title: Contribution of Selective Conditions to Microbial Competition in Four Listeria Selective Enrichment Formulations.
abstract: Microbial competition during selective enrichment negatively affects Listeria monocytogenes populations and may hinder the subsequent detection or recovery of this organism. Competition assays among 10 selected strains of Listeria and Citrobacter braakii were performed in buffered Listeria enrichment broth, 3-(N-morpholino)propanesulfonic acid-buffered Listeria enrichment broth, University of Vermont medium-modified Listeria enrichment broth, and Fraser broth. The individual contributions of each selective agent in these media were also assessed, as well as the contribution of incubation temperature. Acriflavine hydrochloride and sodium nalidixate were ineffective at preventing the overgrowth of C. braakii ; this resulted in substantially lower populations of Listeria than when the competitor was absent. At the higher levels, both of these selective agents were detrimental to Listeria populations. The highest enrichment populations of Listeria were observed when either NaCl or LiCl was present. In the absence of selective agents, the final populations of Listeria following competitive growth with C. braakii were not substantially affected by temperature; however, in the presence of selective agents, the Listeria populations were statistically higher at the higher incubation temperature. There are a limited number of selective agents available for use in Listeria -specific enrichment media, resulting in formulations that are only somewhat selective for this species. The optimization of current formulations may help researchers to improve Listeria recovery, particularly from products with a high microbial load. The understanding of the behavior and interactions between target and nontarget microorganisms in the presence of these available selective agents is a necessary step in the optimization of Listeria selective enrichment formulations.
authors.affiliation: U.S. Food and Drug Administration, Office of Regulatory Affairs, Arkansas Regional Laboratory, 3900 NCTR Road, Building 26, Jefferson, Arkansas 72079.
authors.forename: Ashley L
authors.initials: AL
authors.lastname: Keys
authors.affiliation: U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, 5100 Paint Branch Parkway, College Park, Maryland 20705, USA.
authors.forename: Anthony D
authors.initials: AD
authors.lastname: Hitchins
authors.affiliation: U.S. Food and Drug Administration, Office of Regulatory Affairs, Arkansas Regional Laboratory, 3900 NCTR Road, Building 26, Jefferson, Arkansas 72079.
authors.forename: R Derike
authors.initials: RD
authors.lastname: Smiley
mesh_terms: D000143:Acids; D015169:Colony Count, Microbial; D003470:Culture Media; D005516:Food Microbiology; D008087:Listeria; D008089:Listeria monocytogenes
chemical_list: D000143:Acids; D003470:Culture Media
keywords: Foodborne pathogen; Listeria; Listeria monocytogenes; Microbial competition; Regulatory microbiology; Selective enrichment