Streptococcus parauberis is the main causative bacterial pathogen for streptococcosis in marine fish, causing a significant economic loss to the mariculture industry. In this study, we aimed to investigate the serotypes and antimicrobial susceptibilities of 83 S. parauberis isolates from diseased farmed fish (Paralichthys olivaceus, Platichthys stellatus and Sebastes schlegelii) in Korea. This study categorized the S. parauberis isolates based on PCR serotyping into serotype I (subtypes Ia and Ib/Ic) and serotype II. Serotype Ia was identified to be significantly more prevalent than serotypes Ib/Ic and II. An antimicrobial susceptibility analysis showed that all of the isolates were susceptible to amoxicillin/clavulanic acid. Overall, the isolates studied showed resistance to penicillin and ampicillin (1.2%), tetracycline (68.7%), oxytetracycline (72.3%), doxycycline (53.0%) and erythromycin (37.3%). Interestingly, we found that serotypes Ia and II showed higher degrees of tetracycline resistance than serotype Ib/Ic. The macrolide resistance gene erm(B) was present in 75.8% of the Ia isolates, whereas the mef(A) gene was not detected in any of the isolates. The tetracycline resistance genes tet(S) and tet(M) were detected in 95.2% and 60% of Ia and II isolates respectively. Moreover, β‐lactam antibiotic‐resistant Ia isolates contained a set of 11 mutations in PBP1A and PBP2X. Our study provides a valuable reference for surveillance of antimicrobial resistance and underscores the strict management of antibiotic use to prevent the emergence of resistant of S. parauberis serotypes.

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