Abstract/Summary

Listeria monocytogenes is a ubiquitous foodborne pathogen that can proliferate and survive under stress conditions by adjusting various resistance mechanisms. Consumption of food contaminated with this bacterium can cause listeriosis, which has symptoms ranging from mild stomach issues to more severe conditions like blood infection, stillbirth and miscarriage. L. monocytogenes requires specific nutrients such as carbon, sulphur, amino acids and nitrogen sources for growth, proliferation and survival under stressful conditions. One of the essential amino acids for L. monocytogenes is cysteine. L. monocytogenes can be found in various cysteine-rich foods, including milk and dairy products, meat products, radishes and cabbage in ready-to-eat salads. Several outbreaks of human listeriosis have been associated with some of these foods, such as seafood, hot dogs, and soft cheeses. A review has been conducted (Chapter 1) to investigate cysteine transporters and their roles in the resistance mechanisms of various microorganisms, including L. monocytogenes. CtaP is one of the cysteine transporters in L. monocytogenes and plays a role in growth, host cell attachment and virulence. However, a recent study stated that CtaP does not play a role in cysteine transport, but GSH (glutathione) in L. monocytogenes. A whole transcriptomic analysis was conducted to assess gene expression changes related to ctaP and the effects of exogenous cysteine, providing insight into its potential regulatory functions (Chapter 3). Previous studies have shown that cysteine is necessary for the swarming motility of Salmonella Typhimurium and Serratia marcescens. However, the role of cysteine in the motility behaviour of L. monocytogenes remains poorly understood. Due to the known effects of ctaP on virulence and host cell attachment, we also investigate the role of cysteine transporter ctaP and extracellular cysteine on swimming, swarming and biofilm formation of L. monocytogenes (Chapter 2). Transcriptomic data have shown that different concentrations of cysteine led to varying transcription levels of flagella and chemotaxis genes. Quorum sensing is one of the main pathways affected by the presence of exogenous cysteine in the environment. Moreover, the GAD system, the most important acid resistance mechanism in L. monocytogenes, has been associated with the QS pathway based on transcriptomic data. c-di-AMP, a second messenger, plays a role in various stress responses by influencing the expression of QS-related genes. Therefore, the role of c-di-AMP on the acid resistance (Chapter 4) and oxidative stress resistance (Chapter 5) of L. monocytogenes have been investigated by using pdeA-lacking cells. As part of the ongoing research on oxidative stress and catalase activity, the effects of environmental factors, such as the presence of acid and salt, have also been examined (Chapter 6).