Haemophilus influenzae (HI) naturally resides in the human nasopharynx but, following dissemination into the lower respiratory tract, causes and contributes to many diseases including COPD. Although HI pathogenesis has been studied in detail, the metabolites used for energy generation during infection have not been unambiguously identified.
Metabolomic analyses of media from HI-infected tissue cells revealed that HI consumes lactate and some glucose and produces large amounts of acetate. In the human body, both L- and D-lactate are present and have been shown to support growth of human pathogens.
In HI, lactate utilization genes are highly conserved, with 87% of strains encoding two respiratory enzymes, L-lactate dehydrogenase (LldD) and D-lactate dehydrogenase (Dld), an NAD+-dependent D-lactate dehydrogenase (LdhA), and a lactate permease (LctP). Interestingly, seven strains contained the alternative LutABC lactate utilization system, with the two systems appearing mutually exclusive.
On chemically defined medium, HI2019, a COPD isolate, grew optimally on 10mM DL-lactate, both L- and D-lactate supported growth, and the presence of lactate induced LldD, Dld, and LdhA activity. Despite this, we recorded 2-fold greater respiration in response to L-lactate compared to glucose and D-lactate, indicating LldD is a key enzyme. This is further supported by RNAseq data from HI2019 co-cultured with 16HBE14 cells, where lldD, dld, and lctP were among the most abundantly expressed genes.
HI2019 carrying single gene knock outs in lldD, dld, and ldhA grew on L- and D- lactate, but HI2019∆lldD had ~50% reduced growth on lactate, further supporting the respiration findings. Unexpectedly, the loss of LldD or Dld also reduced activity of the respective other enzyme, despite them oxidising different lactate isomers.
Finally, phenotypic microarray data for several HI strains showed that high growth on lactate is a conserved feature, and we are currently investigating how loss of lactate utilization affects HI interactions with host cells.