Viability staining and detection of metabolic activity of sourdough lactic acid bacteria under stress conditions

Forty-one strains of lactic acid bacteria isolated from wheat sourdoughs were exposed to acid, osmotic and oxidative stresses. Live/DeadÒ BacLightTM Bacterial Viability kit was used to assess cell viability based on membrane integrity and the reduction of the redox dye 2-(iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chlo- ride (INT) was used to detect residual metabolic activity. Comparison between the two different methods and plate counts was carried out for several strains. Complete loss of membrane integrity was observed in Lactobacillus curva- tus strains for all treatments, while Weissella cibaria and Leuconostoc mesenteroides presented a damaged mem- brane only after exposure to acid stress. For most of the strains belonging to Lb. plantarum and Lb. paraplantarum species, a high percentage of cells conserved cell mem- brane integrity after exposure to the different stresses, indicating a high tolerance of all treatments. All species were able to reduce INT after exposure to osmotic stress, but a drastic decrease or a complete loss of cell metabolic activity were achieved after acid and oxidative stress treatments. Live/Dead staining is a rapid method to mon- itor cell injury, but it does not provide a good assessment of cell viability for all stresses. A good agreement was found between cell viability measured by plate count and Bac- Light staining for acid and osmotic stresses, but the latter viability measurement method underestimated the damage caused by oxidative stress when compared to plate count or measurement of metabolic activity.