Metabolic flux analysis revealed key roles of ArcB in ADI pathway and IlvC in BCAA biosynthesis during <i>streptococcus suis</i> 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 <i>Streptococcus suis</i> 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 <i>S. suis</i>. The transcriptomic data were used to reconstruct a genome-scale metabolic network to assess the distribution of metabolic fluxes in <i>S. suis</i> 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 <i>arcB</i> and <i>ilvC</i> participating in these two pathways, respectively, were constructed. Compared to the wild-type strain, the mutant Δ<i>arcB</i> 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 <i>S. suis</i> was cultured under anaerobic conditions, and this effect was attenuated in the mutant Δ<i>arcB</i>. The knockout of IlvC of the BCAA pathway disrupted the normal growth of <i>S. suis</i> in valine- and isoleucine-limited medium under anaerobic conditions. Both Δ<i>arcB</i> and Δ<i>ilvC</i> 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 <i>S. suis</i> survival in response to hypoxia and infection <i>in vivo</i>, with ArcB and IlvC to be promising drug targets.