Evaluation of culture media for counts of Bifidobacterium animalis subsp. lactis Bb 12in yoghurt after refrigerated storage.

Abstract

The agar RCPB pH5 has been considered a good alternative for counts of Bifidobacterium in yoghurt. However, during the refrigerated storage of yoghurt it is extremely difficult to count this microorganism due to the size of the colonies, which are so small they require the aid of a stereoscope to count them. Another agar, MRS-LP, has been also recommended for counts of Bifidobacterium in the presence of yoghurt bacteria. This study evaluated the supplementation of RCPB pH5 agar with dehydrated liver extract and the salts KH2PO4, K2HPO4, FeSO47H2O, MnSO4H2O and MgSO47H2O, aiming at improving the differentiation of Bifidobacterium in yoghurt after refrigerated storage, and also evaluated the selective count of Bifidobacterium in yoghurt using the agar MRS-LP. The agar MRS-LP presented the same cell recovery as non-fortified RCPB pH5 agar, used as a standard medium, thus being considered a good option for counts of Bifidobacterium in yoghurt. The fortified RCPB pH5 also presented the same recovery as the standard RCPB pH5 medium, however, the addition of dehydrated liver extract to the RCPB pH5 agar considerably increased the size of the Bifidobacterium colonies after refrigerated storage, making differentiation of the colonies much easier and reliable when compared to the standard non-fortified RPCP pH5. The addition of the salts (KH2PO4, K2HPO4, FeSO47H2O, MnSO4H2O and MgSO47H2O) had no influence on the performance of the RCPB pH5 agar.