METABOLISM OF A DEUTEROPORPHYRIN-NITROIMIDAZOLE ADDUCT BY GASTROINTESTINAL BACTERIA

This thesis characterised the activity of a previously synthesised deuteroporphyrin-nitroimidazole adduct, ‘P5’. Surprisingly, P5 stimulated the growth of the nitroimidazole-sensitive Bacteroides spp. (five strains). In contrast P5 retained nitroimidazole activity, killing examined Clostridiaceae (five strains). Bacteroidetes incorporate porphyrin into b-type cytochromes, required to functionalise the three-component fumarate reductase operon (frdC, frdA, frdB). Growth of mutant Bacteroides fragilis defective in components of the fumarate reductase operon (frdC and frdB) was suppressed following challenge with P5 indicating the nitroimidazole moiety was activated, albeit with a lower potency than for metronidazole. Importantly, no stimulation of growth was detected for either mutant, indicating P5 was not acting as a source of porphyrin in the absence of a functional fumarate reductase system and that P5 was incorporated into a functional b-cytochrome. The metabolites of P5 incubated with B. thetaiotaomicron were characterised by liquid chromatography coupled mass spectrometry, and a possible structure that is incorporated into a b-cytochrome is proposed. Findings indicate anaerobic Bacteroidetes have efficient porphyrin uptake machinery, including previously unappreciated flexibility for the types of porphyrin that can be taken up and incorporated into b-cytochromes. Both processes appear to be independent of the metal ligand in Bacteroides. Prolonged mucosal inflammation and loss of barrier function, or inflammatory bowel disease, is at least partly mediated by bacteria, such as C. difficile. Since growth of B. thetaiotaomicron was stimulated by therapeutic concentrations (20 nM - 20 µM) of P5, this compound may have a practical use in treating clostridial-mediated IBD, where it is envisaged that the guided restoration of Bacteroides, to the detriment of Clostridia, could be a new treatment modality