Metabolic flux analysis revealed key roles of ArcB in ADI pathway and IlvC in BCAA biosynthesis during streptococcus suis anaerobic growth and infection

Critical metabolic enzymes and pathways specific to bacterial adaptation in different host microenvironments directly contribute to bacterial pathogenicity. In this study, a virulent strain of the important zoonotic pathogen Streptococcus suis was found to show enhanced growth under anaerobic conditions compared to that under aerobic conditions. Transcriptomic analysis found a significant suppression of many central metabolic genes during anaerobic growth of S. suis. The transcriptomic data were used to reconstruct a genome-scale metabolic network to assess the distribution of metabolic fluxes in S. suis under different conditions. Significant activation of the arginine deiminase (ADI) and branched-chain amino acid (BCAA) biosynthesis pathways was identified. The gene deletion mutants of arcB and ilvC participating in these two pathways, respectively, were constructed. Compared to the wild-type strain, the mutant ΔarcB showed more significant growth deficiency under anaerobic conditions than under aerobic conditions. Accumulation of ATP and NH₃, the metabolites of the ADI pathway, were significantly higher when S. suis was cultured under anaerobic conditions, and this effect was attenuated in the mutant ΔarcB. The knockout of IlvC of the BCAA pathway disrupted the normal growth of S. suis in valine- and isoleucine-limited medium under anaerobic conditions. Both ΔarcB and ΔilvC showed attenuation in mice with decreased lethality, bacterial loads in tissues and cytokine levels in serum, with the hypoxia-induced gene up-regulated in tissues. Therefore, ADI and BCAA pathways are critical for S. suis survival in response to hypoxia and infection in vivo, with ArcB and IlvC to be promising drug targets.