Identification of novel genes expressed during host infection in Streptococcus equi ssp. zooepidemicus ATCC35246

• Employed IVIAT to identify novel virulence genes in SEZ.
• Found 43 SEZ candidate virulence genes, some having exceptional research value.
• Verified expression of 6 IVI genes via RT-PCR.
• Characterized the distribution of 10 IVI genes in different pathogenic strains of SEZ.
• Elucidated sagD gene was related to SEZ virulence.

Infection with Streptococcus equi ssp. zooepidemicus (Streptococcus zooepidemicus, SEZ) can cause septicemia, meningitis, and mastitis in domesticated species. Identification of this organism's virulence factors is an effective way of clarifying its pathogenic mechanism. We employed in vivo-induced antigen technology (IVIAT) to find bacterial genes that were only expressed or upregulated in an infected host (IVI genes). Convalescent-phase sera from pigs infected with SEZ were pooled, adsorbed against in vitro antigens, and used to screen SEZ genomic expression libraries. This analysis identified 43 genes as IVI genes. Six of these 43 genes were verified via real-time PCR. Following the analysis, we were able to assign a putative function to 36 of the 43 proteins. These proteins included those involved in virulence and adaptation; formation of intermediary products; gene replication, transcription and expression; energy metabolism; transport and also various proteins of unknown function. The relationship between sagD gene and bacterial virulence was confirmed. This study provides new molecular data for the study of streptococcal disease in swine and is important for identifying the pathogenic mechanisms of SEZ.