Method for the differential enumeration of lactic acid bacteria in a mixture in a food product

A method for distinguishing from one another, and counting, strains of lactic acid bacteria or Bifidobacteria in a known mixture, which are present in different population amounts in a dairy product,

• wherein the diary product to be tested contains at least one strain of Lactobacillus paracasei subsp. paracasei, at least one strain of Lactobacillus delbrueckii subsp. bulgaricus, and at least one strain of Streptococcus thermophilus,
• wherein the method comprises:
• (a) inoculating aliquots of said food product in a series of culture dishes each containing a chemically defined agar M1 medium with the following composition:
• agar 15 g/l, tryptone 2.5 g/l, pepsin-digested meat peptone 2.5 g/l, papain-digested soya peptone 5 g/l, sodium glycerophosphate 19 g/l, lactose 5 g/l, yeast extract 2.5 g/l, meat extract 5 g/l, magnesium sulfate 0.25 g/l, ascorbic acid 0.5 g/l;
• and at least two chromogenic substrates:
• 6-chloro-3-indoxyl-β-D-galactopyranoside (salmon Gal) 0.2 g/l and 5-bromo-4-chloro-3-indolyl-β-D-glucopyranoside (X-Glu) 0.1 g/l producing different colorations, each of said substrates being taken up by at least one of said bacterial strains, and not being taken up by at least one other of said strains;
• and
• (b) optionally, inoculating aliquots of said food product in a series of culture dishes containing a chemically defined agar M2 medium enabling the growth of the lactic acid bacterial strains present in the product to be tested, which strains cannot grow on the M1 medium, and at least one chromogenic substrate taken up by at least one of said bacterial strains;
• and
• c) incubating said dishes for the time necessary to form bacterial colonies, and counting the colonies for each of the colorations observed in each culture dish.