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.