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

Aims 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 Strep. parauberis strains, thus establishing laboratory-specific epidemiological cut-off (COWT) values to distinguish wild-type (WT) and nonwild-type (NWT) strains. Methods and results Using 220 Strep. parauberis isolates obtained from diseased Paralichthys olivaceus, Platichthys stellatus, and Sebastes schlegelii over 6 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. Conclusions Interpretive criteria for Strep. 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.

Automated summary

This study aimed to determine the antimicrobial susceptibility of aquatic Strep. parauberis strains and establish laboratory-specific epidemiological cut-off (COWT) values to distinguish wild-type (WT) and nonwild-type (NWT) strains. Using 220 Strep. parauberis isolates obtained from diseased fish over 6 years in Korea, COWT values for eight common antimicrobial agents were established. Nine NWT isolates with decreased susceptibility to at least two antimicrobials were identified using COWT values based on NRI.