Développement d'un milieu sélectif pour le dénombrement des bifidobactéries dans les laits fermentés

Abstract

Development of a selective culture medium for the enumeration of bifidobacteria in fermented milks. The use of selective culture media is necessary to demonstrate the presence of bifidobacteria in European fermented milks, since Lactobacillus (L.) acidophilus, L. delbrueckii ssp. bulgaricus and Streptococcus (S.) thermophilus usually dominate the flora in these products. But a fully selective medium suppressing these microorganisms does not exist as yet. Therefore, the selectivity and productivity of three media were tested qualitatively and quantitatively: DIC (pH 6.8) and BEE (pH 5.0) were based on TPY and Columbia agar with dicloxacillin and propionic acid respectively, as inhibitory agents. A new combination, DP (pH 6.8), was created on the base of Columbia agar with both propionic acid and dicloxacillin as additives. The elective medium BIO, which is based on RCM agar supplemented with lactose and human erythrocyte concentrate was used as control medium. The samples tested were either single strains of Bifidobacterium spp., L. acidophilus, L. delbrueckii ssp. bulgaricus and S. thermophilus or mixte cultures in form of starter cultures or fermented milks. DIC allowed a good growth of bifidobacteria as well as L. acidophilus. On BEE bifidobacteria as well as L. delbrueckii ssp. bulgaricus grew irregularly. DP allowed a quantitatively sufficient growth of bifidobacteria and prevented the growth of accompanying L. acidophilus, L. delbrueckii ssp. bulgaricus and S. thermophilus strains. Therefore, DP is advantageous over DIC and BEE and can be recommended for the selective isolation of bifidobacteria from starter cultures and from different fermented milks products in Europe.