Minimal Inhibitory Concentration Values of Oxytetracycline for Bacterial Pathogens Isolated from Warmwater Fishes

Abstract

AbstractThe in vitro susceptibility to oxytetracycline (OTC) was determined through minimal inhibitory concentration (MIC) testing of 10 field isolates from each of seven fish pathogens: Edwardsiella ictaluri, E. piscicida, E. tarda, Flavobacterium columnare, Aeromonas hydrophila, Streptococcus iniae, and S. agalactiae. Bacteria from each species were cultured, diluted, dispensed into 96‐well microdilution plates, and incubated at 28°C. The MIC results were read at 48 h for all species except A. hydrophila, for which results were read at 24 h. The MIC ranges of OTC (μg/mL) for these bacteria were as follows: 0.5–64.0 for E. ictaluri; 0.5 to over 64.0 for E. piscicida; 0.5 to ≥64.0 for E. tarda; less than 0.12 (all) for F. columnare; 0.25 (all) for A. hydrophila; 0.5–16.0 for S. iniae; and 0.5–2.0 for S. agalactiae. Escherichia coli isolate ATCC 25922 was used as the MIC quality control throughout testing, and MIC values were within limits established by the Clinical and Laboratory Standards Institute (CLSI). Although there are no CLSI‐published or other standardized epidemiological cutoff values for these fish pathogens, the isolates with MIC values less than 1.0 μg/mL were deemed susceptible to OTC based on the locally established susceptibility ranges at the College of Veterinary Medicine, Mississippi State University. In the present study, 100% of A. hydrophila and F. columnare isolates; 80–90% of E. ictaluri, E. piscicida, and E. tarda isolates; 50% of S. iniae isolates; and 80% of S. agalactiae isolates were considered susceptible to OTC (MIC < 1.0 μg/mL).