Date of Award

Summer 5-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

School

Biological, Environmental, and Earth Sciences

Committee Chair

Dr. Janet Donaldson

Committee Chair School

Biological, Environmental, and Earth Sciences

Committee Member 2

Dr. Fengwei Bai

Committee Member 2 School

Biological, Environmental, and Earth Sciences

Committee Member 3

Dr. Alex Flynt

Committee Member 3 School

Biological, Environmental, and Earth Sciences

Committee Member 4

Dr. YanLin Guo

Committee Member 4 School

Biological, Environmental, and Earth Sciences

Committee Member 5

Dr. Dmitri Mavrodi

Committee Member 5 School

Biological, Environmental, and Earth Sciences

Abstract

The deadly foodborne pathogen Listeria monocytogenes is a gram positive facultative anaerobic bacterium. It is the third leading cause of death from food-borne illnesses. Once ingested, it encounters various stressors in the gastrointestinal (GI) tract, including acidic pH, bile, and alterations in oxygen availability. Various studies have been done regarding the pathogen’s survival mechanism against acid and bile. Since the lower parts of the GI tract are anaerobic, it is imperative to investigate how physiologically relevant anaerobic conditions impact L. monocytogenes’s survival. Transcriptomic analysis of L. monocytogenes under conditions mimicking the GI tract was performed. A large number of genes encoding for pathogenesis, metabolism, transcription factors, stress response, DNA repair, etc., were found to be differentially regulated. Upon exposure to anaerobiosis in acidic conditions, there were variations in the transcript levels for virulence factors such as internalins, listeriolysin O, etc., as well as many histidine sensory kinases. These data indicate that the response to anaerobiosis differentially influences the transcription of several genes related to the survival of L. monocytogenes under acidic and bile conditions. Majority of the above-mentioned biological processes are regulated by a common secondary messenger called cyclic-dimeric-guanosine monophosphate (C-di-GMP).

Therefore, the impact of oxygen availability on the c-di-GMP pathway was analyzed. We found that genes encoding the enzymes involved in c-di-GMP pathway (diguanylate cyclases and phosphodiesterases) were differentially regulated under anaerobic conditions when compared to those in aerobic conditions. It was further found that phosphodiesterase D has a role in regulation of c-di-GMP and that oxygen may influence the regulation of c-di-GMP. pdeD, in particular, was found to be involved in protecting L. monocytogenes against the toxic effects of bile at acidic conditions. As bile in acidic conditions induces oxidative stress, it was next tested whether oxidative stress impacted c-di-GMP. It was found that c-di-GMP concentration increased under oxidative stress, which may help the bacteria survive in host cells. Altogether, it was learned that oxygen availability plays a crucial role in survival of Listeria monocytogenes and the c-di-GMP pathway may contribute to the pathogen’s survival in environments that induce oxidative damage. Further research is needed to determine if the response is specific to reactive oxygen species, or oxidative stress in general.

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