Effect of initial bacterial innoculum density on zones of inhibition in disk diffusion antibiotic susceptibility testing

Effect of initial bacterial innoculum density on zones of inhibition in disk diffusion antibiotic susceptibility testing

Aldijana Hadžić

Nudžejma Kudić

Monia Avdić

Lejla Smajlović-Skenderagić

Alisa Smajović

Samra Međedović

Abstract - The primary objective of this study was to show how the density of the initial bacterial inoculum effects the zone inhibition in Disk diffusion Bauer Kirby (BK) antibiotic susceptibility testing. In this study, three strains of Staphylococcus aureus were tested: a methicillin sensitive clinical strain, a methicillin resistant clinical strain and ATCC 25923. A series of decreasing initial inoculum densities of the three tested strains were prepared and poured onto Mueller Hinton agar plates. After overnight incubation the zones of inhibition around tested antibiotics from different inoculum densities were measured in mm. The results showed that inoculum density does have an effect on the zones of inhibition in BK antibiotic susceptibility testing of S.aureus where in the case of gentamycin sensitivity category change occurred. Correlation analysis showed that there is significant negative correlation between tested inoculum densities and zones of inhibition clinical methicillin sensitive strain of S.aureus after using oxacillin and gentamycin (Pearson coefficient were -0.917 and -0.892, respectively), and between tested inoculum densities and zones of inhibition clinical methicillin resistant strain of S.aureus after using ampicillin (Pearson coefficient was -0.960). Hence, initial bacterial inoculum density can be of high relevance in Bauer-Kirby disk diffusion testing and ought to be precisely determined in purpose of adequate therapy ordination.

Keywords: Staphylococcus aureus, susceptibility testing, McFarland standards, antibiotics, zones of inhibition.

International Burch University

Journal of Natural Sciences and Engineering