Abstract
Listeria monocytogenes is a dangerous bacterium that causes the food-borne disease listeriosis and accounts for nearly 20 % of food-borne deaths. This organism can survive the body's natural defences within the digestive tract, including acidic conditions and bile. Although the bile response has been analysed, limited information is available concerning the ability of L. monocytogenes to resist bile under anaerobic conditions, especially at acidic pH, which mimics conditions within the duodenum. Additionally, it is not known how the bile response varies between serotypes. In this study, the survival of strains representing six serotypes was analysed under aerobic and anaerobic conditions following exposure to bile. Exposure to bile salts at acidic pH increased toxicity of bile, resulting in a significant reduction in survival for all strains tested. However, following this initial reduction, no significant reduction was observed for an additional 2 h except for strain 10403S (P = 0.002). Anaerobic cultivation increased bile resistance, but a significant increase was only observed in virulent strains when exposed to bile at pH 5.5. Exposure to pH 3.0 prior to bile decreased viability amongst avirulent strains in bile in acidic conditions; oxygen availability did not influence viability. Together, the data suggested that being able to sense and respond to oxygen availability may influence the expression of stress response mechanisms, and this response may correspond to disease outcome. Further research is needed on additional strains to determine how L. monocytogenes senses and responds to oxygen and how this varies between invasive and non-invasive strains.
Highlights
Listeria monocytogenes is a Gram-positive, food-borne pathogen that is responsible for nearly 20 % of food-related deaths in the USA (Scallan et al, 2011)
L. monocytogenes must survive a variety of stressors, including acidic conditions of the stomach, bile encountered within the gastrointestinal tract and hypoxic/anoxic conditions
Variation has been observed in bile (King et al, 2003; Merritt et al, 2010; Payne et al, 2013) and acid (King et al, 2003; Koutsoumanis et al, 2003; Liu et al, 2005; Melo et al, 2013; Olesen et al, 2009) resistance between different strains of L. monocytogenes
Summary
Listeria monocytogenes is a Gram-positive, food-borne pathogen that is responsible for nearly 20 % of food-related deaths in the USA (Scallan et al, 2011) This microbe causes the opportunistic infection listeriosis and primarily affects pregnant women, the elderly and the immunocompromised (Farber & Peterkin, 1991). Following exposure to the acidic conditions within the stomach, L. monocytogenes is exposed to bile secreted into the duodenum by the liver during digestion (Monte et al, 2009). This complex fluid is composed primarily of bile acids, cholesterol, phospholipids and bilirubin (Coleman et al, 1979).
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