Sociality in bacteria: Signaling, private, and public goods in Pseudomonas aeruginosa and Escherichia coli

Abstract

Bacteria are social organisms, and they are known to engage in cell-cell signaling, resource sharing, and construction of multicellular structures. These behaviors are important from both ecological and medical perspectives. In this thesis, we study Pseudomonas aeruginosa and Escherichia coli, which both demonstrate social behaviors. P. aeruginosa is well-known for its ability to engage in quorum sensing (QS), a form of cell-cell signaling that allows cells to regulate gene expression based on local cell density. We study a common regulatory component of QS, a positive autoregulatory loop, and work to understand its role in QS activation in populations of P. aeruginosa. We find that positive autoregulation in P. aeruginosa QS is not necessary, but does tightly synchronize QS gene expression across a population. This organism is a devastating pathogen for immunocompromised individuals, and QS regulates numerous virulence factors. Improving our understanding of QS regulation will benefit both medicine and sociomicrobiology studies. In E. coli, we study whether the siderophore enterochelin is shared within well-mixed populations of cells. Sharing of secreted products is an oft-studied form of bacterial cooperation, and studies of this behavior have revealed many mechanisms that bacteria use to stabilize cooperation. We find that enterochelin-producing cells have a growth advantage over non-producing cells when producing cells are scarce. This supports the conclusion that enterochelin is partially privatized in E.coli, and may reveal how enterochelin production has been maintained through evolutionary time. Characterizing secreted products and analyzing the degree of privatization is relevant to evolutionary ecology, and characterizing E. coli iron acquisition is valuable both for treating pathogenic E. coli infections and for understanding the role that E. coli plays in the gut commensal community. Finally, we also construct a synthetic QS strain in E. coli using QS regulatory genes from P. aeruginosa, and we hope to use this strain to study the costs and benefits of using QS to control a single secreted product.