Viability assessment of lactic acid bacteria in commercial dairy products stored at 4 °C using LIVE/DEAD® BacLightTM staining and conventional plate counts

The viability of lactic acid bacteria (LAB) from commercial fermented milks was studied during storage at 4 degrees C. The enumeration of total viable bacteria was assessed using fluorescence microscopy. Plate counting on selective media was used to enumerate LAB. Using LIVE/DEAD (R) BacLight(TM) viability staining, it was observed that bacterial counts decreased gradually after expiry dates, the number of viable bacteria remaining above 10(6) bacteria g(-1) for all of the products. Viable cell counts estimated by plating onto selective media were lower than those obtained by direct microscopic counting. The viability of LAB contained in acid products decreased during their storage period at 4 degrees C. All products contain viable LAB ranging from 10(8) to 10(9) bacteria g(-1) and could be considered as probiotic, given that the recommended minimum number of probiotic bacteria in such food products is approximately 10(7) bacteria mL(-1) product. The number of bifidobacteria in commercial fermented milks declared to contain bifidobacteria varied from 10(4) to 10(7) bacteria mL(-1). This study confirms the usefulness of fluorescent techniques for a rapid and accurate evaluation of bacterial viability in probiotic products.