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This week we profile a recent publication in the Journal of Bacteriology from (pictured, left to right) Zhexian Liu, Sarzana Hossain, Dr. Zayda Morales Moreira, and Dr. Cara Haney from Michael Smith Laboratories and UBC.

Can you provide a brief overview of your lab’s current research focus?

“Symbiosis” means “living together”: sometimes the outcome of host-microbe associations is pathogenic, while in other cases both the host and microbe benefit. The Haney Lab is broadly interested in how bacteria co-exist with plants and animals, and the factors that affect whether the interaction is beneficial or harmful for the host. We study bacteria in the genus Pseudomonas, which includes human and plant pathogens (i.e. P. aeruginosa and P. syringae respectively) and animal and plant commensals (i.e. P. fluorescens). We are interested in both basic mechanisms that regulate host-microbiome and host-pathogen interactions, and to translating these findings to improving agricultural sustainability and the treatment of chronic infections.

What is the significance of the findings in this publication?

Pseudomonas are really good at colonizing plants and animals with outcomes ranging from beneficial to pathogenic. Polyamines (such as putrescine and spermidine) are small molecules made by both plants and animals, and are enriched in host environments ranging from the plant rhizosphere to the lungs of individuals with Cystic Fibrosis. Prior work from the Haney Lab found that the right amount of biofilm formation, driven by putrescine sensing, is necessary for Pseudomonas association with plants. As a result, we hypothesize that polyamines may be a common signal to tell a bacterium that a host is present. Here we used a genetic approach and found that putrescine accumulation, either through disruption of the spermidine biosynthesis pathway or the catabolic putrescine aminotransferase pathway, promoted biofilm formation in the human pathogen Pseudomonas aeruginosa. Consistent with this observation, exogenous putrescine robustly induced biofilm formation in P. aeruginosa that was dependent on putrescine uptake and biosynthesis pathways. We found that both putrescine induced a significant increase in the intracellular level of bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) (c-di-GMP), a bacterial second messenger widely found in Proteobacteria that upregulates biofilm formation. We propose that Pseudomonas species may sense putrescine as a host-associated signal that triggers a lifestyle switching that favors chronic infection.

What are the next steps for this research?

While we have demonstrated that putrescine serves as environmental signals that promote biofilm in Pseudomonas, the identities of potential polyamine sensors are unknown. Our current hypothesis is that c-di-GMP-modulating enzymes can directly binds polyamines via ligand-binding domains. Therefore, a comprehensive screen of mutants in Pseudomonas aeruginosa c-di-GMP-modulating enzymes can identify genes required for sensing and responding putrescine. In addition to identifying the mechanistic link between L-arginine and biofilm enhancement in Pseudomonas, our work lays the foundation for future mechanistic studies that identify such polyamine metabolite sensors, and to study the role of polyamines during associations with hosts.

If you’d like to mention your funding sources, please list them.

This work was supported by an NSERC CGS-M award to Z.L. and NSERC Discovery Grant (NSERC-RGPIN-2016-04121) and CIHR Project Grant (PJT – 169051) awarded to C. H. H.


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