Multiparameter flow cytometric enumeration of probiotic-containing commercial powders

Flow cytometry (FCM) was used for the assessment of viability and number of freeze-dried bacteria such as: i) Lactobacillus sp.; ii) Lactobacillus sp., Bifidobacterium sp.; iii) and Lactobacillus sp., Bifidobacterium sp. and Lactococcus lactis) in commercially available capsules. Samples were analyzed based on two different physiological parameters: membrane integrity and measurement of the activity of cellular oxido-reductases. The total amount of viable bacteria from each probiotic product varied between 70 and 90%, while dead cells were between 10 and 30%. For product containing Lactobacillus sp. only, the number of bacteria assessed by FCM was 1.2 × 1011 AFU/g (active fluorescent units) compared to plate count (PC) method - 1.4 × 1011 CFU/g. For product containing Lactobacillus sp. and Bifidobacterium sp. the difference between CFU/g and AFU/g was 7.3 × 1010 and 1.3 × 1010 for product containing Lactobacillus sp., Bifidobacterium sp. and Lactococcus lactis. A method based on the enzymatic activity of the cells for product containing Lactobacillus sp. gave an average result: 1.4 × 1011 AFU/g, for product containing Lactobacillus sp., Bifidobacterium sp.: 7 × 1010 AFU/g and for product containing Lactobacillus sp., Bifidobacterium sp. and Lactococcus lactis: 3.7 × 1010 AFU/g. FCM allowed to get results after 2 h compared to PC method where result was obtained after 96 h. Moreover no significant differences were found for AFU/g obtained using the two protocols.