Functional analysis of c-di-AMP phosphodiesterase, GdpP, in Streptococcus suis serotype 2

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen that causes serious diseases in pigs and humans. GdpP protein is a recently discovered specific phosphodiesterase that degrades cyclic diadenosine monophosphate (c-di-AMP). It is widely distributed among the firmicutes phylum and altered expression of GdpP is associated with several phenotypes in various bacterial strains. We investigated the role of GdpP in physiology and virulence in SS2. An in-frame mutant of gdpP was constructed using homologous recombination and bacterial growth, biofilm formation, hemolytic activity, cell adherence and invasion, expression of virulence factors, and virulence were evaluated. Disruption of gdpP increased intracellular c-di-AMP level and affected growth and increased biofilm formation of SS2. Simultaneously, the gdpP mutant strain exhibited a significant decrease in hemolytic activity and adherence to and invasion of HEp-2 cells compared with the parental strain. Quantitative reverse transcriptase polymerase chain reaction indicated significantly reduced expression of the known virulence genes cps2, sly, fpbs, mrp, ef and gdh in the gdpP mutant. In murine infection models, the gdpP mutant strain was attenuated, and impaired bacterial growth was observed in specific organs. All these findings revealed a significant contribution of gdpP and its substrate (c-di-AMP) to the biology and virulence of SS2.