Abstract:
The vertebrate gut is a dynamic environment, harboring diverse bacteria that interact with both each other and their host. These interactions can profoundly influence host health, shaping host traits such as development and immunological functions across vertebrates--from zebrafish to humans. Despite its importance, the molecular basis of how microbes interact with each other is still poorly understood. Coaggregation, where genetically diverse microorganisms physically interact with one another to form multi-species clusters, is one type of interaction between microbes that can influence the spatial organization and physiology of microbial communities. Here, I provide mechanistic insights into genetic pathways that shape physical interactions between bacteria that reside in the vertebrate gut and determine how they shape the physiology of other community members.
My studies focused on identifying the molecular mechanisms facilitating coaggregation between two bacteria that coaggregate in the intestine of their host. I used the model bacterial species Aeromonas sp. ZOR0001 (Aer01) and Enterobacter sp. ZOR0014 (Ent14), which form dense coaggregates in the larval zebrafish gut. These bacteria provide an excellent model for studying microbial coaggregation because they are easy to culture outside the zebrafish, are genetically tractable, and coaggregate in culture conditions developed by the Guillemin lab.
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To probe this interaction, I sought to identify the genetic pathways involved in coaggregation for both organisms. First, I leveraged an existing mutant library of Aer01 with aggregation defects in our culture conditions and identified an Aer01 adhesin called MbpA that is necessary for binding Ent14. Using experimental evolution, I generated Ent14 mutants with deficits in coaggregating with Aer01, to identify potential cell surface factors on Ent14 recognized by MbpA. These evolved isolates form large, mucoid colonies, a phenotype that arose rapidly during experimental evolution. Genome analysis of two mucoid isolates revealed these isolates contain different mutations in the Regulator of Colanic Acid Synthesis (Rcs) system. Rcs-mediated upregulation of colanic acid polysaccharide (CAP) production is known to cause mucoidy. Production of this thick polysaccharide capsule can shield indispensable outer membrane factors targeted by host immune system as well as viruses that infect bacteria called bacteriophage. I show my evolved mucoid Ent14 isolates are hyperactive Rcs mutants, suggesting that rather than losing the cell surface factor recognized by MbpA, these mutants shield it from MbpA recognition. Further, I show that Ent14 mucoidy influences the physiology of microbes other than Aer01 that interact with Ent14. Together these data indicate the mucoid phenotype observed in the evolved Ent14 isolates not only alters their interaction with Aer01 but also has broader implications for microbial interactions and community dynamics. My research demonstrates the complex interplay between genetic pathways, phenotypic traits, and ecological consequences within microbial communities.