Culture media for the enumeration of Bifidobacterium bifidum and Lactobacillus acidophilus in the presence of yoghurt bacteria

Abstract

Abstract A proposed methodology for the enumeration of probiotic bacteria (bifidobacteria and Lactobacillus acidophilus ) in the presence of yoghurt bacteria is described. Commercial mixed cultures of yoghurt (four cultures), L. acidophilus (two cultures) and Bifidobacterium bifidum (two cultures) were assayed using a total of 15 culture media from four agar categories: nonselective (Skim Milk (SM), MRS and M17); modified (Galactose MRS (G-MRS), Galactose M17 (G-M17) and Trehalose MRS (T-MRS)); differential (Lactic Bacteria Differential (LBD), Lactic- and L-S); and selective (Nalidixic Paromomycin Neomycin Lithium MRS (NPNL-MRS), Nalidixic Neomycin Lithium MRS (NNL-MRS), Nalidixic Kanamycin Lithium MRS (NKB-MRS), Bile-MRS, Oxgall Gentamicin MRS (OG-MRS) and Lithium Propionate MRS (LP-MRS)). Every culture used was tested for its growth on each medium under three different incubation conditions (aerobic, anaerobic with combustion and anaerobic in GasPak) at 37°C for 72 h. SM and MRS agar were taken as reference media for yoghurt and probiotic bacteria, respectively. For each organism, the cell recovery rate on each medium was compared statistically with that obtained on the reference media. The ability of every medium to differentiate or inhibit the organisms was also tested. SM and LBD agar (aerobic) were useful to enumerate yoghurt bacteria and inhibit bifidobacteria. T-MRS and Bile-MRS agar (aerobic) only allowed the growth of L. acidophilus cultures. For Bifidobacterium bifidum , a selective count could be performed on LP-MRS agar (anaerobic, GasPak). This set of media was demonstrated to be effective for the enumeration of S. thermophilus , Lactobacillus delbrueckii subsp. bulgaricus , L. acidophilus and Bifidobacterium bifidum in commercial dairy products.