Ongoing Evolution of the Lactate Dehydrogenase Reveals the Pleiotropic Effects of Bacterial Adaption to Host Pressure

The bacterial determinants that facilitate (Mtb) adaptation to the human host environment are poorly characterized. We have sought to decipher the pressures facing the bacterium by assessing Mtb genes that are under positive selection in clinical isolates. One of the strongest targets of selection in the Mtb genome is , which encodes a quinone-dependent L-lactate dehydrogenase (LldD2) that catalyzes the oxidation of lactate to pyruvate. Lactate accumulation is a salient feature of the intracellular environment during infection and is essential for Mtb growth in macrophages. We determined the extent of variation across a set of global clinical isolates and defined how prevalent mutations modulates Mtb fitness. We show the stepwise nature of evolution that occurs as a result of ongoing selection in the background of ancestral lineage defining mutations and demonstrate that the genetic evolution of additively augments Mtb growth in lactate. Using quinone-dependent antibiotic susceptibility as a functional reporter, we also find that the evolved mutations functionally increase the quinone-dependent activity of LldD2. Using C-lactate metabolic flux tracing, we find that is necessary for robust incorporation of lactate into central carbon metabolism. In the absence of , label preferentially accumulates in methylglyoxal precursors dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P) and is associated with a discernible growth defect, providing experimental evidence for accumulated lactate toxicity via a methylglyoxal pathway that has been proposed previously. The evolved variants increase lactate incorporation to pyruvate but also alter flux in the methylglyoxal pathway, suggesting both an anaplerotic and detoxification benefit to evolution. We further show that the mycobacterial cell is transcriptionally sensitive to the changes associated with altered activity which affect the expression of genes involved in cell wall lipid metabolism and the ESX-1 virulence system. Together, these data illustrate a multifunctional role of LldD2 that provide context for the selective advantage of mutations in adapting to host stress.