Date on Master's Thesis/Doctoral Dissertation

12-2018

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Microbiology and Immunology

Degree Program

Microbiology and Immunology, PhD

Committee Chair

Schmidt, Nathan

Committee Co-Chair (if applicable)

Egilmez, Nejat

Committee Member

Egilmez, Nejat

Committee Member

Kosiewicz, Michele

Committee Member

Abu Kwaik, Yousef

Committee Member

Joshi-Barve, Swati

Author's Keywords

malaria; plasmodium; microbiome; microbiota; mouse

Abstract

Plasmodium, the parasitic cause of malaria, is a global pathogen, annually causing 216 million infections and 445,000 deaths. As drug resistance continues develop and no effective vaccine is available, it is critical to understand the factors underlying the severity of this disease. Plasmodium is an extra-gastrointestinal tract infection where the parasite infects red blood cells causing clinical malaria. However, recent publications have pointed to interactions between the gut microbiota and malaria. With this in mind, the role of the gut microbiota in malaria infection was studied. C57BL/6 mice from different vendors displayed differential resistance and susceptibility to severe malaria, and cecal contents transplanted from these mice to germ-free mice recapitulated the observed phenotypes. Similarly, resistant mice possessed a much more robust humoral immune response than susceptible mice, which is critical for Plasmodium clearance. When the cecal contents from resistant and susceptible mice were sequenced, Lactobacillus and Bifidobacterium genera were enriched in resistant mice. Moreover, treating susceptible mice with probiotics containing these bacterial genera after antibiotic administration led to a lower parasite burden. These observations point to a previously unknown role for the microbiota in modulating the severity of malaria. To further characterize the interactions between the host and gut microbiota in malaria, different components of gut homeostasis were investigated in both mild and severe disease. While intestinal permeability increased in both resistant and susceptible mice, there were no significant differences between the two groups. However, susceptible mice were shown to have greater numbers of lamina propria immune cells as well as greater abundances of cecal metabolites and bile acids during infection compared to resistant mice. Consistent with the decreased abundance of bile acids, histology showed much greater and prolonged damage and hemozoin deposition in the livers of susceptible mice compared to resistant mice. Despite these differences, the microbiota composition of resistant and susceptible mice became more similar during infection, although these changes were not associated with susceptibility or resistance when the altered cecal contents were transferred into germ-free mice. However, there were distinct differences in the functional capacity of the resistant and susceptible microbiota during infection. Susceptible mice showed significant increases in genes related to bacterial motility and flagellar assembly. Overall, there are profound differences in gut homeostasis during severe and mild Py infection. Finally, it was investigated whether antimalarial drugs, particularly clinically relevant artemisinin combination therapies (ACTs), could disrupt the gut microbiota. As previously shown, the composition of the gut microbiota alone can modulate the severity of Py infection; if ACTs change the microbiota composition, future infections could be more severe. To test this hypothesis, two common ACTs, artesunate plus amodiaquine and artemether plus lumefantrine, were used to orally treat mice while fecal pellets were collected to characterize the gut microbiota before and after treatment. After either ACT treatment, the overall species abundance in mice was similar to baseline. While alpha diversity remained unchanged by any treatment, there were minor, inconsistent changes in beta diversity that returned to baseline. With these findings, it does not appear that ACTs change the gut microbiota. This work has greatly increased the scientific knowledge concerning the three-fold interaction between host, gut microbiota, and Plasmodium. While much work still needs to be done, these findings can provide a contextual foundation on which future work can be built.

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