Honors Theses


Yvonne Sun



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Honors Thesis


Listeria monocytogenes is a Gram-positive bacterium and foodborne pathogen responsible for causing a lethal disease known as listeriosis. Listeriosis tends to affect susceptible populations such as the elderly, pregnant women, and those with compromised immune systems. The infection is often treated with ampicillin and gentamicin, but despite antibiotic treatments, the mortality rate of Listeria infections remains high . In this study, we investigated environmental conditions that may impact bacterial susceptibility to antibiotics in order to increase antibiotic efficacy. Listeria is an enteric bacterium and as it transits through the intestines, it is exposed to short chain fatty acids (SCFAs), such as butyrate, propionate, and acetate under anaerobic conditions. In the presence of these SCFAs, we found that the bacterial membrane fatty acid composition was dramatically altered under aerobic as well as anaerobic conditions. To determine whether exposure to SCFAs also changes antibiotic susceptibility, we performed disc diffusion assays in the presence or absence of SCFAs under aerobic in addition to anaerobic conditions. We found that Listeria was more susceptible to ampicillin under anaerobic conditions with or without the supplementation of SCFAs. Alternatively, gentamicin showed higher efficacy under aerobic conditions. These results suggest that Listeria is more susceptible to ampicillin under anaerobic conditions and gentamicin under aerobic conditions. Additionally, Listeria mutants deficient in electron transport chain enzymes or transcription factors showed no difference in susceptibility between aerobic and anaerobic conditions. In summary, Listeria exposed to SCFAs under anaerobic conditions alters its membrane fatty acid composition as well as antibiotic susceptibility in a manner potentially dependent on the presence of electron transport chain enzymes and transcription factors. Therefore, future mechanistic studies focusing on parameters that will render pathogens more susceptible to antibiotics will contribute to solving the impending antibiotic resistance crisis.

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Undergraduate research