A Metabolomics-Based Analysis of Acyl-Homoserine Lactone Quorum Sensing in Pseudomonas aeruginosa
Pseudomonas aeruginosa is a metabolically versatile environmental bacterium that grows in extremely diverse habitats—from sea water to jet fuel—and is able to infect a large variety of organisms. It is a significant cause of human disease and is one of the most frequent healthcare-associated infections. P. aeruginosa uses a sophisticated gene regulatory network to adapt its growth strategy to these diverse environmental niches and the fluctuating conditions it encounters therein. The las and rhl “quorum sensing” (QS) intercellular communication systems play integral roles in this regulatory network and control the expression of factors important to the bacterium’s ecological fitness, including many secreted factors involved in nutrient acquisition, microbial competition, and virulence. These QS systems use diffusible acyl-homoserine lactone (AHL) signalling molecules to infer environmental parameters, including bacterial population density, and to coordinate behaviour across bacterial communities. This dissertation describes an investigation into the relationship between QS and small molecule primary metabolism, using metabolomic methods based on nuclear magnetic resonance spectroscopy and mass spectrometry. Analysis of extracellular metabolic profiles (the bacteria’s “metabolic footprint”) established that QS can modulate the uptake and excretion of primary metabolites and that this effect was strongest during the transition from exponential to stationary phase cell growth. Analysis of the cellular metabolome and proteome demonstrated that QS affected most major branches of primary metabolism, notably central carbon metabolism, amino acid metabolism and fatty acid metabolism. These data indicate that QS repressed acetogenesis and the oxidative C02-evolving portion of the TCA cycle, while inducing the glyoxylate bypass and arginine fermentation. QS also induced changes to fatty acid pools associated with lower membrane fluidity and higher chemical stability. Elevated levels of stress-associated polyamines were detected in QS mutants, which may be a consequence of a lack of QS-dependent adaptations. These findings suggest that wild-type QS directs metabolic adaptations to stationary phase stressors, including oxidative stress. Previous work, including several transcriptomic studies, has suggested that QS can play a role in primary metabolism. However, there has been no previous study of the global impact of AHL QS on the metabolome of P. aeruginosa. Research presented here demonstrates that QS induces a global readjustment in the primary metabolism and provides insight into QS- dependent metabolic changes during stationary-phase adaptation.
Pseudomonas aeruginosa, quorum sensing, metabolomics, intercellular signalling, bacterial physiology, functional genomics, mass spectrometry, nuclear magnetic resonance spectroscopy, bacterial metabolism
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