Kirby Bauer Test Chart

The Kirby Bauer test chart was devised and is regularly updated by the Clinical Laboratory Standards Institute to ensure uniform sharing of information. It can save lives, shorten the length of illness and suffering, and help scientists understand medicine better. What is this chart, and why do we need it?

Curing Infectious Diseases

For thousands of years, infections in humans were treated by bloodletting. That’s right. Doctors would place leeches on patients to slowly suck the blood from the body and hope that the infection would be sucked out right along with the blood.

Shockingly, while in many cases bloodletting did not stop patients already dying from an infection or other illness, in many cases it did.

Indeed, there is now evidence that shows leeches can actually save limbs and even lives in some medical situations. Leeches, it has been found, will stop coagulation or congestion of blood, allowing it to flow more freely during the body’s natural reparation process.

Fortunately, those situations are pretty rare, so don’t expect your doctor to prescribe leeches next time you get strep throat or pneumonia.

For those conditions, and many other infections, we now have antibiotics.

Antibiotics, another natural occurrence, were discovered one day in 1928 when a microbiologist by the name of Alexander Fleming walked into his lab to find his week-old sandwich covered in mold.

On a hunch, he tested the mold to see how it held up against infection, only to find it highly successful at killing off infections.

Thus, penicillin was born.

From there, a revolution began.

The Prediction

The prediction, thanks to the success of the penicillin against infection, was that we now had the ability to defeat all infections in humans and that humans would remain largely well in the future.

Penicillin, and other antibiotics discovered since, were mass produced and prescribed as a blanket cure-all for myriad infections. The thinking, of course, was that it could certainly do no harm to overprescribe the drug.

The reality was far from that utopian image.

In fact, what did happen was that the bacteria that had previously been beaten back by penicillin and other drugs were now growing resistant to those drugs and greater and greater concentrations were required.

As doctors prescribed still greater concentrations and higher doses, the bacteria grew even stronger.

Another side effect of so many antibiotics in the human system is the destruction of good bacteria in the gut. After all, antibiotics, also known as antimicrobials, kill all the bacteria, not just the bad stuff.

A lack of good bacteria in the gut can lead to all kinds of medical issues in the human system, from headaches and digestive issues to autoimmune problems and diabetes. It’s that serious.

Thus, we needed a better system than simply throwing antibiotics at infections.

The Problem

To try to create a more targeted approach, microbiologists began to culture specific bacteria onto agar plates and then test various antibiotics of differing concentrations on the bacteria.

This process usually took two or three tests to see how effective an antimicrobial would be.

Globally, different labs had different approaches. Different processes were implemented to culture the agar, different agars were used, and different incubation periods and temperatures were used.

This wide variation led to lots of confusion in the medical field, leaving pharmaceutical companies and doctors at a loss as to what exactly to design and prescribe.

And of course, patients suffered.

The Rise of the Kirby Bauer Test

In the 1950s, two doctors, W. Kirby and A. Bauer, tested several different formats and approaches to what had come to be known as the disc diffusion test, and they landed on what they thought was the most effective procedure from start to finish.

At a global conference with multiple admissions, the Kirby Bauer Test was universally approved and adopted and now stands as the globally accepted disc diffusion test.

An agar plate is cultured with bacteria by swiping a cultured Q Tip in a tight zig zag motion back and forth. The plate is then turned 90 degrees and the culture is swiped again in the same way, from edge to edge to ensure the entire plate is covered.

Then, discs loaded with antimicrobial are placed strategically on top of the bacteria on the agar plate, ideally with an automatic disc dispenser.

Then, the lid should be placed on the Petri dish and placed into an incubation chamber at a temperature lower than 35 degrees Celsius for a minimum of 18 hours.

If you are testing for Staphylococcus, oxacillin, or vancomycin, you should wait for 24 hours.

At this point, you will use a ruler that measures in millimeters to measure the zone of inhibition. You will note a space between the discs and the bacteria on the plate. This is the zone of inhibition — the space that is free of bacteria thanks to the antibiotics.

To measure that space, place your ruler on one edge of the zone of inhibition and measure to its opposite edge, ideally crossing the center of the disc. You will include the disc’s diameter in your measurement.

The plate should always be viewed with the naked eye from the bottom, and you’ll round up to the next millimeter.

The Kirby Bauer Test Chart

From there, you can use the Kirby Bauer test chart to calculate how effective the antibiotic is against the bacteria you are testing.

You will see columns telling you which measurements signify resistance, which signify susceptibility, and the grey area in between.

You can find some useful charts here and here.

Be sure you are always looking at the most updated chart for your specific antibiotic and if there are any additional requirements for the specific bacteria you may be seeking to defeat.

In the end, there will always be limitations to every test and every study. Fortunately, the global scientific community is working hard to remain current and create space for real progress.

The Automated Alternative

Oculyze has developed an automated system for quickly and accurately measuring the size of the zones of inhibition on Agar plates for the disc diffusion (aka Kirby-Bauer) test. As already discussed, the susceptibility of each antimicrobial can only be quantified by measuring and comparing the size of the zones of inhibition. However, performing this step manually is tedious and error-prone. And this is where Oculyze comes in.

The Oculyze Disk Diffusion Test Results Analyzer makes accurate calculations for you. You just inoculate and incubate your agar plates and then upload the image of the plate to the Oculyze app. The results show you the diameter of each zone of inhibition, with the largest white number indicating the best reacting one.

In this way, you can not only trust in the measurement of the zone of inhibition, but you can also compare zone sizes across your plate or across multiple plates.

Test our Disk Diffusion Test Results Analyzer! Completely free of charge and with no commitment to purchase. Just please note that the recognition provided here is solely for demonstration purposes and may not accurately represent the performance of our product. Our customers receive customized recognitions tailored to their specific needs, which ensures high levels of accuracy.

Want to know more? Contact us and we’ll be happy to help!


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