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Updated: Jun 24, 2026
A Reference Broth Microdilution Method for Dalbavancin In Vitro Susceptibility Testing of Bacteria that Grow Aerobically
Published on: September 9, 2015
Improved method for determining antibiotic susceptibility of Flavobacterium columnare isolates by broth microdilution
Ahmed M Darwish1, Bradley D Farmer, John P Hawke
1U.S. Department of Agriculture, Agricultural Research Service, Harry K. Dupree-Stuttgart National Aquaculture Research Center, Post Office Box 1050, 2955 Highway 130 East, Stuttgart, Arkansas 72160, USA. ahmed.darwish@ars.usda.gov
Abstract:
A simple and reproducible microdilution method was developed to test the susceptibility of the bacterium Flavobacterium columnare to antibiotics in vitro. The testing was conducted at 28 degrees C for 44-48 h at two dilutions of Mueller-Hinton broth (DMHB) using a standardized inoculum, a reference isolate of Escherichia coli ATCC25922 as a quality control organism, positive and negative control wells, and standardized custom-made microtiter plates. The E. coli ATCC25922 and F. columnare ATCC23463 (the species type strain) had significantly better growth in DMHB at 1:5 (4 g/L) than at 1:7 (3 g/L). The E. coli ATCC25922 was found to be acceptable as a reference isolate and produced minimum inhibitory concentration (MIC) values similar to those in the range published by the Clinical and Laboratory Standards Institute derived using standard Mueller-Hinton broth. The new method was used to determine the MIC of 23 F. columnare isolates (representing the three genotypes of the species) to enrofloxacin, ampicillin, oxytetracycline, erythromycin, florfenicol, flumequine, ormetoprim/sulfadimethoxine, and oxolinic acid.
