Streptococcus suis is one of the major etiologic agents of contagious infections in swine and is also emerged as a zoonotic agent. A recent large scale outbreak has also affected human in China. So far, the accurate information about the macrolide resistant mechanism in S. suis clinical isolates of swine as well as human was not enough. Here we investigated the macrolide resistance in S. suis. MIC, followed by PCR and sequencing, was done to detect the genetic determinants involved in macrolide resistance. MIC results showed a high level resistant rate to macrolides. Likewise, methylation in the 23S rRNA genes by ErmB and efflux mediated by the MefA and MsrD are both found responsible for macrolide resistance in these studied clinical strains of S. suis. Furthermore, the mutants produced from clinical susceptible strains by stepwise induction of resistance were also checked for any other possible mechanism involved in macrolide resistance. Mutations on 23S rRNA, L4 and L22 ribosomal proteins were found responsible for macrolide resistance in passage mutants, these findings were different as compared with studied clinical strains. To our knowledge, this is first to report that the correlation between efflux pump and modifications in L22 ribiosomal protein contributed to develop the macrolide resistance in S. suis isolates.

Key words: Efflux Pump, Macrolide resistance, Methylation, Mutations, Streptococcus suis