Abstract
BackgroundThe defining feature of the cerebrospinal fluid (CSF) collected from infants and children with tuberculous meningitis (TBM), derived from an earlier untargeted nuclear magnetic resonance (NMR) metabolomics study, was highly elevated lactic acid. Undetermined was the contribution from host response (L-lactic acid) or of microbial origin (D-lactic acid), which was set out to be determined in this study.MethodsIn this follow-up study, we used targeted ultra-performance liquid chromatography–electrospray ionization–tandem mass spectrometry (UPLC–ESI–MS/MS) to determine the ratio of the L and D enantiomers of lactic acid in these CSF samples.ResultsHere we report for the first time that the lactic acid observed in the CSF of confirmed TBM cases was in the L-form and solely a response from the host to the infection, with no contribution from any bacteria. The significance of elevated lactic acid in TBM appears to be that it is a crucial energy substrate, used preferentially over glucose by microglia, and exhibits neuroprotective capabilities.ConclusionThese results provide experimental evidence to support our conceptual astrocyte–microglia lactate shuttle model formulated from our previous NMR-based metabolomics study — highlighting the fact that lactic acid plays an important role in neuroinflammatory diseases such as TBM. Furthermore, this study reinforces our belief that the determination of enantiomers of metabolites corresponding to infectious diseases is of critical importance in substantiating the clinical significance of disease markers.Electronic supplementary materialThe online version of this article (doi:10.1186/s12879-016-1597-9) contains supplementary material, which is available to authorized users.
Highlights
The defining feature of the cerebrospinal fluid (CSF) collected from infants and children with tuberculous meningitis (TBM), derived from an earlier untargeted nuclear magnetic resonance (NMR) metabolomics study, was highly elevated lactic acid
Using a spiked sample containing both the L and D forms of lactic acid, and comparing the chromatogram qualitatively with the stable isotope (L-lactic acid-d3), it was evident that the UPLC–ESI–MS/MS method used in this study was able to differentiate between the two enantiomers (Fig. 1a and b)
We compared the total lactic acid concentrations in the corresponding samples with those derived from the UPLC–ESI–MS/MS method used in the current study and with those from the NMR method of Mason et al [14], and found a strong linear relationship between the two sets of results (r = 0.93), with a statistically significant positive correlation (0.86) and a statistically strong validation of the fit with an R2 value of 0.73
Summary
The defining feature of the cerebrospinal fluid (CSF) collected from infants and children with tuberculous meningitis (TBM), derived from an earlier untargeted nuclear magnetic resonance (NMR) metabolomics study, was highly elevated lactic acid. In the case of bacterial meningitis, in which bacteria infect the meninges, with disastrous consequences due to neuroinflammation, there have been numerous reports of highly acidotic CSF owing to the presence of lactic acid [8,9,10]. A unique form of bacterial meningitis, caused by Mycobacterium tuberculosis (Mtb), is tuberculous meningitis (TBM). The most noticeable feature that we detected was highly elevated lactic acid (7.36 ± 2.36 mM) in the CSF of these cases compared to the normal, agerelated reference range (1.65 ± 0.63 mM) [15]. Knowledge of increased CSF lactic acid levels in response to neuropathology is not new; distinguishing the respective roles of the L and D forms has not been attempted hitherto
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