Xu Bin;Ma Zhe;Zhou Hong;Lin Hui-xing;Fan Hong-jie

Jiangsu Acad Agr Sci, Natl Res Ctr Vet Biolog Engn & Technol, Inst Vet Med, Key Lab Vet Biol Engn & Technol,Minist Agr & Rura, Nanjing 210014, Peoples R China;Yangzhou Univ, Jiangsu Coinnovat Ctr Prevent & Control Important, Yangzhou 225009, Jiangsu, Peoples R China;Nanjing Agr Univ, Coll Vet Med, MOE Joint Int Res Lab Anim Hlth & Food Safety, Nanjing 210095, Peoples R China

Streptococcus equi subsp. zooepidemicus;covS;natural variation;virulence;regulation

Streptococcus equi subsp. zooepidemicus (SEZ) is an important zoonotic agent. Here, a virulence-attenuated strain M35246 derived from natural variation of wild-type SEZ ATCC35246 was found. M35246 showed a deletion of 25 contiguous genes as well as a loss-of-function mutation in covS. Subsequently, a 25-gene-deleted strain (Delta PI), a covS-mutant strain (McovS), and relevant complementary strains were constructed and investigated. M35246 and McovS were significantly less encapsulated and exhibited poorer anti-phagocytic capacity compared to wild-type SEZ. McovS was significantly more sensitive to beta-lactams, aminoglycosides, macrolides, and lincosamides than wild-type SEZ. M35246, McovS, and Delta PI exhibited an increase in median lethal dose (LD50) in mice by 105, 105, and 5 times when compared to wild-type SEZ, respectively. Neither M35246 nor McovS were isolated from mice 48 h after being challenged with approximately 2 000 times the LD50 of wild-type SEZ. Transcriptome analysis showed that 668 significantly differentially expressed genes existed between McovS and wild-type SEZ. Numerous virulence factor-encoding genes and anabolic-related genes in McovS that were involved in anti-phagocytosis, capsule formation, pathogenicity, and antibiotic resistance were downregulated significantly relative to the wild-type strain. This study revealed that the CovS plays a vital role in the establishment of SEZ virulence.