Establishment of Tentative Epidemiological Cut-Off Values and Class-Specific Genotype–Phenotype Concordance in Edwardsiella piscicida from Finfish Aquaculture in South Korea

Edwardsiella piscicida causes severe edwardsiellosis in farmed finfish, particularly Paralichthys olivaceus. Lacking clinical breakpoints, epidemiological cut-off values (ECVs) are crucial for distinguishing wild-type (WT) populations from non-wild-type (NWT) strains. We evaluated 68 E. piscicida isolates from South Korea (2022–2025) across six aquaculture antimicrobials. Tentative ECVs derived via Normalized Resistance Interpretation were established at 8/4 µg/mL for amoxicillin/clavulanic acid (64.7% WT), 2 µg/mL for florfenicol (92.6% WT), 0.5 µg/mL for oxytetracycline (38.2% WT), and 0.25/4.75 µg/mL for trimethoprim/sulfadiazine (51.5% WT). Due to pronounced bimodal distributions and mathematical fitting artifacts, objective ECVs could not be reliably determined for enrofloxacin and flumequine via unimodal modeling. ARG screening revealed high prevalence of blaTEM (54.4%) and qnrS (52.9%). Tetracycline resistance genes were harbored by 60.3% of isolates (tetD, 45.6%; tetA, 7.4%; tetB, 5.9%; tetB&tetD, 1.5%; tetE, 0%), showing strong statistical concordance with the oxytetracycline NWT phenotype. Furthermore, floR (7.4%) and sul2 (14.7%) strongly correlated with florfenicol and potentiated sulfonamide NWT phenotypes, respectively. Presumptive multidrug resistance (pMDR) remained low at 4.4% (n = 3). These findings establish empirical baseline ECVs for East Asian aquaculture, supporting responsible antimicrobial stewardship and diagnostic harmonization.