Light and Temperature Cues Elicit Metabolic Reprogramming in the Non-phototrophic Bacterium Pseudomonas aeruginosa

Earth’s organisms are exposed to day-night cycles. These periodic changes in environmental factors, such as temperature and light exposure, trigger regulatory processes that coordinate physiological adaptations in organisms. Circadian organisms, i.e., most eukaryotes and some phototrophic bacteria, undergo autonomous 24-hour biological rhythms that are synchronized to day-night cycles via sensing light cues. However, the extent to which non-phototrophic bacteria tune their physiology to diurnal cycles and exhibit rhythmic behavior has been underexplored. For my thesis work, I investigated how the chemotrophic bacterium Pseudomonas aeruginosa responds to light and temperature signals. This metabolically versatile bacterium regulates its physiology through a vast array of environmental sensing mechanisms and has evolved multiple strategies to cope with redox imbalances. This thesis seeks to address how P. aeruginosa coordinates its metabolic and redox-balancing programs in response to light and temperature changes that occur in its environmental niche. In Chapter 1, I will present background information on relevant concepts such as biological rhythms and photosensory mechanisms and discuss how these principles are connected to physiological adaptations and metabolic plasticity in both phototrophic and non-phototrophic organisms, with a specific focus on chemotrophic bacteria. In Chapter 2, I will demonstrate that P. aeruginosa biofilm development is attenuated by light and that this process is regulated by the integration of light and redox signals. My work presented in Chapter 3 will provide evidence that the transcriptomic and metabolic landscape of P. aeruginosa is vastly reorganized in response to light/dark cycles. In the Chapter 4, I will explore how this reprogramming is manifested through activity by the respiratory machinery and I will demonstrate that P. aeruginosa undergoes intrinsic respiratory oscillations. As an opportunistic pathogen, P. aeruginosa will experience circadian-controlled changes during infection of a (circadian) host through host immune activity as well as exposure to cyclic environmental factors like light and temperature. I will discuss how environmental sensing is relevant for P. aeruginosa’s adaptation to its host-associated lifestyle. In conclusion, the research presented in this thesis establishes that P. aeruginosa exhibits an intricate physiological response to environmental signals, particularly light and temperature. This thesis contributes to a growing body of work that underscores how bacteria have evolved intricate mechanisms to integrate information about their environmental habitat, including host-associated conditions.