Microbiological evaluation and molecular characterization of bifidobacteria strains in commercial fermented milks

The aim of this study was to evaluate the presence of Bifidobacterium animalis subsp. lactis in commercial dairy products using different molecular techniques. We analyzed the microbiological composition of 13 commercial fermented milks available in the Spanish market. Thirteen strains of genus Bifidobacterium were isolated from these products and were identified by genus-specific PCR, by fluorescence in situ hybridization (FISH), by multiplex PCR and amplified ribosomal DNA restriction analysis (ARDRA). The same sets of strains were typed by randomly amplified polymorphic DNA (RAPD) analysis and by amplified fragment length polymorphism technique (AFLP). All strains were identified as B. animalis subsp. lactis using ARDRA and multiplex PCR techniques. Similarity between strains was evaluated based on RAPD and AFLP profiles. The isolated strains showed similar profiles by using these techniques, revealing the reduced genetic variability existing among commercial strains, and all these profiles were reproducible in repeated analysis. ARDRA and multiplex PCR are techniques that allow differentiation of the bifidobacteria at genus and species level, but do not indicate if they are different strains, for which reason the RAPD technique is very useful. All bifidobacteria isolated from commercial fermented milks in Spain belong to the same species B. animalis subsp. lactis. Our results demonstrate the necessity to control the presence of bifidobacteria in commercial fermented milks, not only at species level but also at strain level. Multiplex PCR and RAPDs are the most suitable, rapid and precise techniques to identify all bifidobacteria contained in fermented milk products at genus-, species-, and strain levels.