Pyridoxal phosphate synthases PdxS/PdxT are required for Actinobacillus pleuropneumoniae viability, stress tolerance and virulence.

Pyridoxal 5'-phosphate (PLP) is an essential cofactor for numerous enzymes involved in a diversity of cellular processes in living organisms. Previous analysis of the Actinobacillus pleuropneumoniae S-8 genome sequence revealed the presence of pdxS and pdxT genes, which are implicated in deoxyxylulose 5-phosphate (DXP)-independent pathway of PLP biosynthesis; however, little is known about their roles in A. pleuropneumoniae pathogenicity. Our data demonstrated that A. pleuropneumoniae could synthesize PLP by PdxS and PdxT enzymes. Disruption of the pdxS and pdxT genes rendered the pathogen auxotrophic for PLP, and the defective growth as a result of these mutants was chemically compensated by the addition of PLP, suggesting the importance of PLP production for A. pleuropneumoniae growth and viability. Additionally, the pdxS and pdxT deletion mutants displayed morphological defects as indicated by irregular and aberrant shapes in the absence of PLP. The reduced growth of the pdxS and pdxT deletion mutants under osmotic and oxidative stress conditions suggests that the PLP synthases PdxS/PdxT are associated with the stress tolerance of A. pleuropneumoniae. Furthermore, disruption of the PLP biosynthesis pathway led to reduced colonization and attenuated virulence of A. pleuropneumoniae in the BALB/c mouse model. The data presented in this study reveal the critical role of PLP synthases PdxS/PdxT in viability, stress tolerance, and virulence of A. pleuropneumoniae.