Severe falciparum malaria is a potentially lethal parasitic infection for which the treatment remains suboptimal as the pathophysiology is incompletely understood. Acidosis is the strongest predictor of death, caused by an accumulation of acids of which some are known, such as L-lactate, while others remain unidentified. The aim of this thesis was to further characterise metabolic acidosis in severe falciparum malaria by establishing the identity and source of the unidentified non-lactate acid load. A prospective observational study was conducted among patients with falciparum malaria from Bangladesh to establish the identity of metabolites associated with acidosis through a metabolomic analysis of plasma of 152 participants (Chapter 3). Patients with acidosis had significantly elevated levels of organic acids in their plasma that could be traced to a potential microbial source, either parasitic or bacterial. Additionally, dysregulation of plasma free amino acids was observed. The clearance of newly identified microbial acids was impaired in fatal cases. Follow-up studies were conducted by performing an in vitro metabolomic analysis of the liquid culture medium of Plasmodium falciparum 3D7 strain parasites with the aim to determine if parasites produce any of the microbial acids identified in patients with severe malaria (Chapter 4). There was no evidence for the release of microbial acids by Pf 3D7 in vitro suggesting their source is unlikely to be parasitic. Alongside of this, a prospective study was done to determine factors related to plasma free amino acid dysregulation (Chapter 5). Amino acids were simultaneously quantified in the plasma of 295 participants in relation to clinical syndromes. It was observed that amino acid abnormalities were part of widespread fluctuations in nitrogen balance, whereby the direction of change was related to the severity of malarial disease and the presence of hyperlactataemia, reflecting underlying tissue hypoxia. Finally, the role of the gut microbiota was further investigated as a potential source of microbial acids in 86 participants, including cases with severe falciparum malaria acidosis, by measuring soluble markers of gut integrity in plasma and sequencing the V4 region of the 16S rRNA gene in faecal DNA (Chapter 6). Patients with malaria had signs of impaired gut barrier function with elevated levels of plasma D-lactate linked to an enrichment of lactate-producing species in the gut microbiota. In conclusion, acidosis in severe falciparum malaria is associated with microbial acids and plasma free amino acid derangements. Microbial acids are not produced by Pf3D7 in vitro but instead might derive from the gut microbiota. These findings point towards a leaky gut as a possible source of previously unidentified acids in severe falciparum malaria acidosis with potential implications for treatment.</p
