Relative contributions of the stringent response mediators (p)ppGpp and DksA to Haemophilus ducreyi virulence in humans

Abstract

Haemophilus ducreyi causes chancroid, a sexually transmitted genital ulcerative disease that facilitates the transmission of HIV-1. H. ducreyi also causes non-sexually transmitted cutaneous ulcers in children in tropical regions. During human infection, H. ducreyi is subject to a variety of stresses. The stringent response is a bacterial stress response system induced by nutrient limiting conditions and mediated by guanosine tetra- and pentaphosphate [(p)ppGpp] and the transcriptional regulator DksA. (p)ppGpp and DksA jointly interact with RNA polymerase to regulate genes critical for bacterial survival. We hypothesized that the stringent response is required for H. ducreyi virulence in humans. A ΔrelAΔspoT mutant, which is unable to synthesize (p)ppGpp, was partially attenuated for abscess formation in human volunteers. Loss of (p)ppGpp increased bacterial resistance to phagocytosis and stationary phase survival; however, the mutant was more sensitive to oxidative stress. A ΔdksA mutant was also partially attenuated in humans. The ΔdksA mutant behaved like the (p)ppGpp mutant in stationary phase survival and sensitivity to oxidative stress, but exhibited decreased resistance to phagocytosis. Both mutants had decreased adherence to fibroblasts, but the mechanisms underlying the adherence defect were distinct. To better understand the roles of (p)ppGpp and DksA in regulating gene expression, we performed transcriptome analysis of the parent and mutant strains. (p)ppGpp and DksA deficiency resulted in dysregulation of multiple genes including several known virulence determinants. At stationary phase, (p)ppGpp and DksA targets were not identical but significantly overlapped; as the mutants were phenotypically distinct, this finding underscores both the unique and joint roles DksA and (p)ppGpp play in regulation of H. ducreyi virulence. We conclude that (p)ppGpp and DksA play significant roles in H. ducreyi pathogenesis. This is the first study to show that the stringent response has a direct role in the ability of a bacterial pathogen to cause disease in humans.