Fluorescence microscopy for studing the viability of micro-organisms in natural whey starters

AIMS: The aim of this work was to study the viability and cultivability of microbial populations of different natural whey starters and to evaluate their resistance to thermal treatments (such as exposure to high or low temperatures). METHODS AND RESULTS: Twenty-three natural whey starters for Grana Padano cheese were investigated and subsequently pH measurement, plate count agar using Man-Rogasa-Sharpe (MRS) pH 5.4 agar and whey agar medium (WAM) were performed using these samples. LIVE/DEAD BacLight bacterial viability kit was used. Total count and viability of all the 23 samples were high and similar to each other (CV 20%). However, the cultivable population was lower in terms of cfu ml(-1) and number of cells per millilitre than the viable fraction and highly variable, although its count value was higher in WAM than in MRS pH 5.4. The heating (60 degrees C for 5 min and 54 degrees C for 1 h) and freezing (-20 and -80 degrees C) treatments affected the cultivability and viability of the microbial population. CONCLUSIONS: This study demonstrated the effectiveness of LIVE/DEAD BacLight bacterial viability kit, which has already been used to evaluate bacterial populations, in investigating microbial viability in a complex ecosystem such as a natural whey starter. SIGNIFICANCE AND IMPACT OF THE STUDY: The aim of this study was to quantify the presence of damaged nonviable bacterial cells in natural whey starters. The Thoma Glass is a useful method to obtain fluorescence microscopy counts to evaluate the technological performance of natural whey starters.