Using Imaging Mass Spectrometry to Study Bacterial Sociality: the Role of Secondary Metabolites in Spatial Community Structures

Rachel Gregor gregor@post.bgu.ac.il ^1,2 Sofya Sudin ^1,2 Yonghui Dong ³ Asaph Aharoni ³ Itzhak Mizrahi ^2,4 Pieter C. Dorrestein ⁵ Michael M. Meijler ^1,2

¹Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel
²National Institute of Biotechnology in the Negev, Ben-Gurion University of the Negev, Beer-Sheva, Israel
³Faculty of Biochemistry, Department of Plant Sciences, Weizmann Institute of Science, Rehovot, Israel
⁴Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel
⁵Skaggs School of Pharmacy and Pharmaceutical Sciences, Department of Computer Science, Department of Pediatrics, Center for Microbiome Innovation, and Collaborative Mass Spectrometry Innovation Center, University of California, San Diego, La Jolla, California, USA

Quorum sensing (QS) describes the ability of a population of unicellular bacteria to synchronize their gene expression in a cell-density-dependent manner, through the secretion and recognition of small diffusible molecular signals. QS relies on the increase in physical proximity of bacterial cells to one another and the resulting local increase in concentration of signaling molecules, and therefore is especially relevant to understanding bacterial sociality and the emergence of spatial structure in mixed bacterial communities. Such structure has variously been linked to genetic drift, growth rates, nutrient limitation, and other phenomena.

We are interested in the role of secondary metabolites, especially siderophores, signaling molecules, and other “common goods,” in governing the organization and interactions within a community. Using Pseudomonas aeruginosa as a model system, we study the development of spatial structure and metabolite distribution in mixed communities of wild-type bacteria together with strains in which key QS signaling pathways are disabled. Such mutants are impaired in the production of various common goods, and are therefore dependent on their neighbors under stress conditions in which these secondary metabolites become critical for survival.

To this end, we use novel imaging mass spectrometry (IMS) techniques, specifically matrix-assisted laser desorption/ionization (MALDI) imaging spectrometry. MALDI-IMS has been utilized to provide a macro-scale view of bacterial information flow by directly imaging metabolite secretion from colonies grown on agar, resulting in heat maps of metabolite distribution over a wide range of masses. We are currently using this technique to track the production and diffusion of various relevant secondary metabolites between the wild-type and mutant strains, and to address the question of how such metabolite flow can shape the spatial structure of bacterial communities.