Antimicrobial susceptibility of Peruvian Yersinia ruckeri isolates to oxytetracycline and florfenicol using MIC testing

Yersiniosis, caused by Yersinia ruckeri, has become the most common disease in farmed rainbow trout Oncorhynchus mykiss in Peru, affecting Puno and Junín Regions, important aquaculture areas in the country. Florfenicol (FLO) and oxytetracycline (OXY) are the antimicrobials most frequently used to mitigate losses attributed to this pathogen. This study presents an analysis of the susceptibility patterns of 60 Y. ruckeri isolates (30 isolates each from Puno and Junín), including the type strain CECT 4319^T and the strains CECT 955 and CECT 956, against FLO and OXY. Minimum inhibitory concentrations (MICs) were determined following the guideline for standard broth dilution method published by the Clinical and Laboratory Standards Institute. MIC results ranged from 4.0 to 8.0 μg ml^–1 for FLO and 0.5 to 4.0 μg ml^–1 for OXY. Normalized resistance interpretation (NRI) analysis identified epidemiological cut-off values of ≤16.0 μg ml^–1 for FLO and ≤4.0 μg ml^–1 for OXY. All Peruvian isolates, including the collection strains, were categorized as wild-type for both antimicrobials. Even though the number of Y. ruckeri isolates with MIC values of 8 μg ml^–1 for FLO is more than double in Puno than in Junín (15 vs. 7 isolates), the NRI analysis showed the same epidemiological cutoff of 16 μg ml^–1; while for OXY, it was 4.0 μg ml^–1 for Puno and 2.0 μg ml^–1 for Junín. This study establishes the basis for monitoring susceptibility to FLO and OXY in new Y. ruckeri isolates in Peruvian rainbow trout farming.