Enumeration of bifidobacteria using PetrifilmTM AC in pure cultures and in a fermented milk manufactured with a commercial culture of Streptococcus thermophilus

Abstract

Bifidobacteria are probiotic microorganisms that are widely used in the food industry. With the aim of using of PetrifilmÔ Aerobic Count (AC) plates associated with selective culture media, aliquots of sterile skim milk were inoculated separately with four commercial cultures of bifidobacteria. These cultures were plated by both the conventional method and PetrifilmÔAC, using the culture media NNLP and ABC. The cultures were incubated under anaerobiosis at 37 C for 24, 48 and 72 h. No significant differences (p > 0.05) were observed between the obtained counts at 48 and 72 h. Bifidobacteria counts in ABC were usually higher than in NNLP, independent of the plating method. Subsequently, fermented milk was prepared with a Streptococcus thermophilus strain, and aliquots were inoculated with the same bifidobacteria. Then, the fermented milks were submitted to microbiological analysis for bifidobacteria enumeration using the same culture media and methodologies previously described, incubated under anaerobiosis at 37 C for 48 h. Again, bifidobacteria counts in ABC were higher than in NNLP, with significant differences for some cultures (p < 0.05). The counts obtained by both methodologies presented significant correlations (p < 0.05). The results indicate the viability of PetrifilmÔAC as an alternative method for bifidobacteria enumeration when associated to specific culture media, specially the ABC.