Quality control monitoring of liquid selective enrichment media used for isolating salmonellae

Media for Salmonella

The main categories of selective plating media are reviewed. The selective enrichment media are considered to be of critical importance in salmonella isolation. During the last 10 years Rappaport-Vassiliadis broth has emerged as a new reliable enrichment medium. Likewise motility enrichment is a very promising technique, but will no doubt have its limitations. However, since it is based on a different principle compared with traditional enrichment, it may complement traditional techniques in a useful way. Most important, however, is the insight that the selectivity of a selective procedure is not solely defined by the medium and by incubation conditions. The food sample and its microflora apparently influences the result considerably. This finding may open a new and fascinating field of scientific research; the ecology of selective media.

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.