Title: Redox-Balancing Strategies in Pseudomonas aeruginosa
Description: In natural habitats bacteria predominantly grow and survive as biofilms, which are densely populated assemblages of cells encased in self-produced matrices. Biofilms face the challenge of resource limitation due to poor substrate diffusion and consumption by cells closer to the periphery. When terminal electron acceptors for metabolism, such as oxygen, are limiting, reducing equivalents accumulate in the cell, leading to an imbalanced redox state and disruption of metabolic processes. The opportunistic pathogen Pseudomonas aeruginosa possesses various redox-balancing strategies that facilitate disposal of excess reducing power, including (i) production of phenazines, redox-active compounds that mediate extracellular electron shuttling; (ii) use of nitrate as an electron acceptor via the denitrification pathway, and (iii) fermentation of pyruvate. However, if the biofilm grows to a point where these metabolic strategies become insufficient, the community adopts a “structural” strategy: the cells collectively produce extracellular matrix to form wrinkle features, which increase surface area and oxygen availability, ultimately oxidizing (i.e., rebalancing) the cellular redox state.

Though the broad physiological effects of these metabolic and structural strategies are known, details of their regulation and coordination in biofilm communities have remained elusive. The work presented in this thesis was aimed at elucidating the (cross-)regulation and coordination of different redox-balancing strategies in biofilms of P. aeruginosa strain PA14. Studies described in Chapter 2 demonstrate novel regulatory links between phenazines and microaerobic denitrification, including a redox-mediated mechanism for control of the global transcription factor Anr, which is traditionally thought to be regulated solely by oxygen. This chapter also presents observations of the spatial segregation of denitrification enzymes in a colony biofilm, which is suggestive of metabolic specialization and substrate crossfeeding between different groups of cells. Chapters 3 and 4 describe work examining the physiological functions and regulation of pyruvate and lactate metabolism in P. aeruginosa.

These studies were motivated by pyruvate’s role as a “hub” for central metabolism, the unique structural biochemistry of the P. aeruginosa pyruvate carboxylase, and the intriguing complement of “lactate dehydrogenase” genes in P. aeruginosa. These genes include two that encode canonical and non-canonical respiration-linked L-lactate dehydrogenases. My results in Chapter 3 show that the non-canonical L-lactate dehydrogenase gene can substitute for the canonical one to support aerobic L-lactate utilization and that it is induced specifically by the L- enantiomer of lactate. This enzymatic redundancy for L-lactate utilization could be an adaptation that enhances virulence, given that host organisms (e.g. humans and plants) produce L-lactate but not D-lactate. In addition, Chapter 3 includes studies of pyruvate-lactate metabolism in the context of biofilm communities, where aerobic and anaerobic zones coexist in proximity. Evidence is provided that cells in biofilms have the potential to engage in crossfeeding of anaerobically generated D-lactate, which would constitute a new instance of bacterial multicellular metabolism.

Finally, Chapter 4 shows that mutants of pyruvate carboxylase, which converts pyruvate to oxaloacetate, have a matrix-overproducing, hyperwrinkling biofilm phenotype indicative of an imbalanced cellular redox state. This result suggests that disruption of pyruvate carboxylase shunts metabolic flow through pyruvate dehydrogenase, converting pyruvate to acetyl-CoA and generating an excess of reducing power. Together, the findings presented in Chapter 3 and 4 underscore the importance of pyruvate metabolism in the contexts of redox homeostasis and community behavior. When metabolic strategies are insufficient to balance the redox state, biofilms can ameliorate the problem of electron acceptor limitation by forming wrinkle structures, which increase the community’s surface area-to-volume ratio. Wrinkle formation depends on the production of extracellular matrix. Matrix production is also required for the formation of pellicles, biofilms that reside at air-liquid interfaces. Experiments described in Chapter 5 investigate properties of the P. aeruginosa matrix from a socio-evolutionary perspective.

My results show that matrix production confers a competitive advantage in pellicle biofilms but not in colony biofilms. The evolutionary landscape of matrix production in biofilms is complex and context-specific; i.e., each microenvironment selects for a subset of phenotypes that confers fitness only in that specific microenvironment. Chapter 6 describes the dynamic processes of pellicle formation in the gram-positive bacterium Bacillus subtilis as well as the gram-negative P. aeruginosa in a time-resolved manner. In these two distantly related species, we observed a conserved mechanism for pellicle formation that involves motility, chemotaxis and aerotaxis. These findings indicate that motility is more than just a unicellular behavior: cells collectively migrate to a microniche and initiate biofilm formation. Finally, Appendix A describes efforts to characterize proteinaceous components of the matrix isolated from P. aeruginosa PA14. In conclusion, this work has elucidated mechanistic details of various redox-balancing strategies in P. aeruginosa, particularly from the perspective of multicellular community development.