Reduced virulence is an important characteristic of biofilm infection of Streptococcus suis.

Streptococcus suis 2 (SS2) is a zoonotic pathogen that can participate in biofilm formation to survive in hostile environments. In this study, virulent SS2 strains HA9801 and ZY05719 displayed increased biofilm formation compared with SS2 avirulent strain T15. In addition, a 58% reduction in adherence to HEp-2 cells was observed for HA9801 biofilm cells, compared with HA9801 planktonic cells. The 50% lethal dose (LD(50) ) of biofilm cells was 40-fold greater than that of planktonic cells. Quantification of expression levels of known virulence genes by real-time PCR revealed that the transcription levels of the gdh, cps2 and mrp genes in biofilm cells were downregulated, while the sly and gapdh genes were upregulated. HA9801 biofilm and planktonic vaccines provided 60% and 46% protection, respectively, when challenged with 50 times the LD(50) of the HA9801 strain. These results suggest a possible connection between virulence and the ability of biofilm formation; cell adhesion, transcription levels and virulence properties are different between biofilm cells and planktonic cells. Furthermore, this work offers a novel insight into bacterium infection mechanisms, which suggests that a virulent strain may be able to decrease its virulence by forming a biofilm so that it can achieve persistent infection in vivo.