The PdxR-PdxKU locus involved in vitamin B6 salvage is important for group A streptococcal resistance to neutrophil killing and survival in human blood.

Abstract

Streptococcus pyogenes (Group A Streptococcus, GAS) is a Gram-positive bacterium that inflicts both superficial and life-threatening diseases on its human host. Analysis of fitness using a transposon mutant library revealed that genes predicted to be involved in vitamin B₆ acquisition are associated with fitness in whole human blood. Vitamin B₆ is essential for all life and is important for many cellular functions. In several streptococcal species, it has been shown that mutants in B₆ acquisition exhibited reduced virulence phenotypes and were attenuated during infection. In GAS, B₆ acquisition is believed to be controlled by the pdxR-pdxKU locus, where PdxR is a positive regulator of pdxKU, which encodes for a B₆-substrate kinase and permease, respectively. Mutants in the regulator (ΔpdxR) and salvage machinery (ΔpdxKU) both exhibited modest growth defects when grown in oxygenated conditions with limited vitamin B₆ precursors. ∆pdxR and ∆pdxKU mutants also exhibited an impaired ability to survive when challenged with whole human or mouse blood. This defect was characterized by reduced survival in the presence of human neutrophil-like HL60s, primary polymorphonuclear leukocytes, and antimicrobial peptide LL-37. Promoter analysis showed that PdxR is an autoregulator and activated pdxKU in the absence of B₆. Interestingly, ∆pdxR and ∆pdxKU mutants were not attenuated in mouse models of infection, suggesting a species-specific impact on virulence. Overall, it appears that pdxR-pdxKU is associated with GAS vitamin B₆ metabolism as well as pathogen survival during encounters with the human innate immune system.IMPORTANCEBacterial pathogens such as Streptococcus pyogenes (Group A Streptococcus, GAS) must be able to obtain needed nutrients in their human host. Vitamin B₆ or pyridoxal 5' phosphate is essential for all life and is important for many cellular functions. In other streptococcal pathogens, B₆ acquisition has been shown to be important for their ability to cause disease. Here, we show that loss of the putative vitamin B₆ salvage pathway locus pdxR-pdxKU affects GAS pathogenesis when encountering innate immune responses from phagocytic neutrophils and antimicrobial peptides within the host. pdxR-pdxKU may contribute to oxygen tolerance through B₆; however, there appear to be other mechanisms for salvaging vitamin B₆. Overall, pdxR-pdxKU is associated with GAS resistance to the human innate immune response and oxygen tolerance and contributes modestly to B₆ metabolism.