The Kirby Bauer Test, a Way to Determine Whether or Not Bacteria are Affected by Antibiotics

The Kirby Bauer test is an inexpensive way to test whether not bacteria are affected by antibiotics. This test was conducted on eight antimicrobials to test and compare the effectiveness, susceptibility, and zones of inhibition During this experiment Mueller- Hinton agar plates were used for the testing, and the two bacterial cultures compared were Escherichia coli and Staphylococcus aureus. The eight antimicrobials tested were Ampicillin (10 pg, AM 10), Chloramphenicol, Ciprofloxacin, Erythromycin (15 pg, E 15), Kanamycin (30 pg, K 30), Rifampin (5 pg, RA 5), Trimethoprim (5 pg, TMP S), and Streptomycin.

The zone of inhibition was determined by observing a clearing around the antimicrobial, the diameter of the clearing was then measured in millimeters, Scientists are able to relate the zone of inhibition to antimicrobial effectiveness by using the concept that.

If the observed zone of inhibition is greater than or equal to the standard size, the organism’s relationship to the antimicrobial is considered to be sensitive. If the zone of inhibition is determined to be less than the standard measurement, the relationship to the organism is considered to be resistant to the antimicrobial (Rollins, DM.

and SW, Joseph), The measurement of the diameter, or the zone of inhibition was dependent on diffusion rate of the antimicrobial, degree of sensitivity of the microorganism, and the growth rate of the bacterium. A standard zone diameter interpretation chart is used to interpret whether the microbial is susceptible, intermediate or resistant. Ciproflaxacin‘s (CIP 30) zone of inhibition had the diameter of 35mm, making it susceptible to E.coli, In comparison, the disk size around CIP 30 on the Staphylococcus aureus culture measured 30mm, still making it susceptible to the bacterial culture but slightly less.

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Clinically, being that the larger the disk size the better (Ericsson JM), this means that this would be a fairly effective compound to use to fight S. aureus at the correct dosage, and an even better option for E coli, Kanamycin (K30) had the disk size of 22mm, meaning it was susceptible to E.coli. On the S, aureus plate, the disk was just 2 millimeters more at 24mm. The antimicrobial TMP 5 had a diameter of 26mm, and was susceptible, and on S, aureus the diameter was 22. The Disk Diameter of Rifampin (RAB) was 11mm, turning out to be resistant to E. coll. Clinically, this would not be a viable choice to treat E. coli because it would not respond to it, regardless of the dosage, RAS‘s diameter on S. aureus was 30mm, a huge difference when compared to Eicoli. This translates to Rifampin being way more effective in treating a Staphylococcus aureus infection.

Streptomycin’s (510) disk size was 20mm on ECU“, and 18mm on S. aureus, meaning it was susceptible both times and a fairly good approach to treatment. Ampicillin (AMIO) had a 22mm diameter, translating to being susceptible to the Etcoli culture. On the S. aureus culture, the diameter reached 37mm, which is a sign of it being the most effective out of all eight microbials tested on Staphylococcus aureus. The large diameter matches the original hypothesis of this antimicrobial being bactericidal, meaning that it kills bacteria and completely strips it’s reproduction capabilities.

Chloramphenicol (C30) had a fairly large diameter of 29mm on the Ecoli culture, and on Staphylococcus aureus it was measured as 26mm. Lastly, on Etcoli Erythromycin (E15) had a disk diameter of 15mm, translating to intermediate and falls under the category of bacteriostatic. For Etcoll this would be a moderate approach to treatment. The diameter on Staphylococcus aureus was 26mm, interpreting to susceptible – this antimicrobial would work better to treat S. aureus better than[i. All eight of the antimicrobial disk diameters for both pure cultures matched the expected range on the standardized zone of inhibition chart. Overall, AMlO was determined as the most effective antimicrobial for Staphylococcus aureus, and 510 was the worse. For E. coll, CI? 5 was deemed the best antimicrobial for treatment, and RA 5 was the least.

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The Kirby Bauer Test, a Way to Determine Whether or Not Bacteria are Affected by Antibiotics. (2023, May 15). Retrieved from

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