Evaluation of Antibiotic Susceptibility Tests: Agar Diffusion and Broth Dilution Assay Against Enterococcus faecalis

Abstract

Background: Antibiotic susceptibility testing (AST), a technique to measure antibiotic susceptibility to different infections, is used for drug invention, estimation of therapeutic outcomes, and evaluation of their ability to withhold bacterial growth. This study aimed to compare the antibiotic susceptibilities of various important antibiotics using agar diffusion and broth dilution assays against the growth of Enterococcus faecalis (E. faecalis).

Methods: This experiment was carried out in the Microbiology Laboratory at the Birmingham Dental Hospital, Birmingham. In the Agar Diffusion Assay, different solutions of concentrations (50mg/ml), punch into nutrient agar dishes in two groups, n=15(peer A1) and n=15 (peer A2), inoculated with strains E. faecalis. Inhibitory zones were measured under European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines. While in the Broth assay, bacteria were inoculated in 100 µL in the presence of multiple concentrations of antimicrobial solutions and bacterial growth was assessed using Optical Densities (OD) measurement.

Results: Agar diffusion assay showed the susceptibility of E. faecalis against Ampicillin, Gentamycin, and Erythromycin (OD< 0.1), whereas, it was found resistant with no zone of inhibition by Metronidazole (OD > 0.1). Similarly, broth dilution assay resulted in marked E. faecalis susceptibility to Ampicillin and Gentamycin at a minimum inhibitory concentration (5 mg/dl and 0.5mg/dl), but was not responsive to Metronidazole. When compared statistically with peer A2 non-significant values were obtained for Gentamycin, Ampicillin, and Erythromycin (p-value=0.5, 0.28, 0.23 respectively).

Conclusion: Antibiotics susceptibility measured by Broth Dilution Assay showed more authentic results in terms of minimum inhibitory concentration and optic density compared to Agar Diffusion Assay.

Keywords: Bacterial Sensitivity Tests; Microbiological Techniques; Serum Bactericidal Test; Minimum Inhibitory Concentration; Enterococcus faecalis; Zone Diameter Breakpoints.