Listeria monocytogenes in foods. Isolation, characterization and control

Microbiological assessment of culture media: comparison and statistical evaluation of methods

In this review methods for the quality control of media were compared, taking the following questions as a guidelines. (i) Which methods are easy to use and give reliable results? (ii) Which experimental design should be used in order to obtain reliable data with a minimal input of resources (staff and materials)? These questions can be answered satisfactorily using statistical methods. This review shows that solid media can be assessed with acceptable accuracy using well established methods like the spread plate technique. In order to assure a minimum of statistical error, at least two plates with an average count of 100 colonies per plate seems to be the best design. This also applies to the ecometric streaking technique, a good alternative to the more quantitative methods. For an accurate assessment of liquid media, large numbers of tubes need to be tested. This is very expensive in terms of laboratory resources and therefore unlikely to be used routinely. Therefore it is proposed to use the serial dilution technique, in which the broths are tested in triplicate (Richard, N. (1982) Monitoring the quality of selective liquid media used in the official serial dilution technique for the bacteriological examination of food. In: J.E.L. Corry (Ed.). Quality Assurance and Quality Control of Microbiological Culture Media. Proceedings of the Symposium held on 6-7 September 1979, Callas de Mallorca, Spain, G.I.T.-Verlag Ernst Giebeler, Darmstadt. pp. 51-58). The recommendations in this review can be used together with the methods recommended by the International Committee for Food Microbiology and Hygiene, Working Party on Culture Media (ICFMH, WPCM: Baird et al., 1987) to assist laboratories setting up QC tests for culture media.

Evaluation of monoclonal antibodies to Listeria monocytogenes flagella by checkerboard ELISA and cluster analysis

A simple ELISA technique is described which utilizes whole bacteria as solid phase antigens for the evaluation of monoclonal antibodies against flagellar proteins of Listeria monocytogenes. Cluster analysis of the ELISA extinction values gave distinct reaction patterns from 23 tested supernatants. The method offers the possibility of screening supernatants without the necessity for standardization of antibody concentration or the labelling of established monoclonal antibodies.

Detection of Listeria monocytogenes in foods by immunomagnetic separation

Immunomagnetic separation with immunomagnetic beads was used to isolate strains of Listeria monocytogenes both from pure cultures and from heterogeneous suspensions. The monoclonal antibodies used recognized all six strains of serotype 4 but only one of three strains of serotype 1. Coating procedure, incubation time, and number of immunomagnetic beads influenced the sensitivity of the isolation method. Less than 1 x 10(2) bacteria per ml in pure cultures and less than 2 x 10(2) bacteria per ml in enriched foods could be detected. The method represents a new approach to extraction and isolation of pathogenic bacteria directly from foods, after resuscitation, or from enrichment broths.