Viability and diversity of probiotic Lactobacillus and Bifidobacterium populations included in commercial fermented milks

Abstract

A selection of commercial fermented milks was evaluated for the presence and viability of probiotic lactobacilli and bifidobacteria. Counts of Lactobacillus spp. always remained higher than 10 5 CFU ml −1, whereas the population of Bifidobacterium spp. decreased below this level in two products. All the probiotics announced on the label were present in commercial products, yet in two fermented milks one additional group of microorganisms was also found. The yogurt microorganism Streptococcus thermophilus was present in all cases, whereas Lactobacillus delbrueckii ssp. bulgaricus was only detected in two products. Bifidobacterium was the most frequently isolated group, all commercial strains being identified as Bifidobacterium animalis/ Bifidobacterium lactis by analysis of partial sequences of the 16 rRNA gene. The same technique allowed the identification of members of Lactobacillus casei (species Lactobacillus casei/ Lactobacillus paracasei/ Lactobacillus zeae) and Lactobacillus acidophilus (species L. acidophilus sensu stricto and Lactobacillus johnsonii). The analysis of macrorrestriction profiles by pulsed-field gel electrophoresis evidenced the reduced genetic variability existing among commercial Lactobacillus and Bifidobacterium strains. The combined use of macrorrestriction analysis and carbohydrate fermentation profiles enhanced the discriminatory power of the first technique for strains differentiation. None commercial Bifidobacterium strain presented the harmful β-glucuronidase activity whereas all of them displayed β-galactosidase, α-glucosidase and α-galactosidase activity.