The impact of bile and anaerobic stress on Listeria monocytogenes isolates

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Lynch, Mary Jane
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University College Cork
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Listeria monocytogenes is regarded as an important food pathogen that can cause serious illness. All individuals are susceptible to L. monocytogenes infection with pregnant women, immunocompromised people and the elderly particularly at high risk. L. monocytogenes infection can cause febrile gastroenteritis and a more serious invasive form where the bacteria cross the blood/brain barrier or the blood/foetal barrier. In this study the effect of bile and anaerobic stress on L. monocytogenes was assessed. Bile has antimicrobial properties and it is one of the stressors that L. monocytogenes must overcome in the human gastrointestinal tract to establish infection. L. monocytogenes uses an enzyme, bile salt hydrolase (BSH), to detoxify bile. The bile resistance response may be triggered by anaerobic conditions. In total 154 food/environmental isolates and 74 clinical isolates were screened. A bile tolerance assay and a BSH enzymatic agar plate assay were developed. Experiments were carried out at a lowered pH and/or anaerobic conditions to simulate conditions in the human gastrointestinal tract where L. monocytogenes is exposed to bile. Results indicate that there are bile tolerance variations in L. monocytogenes strains from food, environmental and clinical sources. In this large-scale study, cultures grown anaerobically were less tolerant to bile than cultures grown aerobically and generated more varied resistance patterns. The effect of anaerobiosis is not as well understood as aerobic and results provide an insight into the influence of environmental factors. Interestingly, the bile resistance assay revealed that the clinical isolates were significantly more tolerant to bile than the food and environmental isolates (P < 0.001). This important finding demonstrates the evolutionary pressure on these strains to survive in the gut. Correlation of results from the BSH agar plate assay with the bile tolerance assay demonstrated that the food and environmental isolates had slightly higher levels of BSH activity than the clinical isolates. However, the difference between the groups was not as significant as the bile resistance results. Results were also analysed on the level of lineage and clonal complex to identify possible links to the bile response. The results generated from this study provide insight into important associations between bile tolerance of individual strains/lineages and efficacy of L. monocytogenes infection.
Microbiology , Listeria monocytogenes , Bile , Anaerobic Stress
Lynch, M. J. 2023. The impact of bile and anaerobic stress on Listeria monocytogenes isolates. MRes Thesis, University College Cork.
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