against common infections, offering a reliable, cost-effective, and simple way to gather this data.
Performing the Disk Diffusion Test
The accuracy of the Kirby-Bauer test relies on a highly standardized methodology to ensure reproducible results across different laboratories. The process begins with preparing a standardized bacterial suspension, called the inoculum. Colonies of the isolated bacterium are suspended in a liquid medium, and the turbidity, or cloudiness, is adjusted to match the 0.5 McFarland standard.
This specific turbidity is equivalent to a bacterial concentration of approximately 150 million cells per milliliter, which ensures a consistent and uniform layer of growth on the plate. Next, a sterile swab is dipped into this standardized inoculum and used to evenly coat the entire surface of a Mueller-Hinton agar plate. Mueller-Hinton agar is the medium of choice because its controlled composition and pH do not interfere with the activity of most antibiotics.
Once the plate is inoculated and allowed to dry briefly, small paper disks impregnated with specific, known concentrations of different antibiotics are placed on the surface. These disks are spaced far enough apart to prevent the diffusion of antibiotics from overlapping. The plate is then incubated under standard conditions, allowing the bacteria to grow and the antibiotics to diffuse into the agar.
Reading and Classifying Results
After the incubation period, the results of the Kirby-Bauer test are determined by observing the growth pattern around each antibiotic disk. If the antibiotic is effective, a circular, clear area known as the “Zone of Inhibition” will surround the disk, indicating where bacterial growth was prevented. A larger zone suggests a greater inhibitory effect of the drug.
The diameter of this clear zone is precisely measured in millimeters using a ruler or caliper. The size of the zone is a raw measurement that must be compared against standardized clinical criteria to be clinically meaningful. These criteria, or interpretive breakpoints, are published by organizations such as the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST).
These standardized tables convert the zone size measurement into one of three clinical classifications: Susceptible (S), Intermediate (I), or Resistant (R). A Susceptible result indicates that the infection is likely to respond to treatment with that drug at the normal dosage. A Resistant result means the organism is not inhibited by the drug concentration, and treatment with that antibiotic will likely fail. The Intermediate classification suggests the drug might be effective if given at a higher dose or if used in combination with other agents.