A Comparison of Disc Diffusion and Microbroth Dilution Methods for the Detection of Antibiotic Resistant Subpopulations in Gram Negative Bacilli

Abstract

Two of the different types of antibiotic susceptibility tests available are the solid media based Kirby Bauer method and the liquid media based microbroth dilution method. The microbroth dilution method has become more widely used due to its ability to become automated, however, the Kirby Bauer method may be able to detect subpopulations of resistant bacteria that would be visualized as inner colonies in the zone of diffusion. Using Gram negative rod isolated collected from the UWMC clinical microbiology laboratory, the presence of inner colonies was screened and both methods were compared to determine if the microbroth dilution method was able to detect the additional resistance of the subpopulations. The data went to show that the microbroth dilution method was not detecting the additional resistance in about 68% of the isolates tested. It was unable to be determined if the results were due a limitation of the microbroth dilution method or whether it is an artifact of the testing methods, but it does open up the possibilities of further testing to determine the cause of the disparity in results.