title: Reduced detectability of Listeria monocytogenes in the presence of Listeria innocua.
abstract: Recent foodborne crises have demonstrated the importance of monitoring food safety. In terms of microbiological criteria, food safety requires the reliable detection of pathogens such as Listeria monocytogenes along the food chain by appropriate analytical methods. However, indications exist that accompanying Listeria innocua strains suppress the growth of L. monocytogenes during selective enrichment, which may cause reduced or even inhibited detection. To study these effects, the limit of detection of L. monocytogenes was investigated in the presence of L. innocua using the International Organization for Standardization standard method ISO 11290-1 and the VIDAS LDUO system, an automated method based on enzyme-linked fluorescence technology. The challenge was to provide low initial Listeria concentrations at sufficient precision to quantify the influence on the probability of detection of L. monocytogenes. The application of reference materials appropriate for quantitative test methods and a standardized dilution procedure were necessary to ensure accurate CFU levels of defined proportions of mixtures of both Listeria species. During selective enrichment, overgrowth of L. monocytogenes by L. innocua could be confirmed, leading to high rates of false-negative results. Moreover, with both methods, a significant decrease in the detectability of L. monocytogenes could be quantified at ratios of 2:1 at very low concentrations representative of natural contamination levels often found in foods and environments. It is concluded that there is a need to improve existing procedures with respect to selective enrichment, as well as the detection techniques.
authors.affiliation: Department of Food Sciences and Food Technology, Institute of Food Sciences, University of Natural Resources and Life Sciences (BOKU), Muthgasse 18, 1190 Vienna, Austria. zitz@boku.ac.at
authors.forename: Ulrike
authors.initials: U
authors.lastname: Zitz
authors.forename: Marija
authors.initials: M
authors.lastname: Zunabovic
authors.forename: Konrad J
authors.initials: KJ
authors.lastname: Domig
authors.forename: Peter-Theodor
authors.initials: PT
authors.lastname: Wilrich
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Kneifel
mesh_terms: D015373:Bacterial Typing Techniques; D015169:Colony Count, Microbial / Q000379:methods* / Q000592:standards*; D003257:Consumer Product Safety; D003470:Culture Media / Q000737:chemistry*; D005516:Food Microbiology; D008087:Listeria / Q000145:classification / Q000302:isolation & purification*; D008089:Listeria monocytogenes / Q000145:classification / Q000302:isolation & purification*; D012680:Sensitivity and Specificity; D013045:Species Specificity
chemical_list: D003470:Culture Media