Page 56 - 《渔业研究》2025年第6期

第 6 期 戴景辉等：蛙源与鱼源无乳链球菌的遗传分化、基因分型、毒力谱、耐药性与致病性比较研究 747

Comparative studies on genetic differentiation, genotyping, virulence profiles,
drug resistance and pathogenicity of Streptococcus agalactiae of frog
and fish origin

DAI Jinghui，ZHOU Dan，BAI Lin，YANG Tao *
(Key Laboratory of Aquatic Nutrition and Smart Aquaculture, Ministry of Agriculture and Rural Affairs, Tongwei Agricultural
Development Co., Ltd., Chengdu 610041, China)

Abstract: [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 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-de-
rived) 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
bca, bibA and fbsB, but lacked cylE and scpB. O. niloticus-derived strains carried a broader spectrum of vir-
ulence 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 ex-
hibit 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 tar-
geted prevention and control strategies of S. agalactiae in aquaculture.
Key words: Streptococcus agalactiae; phylogeny; multilocus sequence typing (MLST); capsular polysaccharide
serotype; virulence genes; drug resistance; pathogenicity