Abstract
Mycobacterium tuberculosis (Mtb) uses alveolar macrophages as primary host cells during infection. In response to an infection, macrophages switch from pyruvate oxidation to reduction of pyruvate into lactate. Lactate might present an additional carbon substrate for Mtb. Here, we demonstrate that Mtb can utilize L-lactate as sole carbon source for in vitro growth. Lactate conversion is strictly dependent on one of two potential L-lactate dehydrogenases. A knock-out mutant lacking lldD2 (Rv1872c) was unable to utilize L-lactate. In contrast, the lldD1 (Rv0694) knock-out strain was not affected in growth on lactate and retained full enzymatic activity. On the basis of labelling experiments using [U-13C3]-L-lactate as a tracer the efficient uptake of lactate by Mtb and its conversion into pyruvate could be demonstrated. Moreover, carbon flux from lactate into the TCA cycle, and through gluconeogenesis was observed. Gluconeogenesis during lactate consumption depended on the phosphoenolpyruvate carboxykinase, a key enzyme for intracellular survival, showing that lactate utilization requires essential metabolic pathways. We observed that the ΔlldD2 mutant was impaired in replication in human macrophages, indicating a critical role for lactate oxidation during intracellular growth.
Highlights
Mycobacterium tuberculosis (Mtb) uses alveolar macrophages as primary host cells during infection
Essentiality of gluconeogenesis for intracellular survival was established in Mtb lacking phosphoenolpyruvate carboxykinase (PckA) activity[2], highlighting the important role of carbon substrates oxidized in the tricarboxylic acid (TCA) cycle and subsequently carboxylated for gluconeogenesis
We analysed the role of lactate, another metabolite feeding in the TCA cycle, as an additional carbon source
Summary
Mycobacterium tuberculosis (Mtb) uses alveolar macrophages as primary host cells during infection. Lactate might present an additional carbon substrate for Mtb. Here, we demonstrate that Mtb can utilize L-lactate as sole carbon source for in vitro growth. During infection Mtb depends on carbon substrates feeding in the central carbon metabolism (CCM) down-stream of PEP. Growth on these substrates demands gluconeogenesis for biomass formation. Because Mtb is able to co-metabolize multiple carbon substrates[6], additional gluconeogenic carbon sources might be catabolized in a PckA-dependent manner and thereby influence intracellular growth and survival. Lactate is incorporated in the CCM through conversion into pyruvate, entering metabolism down-stream of PEP Innate immune cells such as neutrophils, dendritic cells and macrophages produce lactate by the so-called Warburg effect[7]. Two genes encoding for potential quinone-dependent L-lactate dehydrogenase are annotated in the genome of Mtb, lldD1 (Rv0694) and lldD2 (Rv1872c)[13]
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