Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is one of the important zoonotic and opportunistic pathogens. In recent years, there has been growing evidence that supports the potential role of S. zooepidemicus in severe diseases in horses and other animals, including humans. Furthermore, the clinical isolation and drug resistance rates of S. zooepidemicus have been increasing yearly, leading to interest in its in-depth genomic analysis. In order to deepen the understanding of the S. zooepidemicus characteristics and genomic features, we investigated the genomic islands, mobile genetic elements, virulence and resistance genes, and phenotype of S. zooepidemicus strain ZHZ 211 (ST147), isolated from an equine farm in China. We obtained a 2.18 Mb, high-quality chromosome and found eight genomic islands. According to a comparative genomic investigation with other reference strains, ZHZ 211 has more virulence factors, like an iron uptake system, adherence, exoenzymes, and antiphagocytosis. More interestingly, ZHZ 211 has acquired a mobile genetic element (MGE), prophage Ph01, which was found to be in the chromosome of this strain and included two hyaluronidase (hyl) genes, important virulence factors of the strain. Moreover, two transposons and two virulence (virD4) genes were found to be located in the same genome island of ZHZ 211. In vitro phenotypic results showed that ZHZ 211 grows faster and is resistant to clarithromycin, enrofloxacin, and sulfonamides. The higher biofilm-forming capabilities of ZHZ 211 may provide a competitive advantage for survival in its niche. The results expand our understanding of the genomic, pathogenicity, and resistance characterization of Streptococcus zooepidemicus and facilitate further exploration of its molecular pathogenic mechanism.

The zoonotic pathogen Streptococcus equi subspecies zooepidemicus (SEZ) frequently colonizes equines harmlessly but can occasionally cause disease or cross species barriers. Currently, growing evidence suggests SEZ can lead to severe clinical manifestations in horses and other animals, posing a threat to human and companion animal health. In this study, we sequenced the complete genome of the SEZ strain HT321, a novel sequence type 420 isolated from a donkey with a respiratory infection in China. Subsequently, we conducted comparative genomics, core genome single nucleotide polymorphisms (cgSNP), phylogenetic analysis multilocus sequence typing (MLST), and in vitro pathogenic analysis of this isolate. 118 genes in HT321 were annotated as antibiotic resistance genes (ARGs) and comparative genomics revealed that HT321 contained more lincosamide ARGs compared to other strains. The genomic island of HT321 carried more defensive virulence genes than that in the horse strain JMC111. Furthermore, compared to the reference equine strain JMC111, HT321 exhibited superior antimicrobial resistance and biofilm formation capability but lower pathogenicity. Interestingly, core genome single-nucleotide polymorphism phylogenetic analysis of 51 SEZ strains demonstrated that HT321 clustered with horse and donkey SEZ strains as well as S. canis strains. Notably, MLST analysis of the HT321 and 116 SEZ strains indicated that the HT321 donkey strain was related to SEZ canis isolates. These findings provide valuable insights for understanding, tracking, controlling, and preventing diseases caused by SEZ in donkeys.