Thesis (Ph. D.)--Massachusetts Institute of Technology, Microbiology Graduate Program, 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 91-96). / Integrative and conjugative elements (ICEs) are widespread mobile genetic elements that are integrated in bacterial chromosomes, but can excise and transfer to a recipient through conjugation. ICEs are important agents of evolution, contributing to the acquisition of new traits, including antibiotic resistance. Many ICEs are site-specific in that they integrate preferentially into a primary attachment site in the bacterial chromosome. Site-specific ICEs can integrate into secondary locations, but little is known about the consequences of integration. Using ICEBs1, a site-specific ICE from Bacillus subtilis, I found that integration into secondary attachment sites is detrimental to both ICEBs1 and the host cell. Integration at secondary locations is detrimental to ICEBsJ. Once integrated in the chromosome, excision of ICEBs1 from all secondary attachment sites analyzed was either reduced (4 sites) or undetectable (3 sites) compared to ICEBs1 excision from the primary site. Additionally, from two of the four secondary sites that exhibited reduced but detectable excision, the excised, circular form of ICEBs1 was present at lower levels than expected, indicating that circular ICEBs1 may be unstable. Defects in excision and stability of ICEBs] severely limit its ability to spread to other cells. Integration at secondary locations is detrimental to the host cell. Induction of ICEBs1 gene expression in secondary integration sites resulted in a defect in cell proliferation and/or viability, as well as induction of the SOS response. These effects are likely due to DNA damage resulting from plasmid-like, rolling-circle replication of the excision-defective ICEBs1 in the chromosome. Consistent with this model, deletion of ICEBs] replication genes (nicK and helP) alleviated the proliferation and viability defects. Implications for the evolution of ICEs. These previously unrecognized detrimental effects may provide selective pressure against propagation of ICEBs1 in secondary attachment sites. Such detrimental effects could explain the maintenance and prevalence of site-specific integration among ICEs. / by Kayla L. Menard. / Ph.D.

Thesis: Ph. D. in Microbiology Graduate Program, Massachusetts Institute of Technology, Department of Biology, 2019 / Cataloged from PDF version of thesis. / Includes bibliographical references. / Microbes have adapted to life in complex microbial communities in a large variety of ways, and they are continually evolving to better compete in their changing environments. But identifying the conditions that a particular microbe thrives under, and how they have become adapted to those condition can be exceedingly difficult. For instance, Clostridium difficile became widely known for being the world's leading cause of hospital associated diarrhea, but people can also have C. difficile in their gut without developing diarrhea. Although these asymptomatic carriers are now thought to be the largest source of infection, we know very little about how these people become colonized. In the first chapter of my thesis I use publicly available microbiome survey data and a mouse model of colonization to show that C. difficile colonizes people immediately after diarrheal illnesses, suggesting C. difficile is a disturbance adapted opportunist. / However, the differences between very recently diverged microbial populations that are adapted for growth in different conditions can be very difficult to detect. To address this limitation, I developed a method of identifying regions that have undergone recent selective sweeps in these populations as a means of distinguishing them, and specifically quantifying their abundance in complex environments. But part of what makes microbial evolution so difficult to interpret is the vast diversity of genes that are only shared by a fraction of all the members in a population. To better understand how these flexible regions are structured, I systematically extracted all contiguous flexible regions in nine marine Vibrio populations and compared their organization and evolutionary histories. / I found that horizontal gene transfer and social interactions have led to the evolution of modular gene clusters that mediate forms of social cooperation, metabolic tradeoffs, and make up a substantial portion of these flexible genomic regions. The observations made in these studies help us understand how microbes are organized into socially and ecologically cohesive groups, and how they have evolved to interact with complex and changing environments. / by David VanInsberghe. / Ph. D. in Microbiology Graduate Program / Ph.D.inMicrobiologyGraduateProgram Massachusetts Institute of Technology, Department of Biology