15D-21 |
Bile tolerance of Listeria monocytogenes: A potential virulence factor |
T. ANNAMALAI, P. Marek, and K. Venkitanarayanan. Animal Science, University of Connecticut, 3636 horse barn hill road extension, G. white building, Storrs, CT 06269 Listeria monocytogenes is a gram-positive foodborne pathogen. Gastric acidity and bile toxicity are two barriers in the human gastrointestinal tract against pathogen invasion. Bile secreted into the small intestine is encountered by L. monocytogenes in the early stages of infection. Gram-negative enterics are more bile tolerant than gram-positive bacteria, with the exception of some lactic acid bacteria and Clostridium species. Although L .monocytogenes is acid tolerant, its ability to withstand bile has not been investigated. L. monocytogenes has been isolated from bile of listeriosis patients, and biliary excretion is considered a possible origin of L. monocytogenes in fecal carriers. The objective of this study was to determine the tolerance of L. monocytogenes to bile. Five strains of L. monocytogenes were inoculated separately to 104 CFU/ml in Tryptic soy broth (pH 6.0) containing oxgall (0, 1 or 2%) and 2% NaCl, representing the normal bile concentrations, pH and electrolytic balance in human duodenum. The samples were incubated at 37ºC for 0, 1, 2, 4, 6, 8, 24 hours, and L. monocytogenes was enumerated on tryptic soy agar after each incubation. All five strains survived 1 and 2% bile throughout the incubation period. In samples containing 0% bile, L. monocytogenes had a lag phase of 2 hours and increased by ~4 log CFU/ml at 24 hours. All strains exhibited a longer lag phase of 8 hours in bile containing samples. Three strains increased by 3 log CFU/ml at both concentrations of bile, whereas two strains increased by ~1 log CFU/ml in 1% bile. This is the first report on bile tolerance of L. monocytogenes. Bile tolerance may be a potential virulence factor in L. monocytogenes. Recently published genome of L. monocytogenes revealed three probable bile salt hydrolase genes and indicated them as potential virulence factors, thereby strengthening the results of this study.
Session 15D, Food Microbiology: Fruits and vegetables
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