Evaluation of MIC distribution and establishment of epidemiological cut-off values for Streptococcus parauberis isolated from aquatic animals.

Abstract

Streptococcus parauberis is responsible for the development of streptococcosis in marine fish. The aim of the current study was to determine the antimicrobial susceptibility of aquatic S. parauberis strains, thus establishing laboratory-specific epidemiological cut-off (COWT) values to distinguish wild type (WT) and non-wild type (NWT) strains. Using 220 S. parauberis isolates obtained from diseased Paralichthys olivaceus, Platichthys stellatus, and Sebastes schlegelii over six years from seven different locations in Korea, we established COWT values for eight common antimicrobial agents using the standard broth microdilution method. The COWT values calculated using MIC distribution with the NRI and ECOFFinder methods were the same or within one dilution step for the eight antimicrobials tested. Nine NWT isolates with decreased susceptibility to at least two antimicrobials and one of these isolates exhibited decreased susceptibility to six antimicrobial agents were identified using COWT values based on NRI. Interpretive criteria for S. parauberis have not yet been established, and the findings of this study provide putative COWT values for eight antimicrobial agents frequently used in aquaculture in Korea.