Suitability of MRS-bile agar for the selective enumeration of mixed probiotic bacteria in presence of mesophilic lactic acid cultures and yoghurt bacteria

Abstract

Measuring the viability of probiotic microorganisms in food products using plate count methodology is a common practice due to the simplicity (ease of performance), inexpensive and routine testing characters of<br />this method. In present study, the suitability of de man rogosa and sharpe agar (MRS) bile agar medium for<br />the selective enumeration of mixed probiotic bacteria (Lactobacillus acidophilus LA-5, L. casei 431 and<br />Bifidobacterium lactis BB-12) in presence of mesophilic lactic cultures (Lactococcus lactis ssp. lactis<br />and Lactococcus lactis ssp. cremoris) and yoghurt bacteria (Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus) was investigated. Yoghurt bacteria did not grow neither in presence of 0.15% nor 0.30% of bile salts, as was expected. Mesophilic lactic starters could grow at both concentrations of bile salts at all incubation temperatures except 37°C. According to these results, MRS-bile agar (0.15 bile salts) could be successfully used for selective enumeration of mixed probiotic cultures in presence of mesophilic culture and/or yoghurt bacteria when plates were incubated at 37°C for 72 h.