Honors Theses


Yvonne Sun, Ph.D.



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Honors Thesis


Antibiotics can be found in sublethal levels environmentally as pollution and within the body when used as medication. Antibiotics induce stress responses and sometimes act as signaling molecules in bacteria. In order to better understand the relationship between antibiotics and bacterial infections, the effects of sublethal antibiotic exposure on the ability of Listeria monocytogenes to infect macrophages was studied. Listeria monocytogenes is an intracellular pathogen of the gastrointestinal tract that is facultatively anaerobic. Two strains of Listeria monocytogenes were tested: a lab strain and a clinical cardiotropic strain. The strains were grown in liquid media overnight aerobically or anaerobically with either no antibiotics or with a 0.05 μg/mL concentration of ampicillin, which is the main antibiotic used to treat Listeria infections. Bacteria from the overnight cultures were used to infect RAW264.7 cells (macrophages) at a multiplicity of infection of 10. The number of intracellular colony forming units (CFUs) were measured at 2 and 24 hours post-infection by plating the cellular lysate. Using the number of intracellular CFUs, the percent input was calculated to measure amount of Listeria present inside the macrophages at each timepoint. The results for both strains showed no significant difference in the ability to infect and replicate inside macrophages between Listeria grown with or without ampicillin. Interestingly, the aerobically grown cardiotropic strain had a significantly lower percent input than the lab strain at 2 hours post infection, but there was no significant difference in percent input between the strains at 24 hours post infection. This suggests that although the cardiotropic strain cannot infect macrophages as well as the lab strain, it can either grow better or resist macrophage killing better than the lab strain. Future directions include testing higher levels of antibiotics and exploring the differences in virulence between strains of Listeria monocytogenes.

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Undergraduate research



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