Haemophilus influenzae tryptophan biosynthesis is required for lung infection

Tryptophan plays a key role in regulating human lung homeostasis and immunity through the indoleamine 2,3-dioxygenase (IDO) and aryl hydrocarbon receptor (AhR) signalling pathways. In patients with chronic obstructive pulmonary disease (COPD), altered IDO and AhR activity is observed, potentially favouring infection by pathogens capable of synthesizing their own tryptophan. In this study, we investigated the contribution of tryptophan availability to lung infection by Haemophilus influenzae, a tryptophan synthesizing pathobiont associated with COPD exacerbations.

A chemically defined medium with controlled tryptophan levels was developed, and used to determine bacterial (i) metabolite consumption/excretion; (ii) genome-wide differential gene expression and post-transcriptional regulation; (iii) in vivo growth in a murine model of lung infection; and (iv) growth upon tryptophan biosynthesis allosteric inhibition.

Under tryptophan-rich conditions, we observed up-regulation of tnaA (encoding a tryptophanase) and tnaB (encoding a tryptophan transporter), accompanied by down-regulation of tryptophan biosynthetic genes. Furthermore, transcriptomic analysis combined with the ExcludonFinder computational tool generated an excludon map of the H. influenzae genome, and identified that the 3´-UTR regions of the convergent mtr tryptophan transporter and the sdaCA serine transporter-deaminase genes overlap, suggesting a post-transcriptional regulatory link between tryptophan and serine metabolism. In vivo, dietary modulation of tryptophan availability in a murine model supported effective lung infection as long as the bacterial tryptophan biosynthetic pathway remains functional. This biosynthetic requirement is supported by the in vitro growth inhibitory effect of indole propionic acid, a tryptophan derivative acting as TrpE allosteric inhibitor.

These findings demonstrate that H. influenzae's capacity to synthesize tryptophan is required for infection under host-imposed nutrient limitations, and highlight the potential of tryptophan biosynthesis as a target for antibacterial intervention.