The QseB/QseC two-component system contributes to virulence of Actinobacillus pleuropneumoniae by downregulating apf gene cluster transcription

Actinobacillus pleuropneumoniae(APP)is the major pathogen of porcine contagious pleuropneumoniae(PCP).The QseB/QseC two-component system(TCS)consists of the regulator QseB and the kinase QseC,which relates to quorum sensing(QS)and virulence in some bacteria.Here,we investigated the role of QseB/QseC in apf gene cluster(apfABCD)expression of APP.Our results have showed that QseB/QseC TCS can potentially regulate the expression of apf gene cluster.The△qseBC,△apfA,△apfB,△apfC and△apfD strains are more sensitive to acidic and osmotic stressful conditions,and exhibite lower biofilm formation ability than wild-type(WT)strain,whereas the complemented strains show similar phenotype to the wr strain.In additon,the mutants have defective antiphagocytosis,adhesion and invasion when they come into contact with the host cells.In experimental animal models of infection,mice infected with△qseBC,△apfA,△apfB,△apfC and △apfD strains showed lower mortality and bacterial loads in the lung and the blood than those infected with wr strain.In conclusion,our results suggest that QseB/QseC TCS contributes to stress resistance,biofilm formation,phagocytosis,adhesion,invasion and virulence by downregulating expression of apf gene cluster in A.pleuropneumoniae.