Qualitative microbiological measurement methods in which the measurement results are either 0 (microorganism not detected) or 1 (microorganism detected) are discussed. The performance of such a measurement method is described by its probability of detection as a function of the contamination (CFU/g or CFU/mL) of the test material, or by the LODp, i.e., the contamination that is detected (measurement result 1) with a specified probability p. A complementary loglog model was used to statistically estimate these performance characteristics. An intralaboratory experiment for the detection of Listeria monocytogenes in various food matrixes illustrates the method. The estimate of LOD50 is compared with the Spearman-Kaerber method.
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.
ZusammenfassungEmil J. Gumbel ist der Namensgeber der jährlichen Gumbel-Vorlesung auf der Statistischen Woche. Leider ist der Namensgeber dieser Vorlesung nur noch wenigen Teilnehmern und Vortragenden bekannt. Dieser Artikel möchten diesem Defizit abhelfen. Denn Gumbel war nicht nur der Namensgeber irgendeiner statistischen Verteilung.Der Aufsatz beschreibt den Lebensweg Gumbels vom Weltkriegsteilnehmer zu einem weithin bekannten Pazifisten, der die politischen Morde in der frühen Weimarer Republik mit statistischen Schlußweisen analysierte. Seine Zahlenwerke legten die Defizite der Rechtssprechung in der Weimarer Republik offen. Die Richter kompromittierten sich mit ihren Urteilen. Seine Statistik-Professur an der Universität Heidelberg verlor er nach Angriffen der NS-Studenten und einer deutsch-nationalistischen Universitätsleitung noch vor Beginn der NS-Herrschaft. Wir schildern Gumbels Emigration erst nach Frankreich und von da in die USA und seine vergeblichen Versuche, wieder an deutschen Universitäten aufgenommen zu werden.Im zweiten Teil skizzieren wir Gumbels wissenschaftliches Werk im Bereich der Statistik der Extremwerte. Gumbel schrieb nicht nur den Klassiker dieses Statistik-Bereichs. Er baute auch vielen Ingenieuren einen fachlichen Zugang mit leicht berechenbaren und gut interpretierbaren Diagrammen.Schließlich schildert einer der Autoren, der Gumbel noch persönlich kannte, seine Erinnerungen an den freundlichen und interessierten Gastprofessor aus den USA.
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