Comparison of selective and differential media for the accurate enumeration of strains of Lactobacillus acidophilus, Bifidobacterium spp. and Lactobacillus casei complex from commercial yoghurts

Various selective and differential media were evaluated for their suitability to provide reliable counts of Lactobacillus acidophilus, Bifidobacterium spp. and L. casei from nine different commercial probiotic yoghurts. Selective media reported to develop only single type of colonies, demonstrated two types of colonies with some yoghurts, making it difficult to conclusively enumerate the probiotic colonies. Similarly, colony characteristics on the differential media were found too subjective to suitably distinguish the probiotic colonies. Counts of each probiotic strain from the same yoghurt sample also varied on the different media. Except for LC agar, no selective or differential medium provided reliable counts of probiotic bacteria in all yoghurts. This study demonstrates the urgent need to develop reliable techniques to accurately enumerate probiotic bacteria in commercial yoghurts.