Actinobacillus pleuropneumoniae, a major bacterial porcine respiratory tract pathogen causing pig pleuropneumonia, has resulted in high economic losses worldwide. The mutation of the two-component system CpxAR strongly impacted the virulence of A. pleuropneumoniae, but the underlying regulatory mechanism remained unclear. Here, we found that CpxAR positively regulated the cpxDCBA gene cluster involved in polysaccharide capsule export. A capsular layer was confirmed in wild-type cells by transmission electron microscopy, whereas cpxAR and cpxD mutants were non-capsulated. The mutants for polysaccharide capsule export gene cpxD exhibited non-capsulated and were strongly impaired in virulence for mice, indicating a major role of CPS export system in virulence. We then demonstrated that CpxR directly regulated the transcription of the CPS export gene cluster cpxDCBA. Taken together, our data suggested that CpxAR is a key modulator of capsule export that facilitates A. pleuropneumoniae survival in the host.

IntroductionTo survive in various hostile environments, two-component system is an adaptive mechanism for diverse bacteria. Activity of the CpxA/CpxR two-component system contributes to coping with different stimuli, such as pH, osmotic and heat stress.MethodsHowever, the role of the CpxA/CpxR system in cold resistance is little-known. In this study, we showed that CpxA/CpxRwas critical for A. pleuropneumoniae growth under cold stress.Resultsβ-Galactosidaseanalysis showed that CpxA/CpxR positively regulated the predicted cold stress gene cspC. The mutant for cold stress gene cspC was impaired in the optimal growth of A. pleuropneumoniae under cold stress. Furthermore, electrophoretic mobility shift assays demonstrated that CpxR-P could directly regulate the transcription of the cold stress gene cspC.DiscussionThese results presented in this study illustrated that the CpxA/CpxR system plays an important role in cold resistance by upregulating expression of CspC. The data give new insights into how A. pleuropneumoniae survives in cold stress.