Use of modified Lactobacillus selective medium and Bifidobacterium lodoacetate medium for differential enumeration of Lactobacillus acidophilus and Bifidobacterium spp. in powdered nutritional products

Modified Lactobacillus selective agar (APT agar + sodium acetate and glacia l acetic acid; mLBS) was compared to selective modified Lactobacillus selec tive medium (LBS agar + tomato juice and acetic acid; mLSM) and nonselectiv e de Man Rogosa Sharpe (MRS) agar for the enumeration of Lactobacillus acid ophilus in probiotic-containing powdered nutritional products. The mLBS aga r was equivalent to MRS agar and superior to the mLSM agar for enumerating L. acidophilus in products stored in sealed cans at 22 degrees C. When samp les were analyzed for L. acidophilus concentration after high temperature s torage in sealed cans or storage in open cans at high relative humidity, th e mLBS and MRS agars were highly correlated (r(2) = 0.93). Modified Bifidob acterium iodoacetate medium (12.5 mg iodoacetic acid/liter; mBIM) was compa red to MRS agar + bile, cysteine, and dicloxacillin (MRS + BCD) for enumera ting Bifidobacterium infantis or Bifidobacterium lactis in the nutritional products. The two media were equivalent for enumerating B. infantis in prod uct stored at 22 degrees C in sealed cans. However, the two media were poor ly correlated (r(2) < 0.50) for enumeration of B. infantis and B. lactis in products stored in sealed cans at high temperature or in open cans at high relative humidity. The mLBS medium has potential industry application as a relatively inexpensive, convenient differential enumeration method for L. acidophilus. The mBIM medium cannot be recommended as a sole medium for enu meration of probiotic Bifidobacterium spp. in powdered nutritional products stored under high temperature and/or high relative humidity conditions.