The competitive growth dynamics and effects of reserpine on Listeria species in Fraser broth
LE3 .A278 2012
Bachelors of Science
Listeria monocytogenes (Lm) is a Gram-positive intracellular pathogen and causal agent of food-borne listeriosis. The primary vehicle for Listeria transmission is processed food, especially dairy items and deli meats. In nature, Lm often persists in mixed microbial communities with other Listeria spp., making isolation and detection by standard methods difficult. Non-pathogenic Listeria spp. often out-compete Lm during selective enrichment in Fraser broth (FB). This phenomenon may be due to; 1) increased ability for nutrient acquisition, and/or 2) variation in sensitivities to FB inhibitory compounds. Growth curves were constructed in FB for two Lm strains (Lm74 and Lm77) paired with a number of L. innocua (Lin) and L. welshimeri (Lw) strains to visualize the overgrowth trend. To test the role of nutrient acquisition, bulk transfer of nutrients was restricted by adding solidifying agents (agar, agarose, or κ carageenan) to FB at 1.0% or 1.5%. Results showed that an increase in solidifier percentage increased growth of Lm while in combination with certain Lin or Lw strains. To evaluate application of this method to food testing, Havarti cheese and salami samples were inoculated with Lm/Lw or Lm/Lin prior to adding agar/broth mixture; an increase in Lm was observed. To elucidate effects of inhibitory agents in FB, disk diffusion assays were used to show Lm was more sensitive than Lin or Lw to acriflavine. Since acriflavine resistance may be via an efflux mechanism, growth curves were constructed for paired strains in the presence of acriflavine and efflux pump inhibitor, reserpine. Reserpine significantly (P < 0.05) impacted the growth for Lm but not Lin, indicating a different mechanism for resistance may exist in Lin. These results may help elucidate the overgrowth phenomenon in FB and assist in the development of better isolation methods for Listeria monocytogenes.
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