Quantitative analysis and molecular identification of bifidobacteria strains in probiotic milk products
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The aim of the present study was to evaluate an agar medium for the quantitative analysis of bifidobacteria in probiotic milk products on the market, and to investigate the identities of active strains. For this purpose, three sour milk products and a soft cheese brand were each analysed ten times with Wilkins-Chalgren agar (WCA) supplemented with mupirocin, and with MRS agar supplemented with dicloxacillin at two different concentrations. By statistical analysis of the bacterial counts obtained, WCA was shown to have the best detection rate. All the sour milk products examined conformed to Swiss food legislation and contained more than 106 cfu of bifidobacteria per gram of living bifidobacteria. All the samples of soft cheese analysed revealed counts that were clearly (approximately 2 logs) below this minimum requirement. Isolated bifidobacteria strains were all identical with reference strains from producer companies shown by pulsed-field gel electrophoresis (PFGE) profiling. According to the labels, the three brands of sour milk examined contained different strains of bifidobacteria (Bifidobacterium lactis Bb 12, Bifidobacterium animalis DN 173010 and Bifidobacterium species 420). However, all the strains revealed repeatedly identical PFGE-patterns, thus showing a close relationship. These findings show that a better taxonomical definition of commercially used bifidobacteria strains is needed.
KeywordsBifidobacteria Probiotic milk products Pulsed-field gel electrophoresis Quantification Identification
We are grateful to DANONE Vitapole France, Chr. Hansen Danemark, Emmi Schweiz AG (Wisby) for making reference strains available to us. We also thank PD Dr. Patrick Boerlin, Institute of Veterinary Bacteriology, University of Bern for his technical advice related to PFGE, PD Dr. Urs Pauli, SFOPH for his assistance in the digital documentation of electrophoresis gels and Max Haldimann, SFOPH for the statistical evaluation.
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