Objective This study aims to investigate the differences in biological characteristics, molecular typing, and pathogenicity to bullfrogs among Streptococcus agalactiae strains from different host sources, providing a theoretical basis for aquaculture disease prevention and control.

Methods Five frog-derived strains one from Pelophylax nigromaculatus, 14^#; four from Rana catesbeiana, 19^#, 20^#, 24^#, 25^# and two Nile tilapia (Oreochromis niloticus)-derived strains (TW2018-78, TW2021-21) were selected. Species identification was conducted using 16S rRNA phylogenetic analysis and detection of the cfb gene. Further analyses included growth characteristics, capsular polysaccharide (CPS) serotyping, multilocus sequence typing (MLST), virulence and antibiotic resistance gene profiling, and evaluation of pathogenicity to bullfrogs through experimental infection.

Results All seven strains were identified as S. agalactiae. Phylogenetic analysis revealed that frog- and O. niloticus-derived strains belonged to distinct evolutionary lineages. The frog-derived strains were all of the Ib-ST261 type, optimally grew at 28 °C, and tested negative for both haemolysis and synergistic haemolysis (CAMP test). In contrast, the O. niloticus-derived strains were of the Ia-ST7 type, grew well at 37 °C, and tested positive for β-haemolysis and the CAMP test. Experimental infections showed that strain 25# (frog-derived) was highly pathogenic to bullfrogs, while O. niloticus-derived strains were non-pathogenic. Virulence gene profiling showed that frog-derived strains commonly carried cfb, hylB, and sip, with some also harboring bibA and fbsB, but lacked cylE and bca. O. niloticus-derived strains carried a broader spectrum of virulence genes. Antibiotic resistance gene analysis revealed that frog-derived strains only carried ermA, while O. niloticus-derived strains carried multiple resistance genes including ermA, sul1, qnrS, and pbp2x, with observed inconsistencies between genotype and phenotype.

Conclusion S. agalactiae strains from different hosts exhibit significant differences in genetic background, physiology, virulence profiles, antimicrobial resistance, and pathogenicity, indicating potential host adaptation. The results of the study provides a theoretical basis for targeted prevention and control strategies of S. agalactiae in aquaculture.