Characterization of Streptococcus agalactiae serotype Ia isolates from aquaculture in Israel

In 2024, abnormal morbidity and mortality events were documented in three commercially important fish species cultured in Israel: adult hybrid tilapia (Oreochromis spp., 800–1000 g), flathead grey mullet (Mugil cephalus, 40 g), and hybrid striped bass (Morone spp., 120 g), originating from both earthen and concrete pond systems. Aseptically collected brain, kidney, and spleen samples from moribund individuals were cultured on tryptic soy agar (TSA) supplemented with 5% sheep blood. Bacterial isolates were identified as Streptococcus agalactiae by species-specific PCR. Subsequent serotyping confirmed all isolates as serotype Ia. Multilocus sequence typing (MLST) was performed for genetic characterization, and whole genome sequencing (WGS) was conducted on isolates from each fish species to assess phylogenetic relatedness and genetic diversity. This investigation represents the first documented isolation and genomic characterization of S. agalactiae serotype Ia from fish in Israel, with findings consistent with globally reported piscine-associated serotype Ia strains.