Honors Theses


Yvonne Sun, Ph.D.



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Document Type

Honors Thesis


Listeria monocytogenes (L. monocytogenes) is a deadly food borne pathogen that causes listeriosis infection in humans with a high mortality rate. L. monocytogenes can form biofilms in food processing environments and become resistant to antimicrobial agents such as benzalkonium chloride (BC) and other quaternary ammonium chlorides (QACs). BC is used for cleaning and disinfection of food processing areas and is known to disrupt cell membranes of L. monocytogenes, causing cytosol leakage and the degradation of proteins and nucleic acids. Previous research shows that QAC resistance is associated with the upregulation of certain efflux pump genes (mdrL, brcABC, qacH, and emrE). Moreover, biofilm formation can also contribute to QAC resistance and subsequent persistence of L. monocytogenes in the environment. In addition, propionate is a commonly used food additive for flavoring and spoilage prevention that can potentially regulate L. monocytogenes biofilm formation. Biofilm formation and the expression of efflux pumps can both be regulated by the stress response sigma factor SigB in L. monocytogenes. However, it is not clear whether propionate affects this regulatory pathway. Therefore, my Honors Thesis research aims to investigate whether propionate can be used to increase L. monocytogenes susceptibility to BC and to determine the role of transcription factors, such as SigB, in conferring BC resistance. Results indicate that BC decreases planktonic growth in the presence of propionate in aerobic conditions, but not anaerobic conditions. Additionally, the growth of the ΔsigB mutant is significantly reduced by BC under anaerobic but not aerobic conditions. These results highlight that SigB as well as the presence or absence of oxygen all play critical roles in regulating L. monocytogenes susceptibility to BC. Therefore, environmental conditions and genetic composition of L. monocytogenes can both contribute to the efficacy of our antimicrobial efforts in the food processing industry.

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Undergraduate research