The zoonotic pathogen Coxiella burnetii continues to pose a serious global public health concern. C. burnetii replicates inside host cells within a unique vacuole derived from the host phagolysosome, known as the Coxiella-containing vacuole (CCV). In order to cause disease, C. burnetii must not only survive the host defence mechanisms inherent in this vacuole, but also obtain the necessary energy and nutrients to replicate. Investigating the metabolic pathways required by C. burnetii to survive inside the host may identify novel therapeutic targets.
Recent stable isotope labelling studies by ourselves and others revealed that C. burnetii is capable of synthesising lactate. The C. burnetti genome lacks an annotated lactate dehydrogenase (LDH), implying the presence of an alternative mechanism for synthesizing lactate, which does not involve conversion from pyruvate. Lactic acid bacteria contain malolactic enzymes which convert malate to lactate. Our bioinformatic analysis revealed that CBU_0823, currently annotated as the NAD-dependent malic enzyme, SfcA, possesses 43% identity with the malolactic enzyme of the lactic acid bacterium Oenococcus oeni. As C. burnetii already possesses a putative malate dehydrogenase, MDH (CBU_1241), SfcA may function as a malolactic enzyme in C. burnetii, converting malate generated from the TCA cycle into lactate. Alternatively, the annotated MDH may perform a dual function and also possess LDH activity, as some MDH enzymes also have LDH activity.
We have successfully expressed and purified recombinant CBU_0823 and CBU_1241 as 6xHis N-terminal fusion and GST N-terminal fusion proteins respectively. Current work is focused on characterising the in vitro activity of these recombinant proteins, to identify if either is responsible for the observed lactate production. Future work will also aim to examine if this novel pathway is important for C. burnetii replication inside cells.