Publication Date


Advisor(s) - Committee Chair

Paul Tanner, David Dunn, Robert Baum

Degree Program

Department of Public Health

Degree Type

Master of Science


The Kirby-Bauer test for determining antibiotic effectiveness is widely used in laboratories. The 10 to 20 hour incubation time needed to obtain useful results is a disadvantage of that test. This experimental research was developed to test a modification which could provide useful results in 5 hours.

The modification employed in this experimental technique used an increased inoculum at a 1.0 McFarland standard instead of the customary 0.5 standard. The 2 to 5 hour incubation period in the trypticase soy broth was deleted. The Mueller Hinton plates were incubated for 5 hours and then observed for resistant and/or sensitive patterns.

Controls for this experimental study were the results of the standard Kirby-Bauer test as recorded by the day and night shift personnel of the Medical Center at Bowling Green. Bowling Green, Kentucky. Tested were 33 cultures of Escherichia coil, 33 cultures of Pseudomonas aeruginosa, and 33 cultures of Staphylococcus aureus. The same cultures of each organism were tested using the 5 hour experimental procedure. A pure culture was inoculated in a tube of trypticase soy broth to a final turbidity equal to a 1.0 McFarland standard. A portion of this inoculum was swabbed onto the entire surface of a Mueller Hinton plate. Antibiotic discs were placed on the agar surface and tapped gently to insure contact. The plates were put into a 37°C incubator for 5 hours then removed to observe zones of no growth. Results were classified as either "resistant" or "sensitive"; "intermediate" was deleted. If a zone of no growth was closer to the sensitive reading than the resistant reading for an antibiotic, the bacterium was considered sensitive to that antibiotic. The same was true for resistant readings. Measurements were taken with a caliper dial.

For the two procedures, identical results occurred 99.7% of the time for Pseudomonas aeruginosa. For Escherichia coli 96.8% of the tests were identical, and with Staphylococcus aureus 93.2% of the tests were identical. Strains of Staphylococcus aureus that were sensitive to penicillin G and ampicillin with the standard Kirby-Bauer test were resistant with the 5 hour test. It occurred 10 times with a quality control stock culture and 1 time with a clinical isolate for ampicillin. It occurred 9 times with a quality control stock culture and 1 time with a clinical isolate for penicillin G. It is likely that the differences with Staphylococcus aureus for ampicillin and penicillin G are due to the interaction between the organism and the two antibiotics. Further studies are needed to determine whether or not a 1 to 2 hour extension of the incubation time could alleviate this problem.


Medicine and Health Sciences | Public Health

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