The influence of fermentation parameters and heat-shock treatment on the viability of Lactobacillus fermentum PC1 during stress conditions

Lactobacilli need to be viable to be efficacious as probiotics. The aim of this study was to determine factors that influenced viability of the probiotic strain Lactobacillus fermentum PC1. The study was designed to investigate the influence of pH control during fermentation, the growth phase of the harvested cells and a heat-shock treatment (exposure to 52°C for 30 min) on viability of freeze-dried L. fermentum PC1 during different stress conditions. Protein profiling was used to understand the cellular mechanisms involved. For cells grown without pH control, early stationary and mid-stationary phase cells had better viability than late-log phase cells during all stress conditions except freeze-drying, while only early stationary phase cells had better viability during freeze-drying, acid and bile stress conditions when grown with pH control. Stationary phase cells had better resistance to acidic and storage conditions when grown without pH control, but had better resistance during freeze-drying when grown with pH control. Late-log phase cells grown with and without pH control had similar resistance during bile and acid stress conditions and storage at 75% relative humidity (RH). When cells were grown without pH control, heat-shock treatment improved viability of late-log phase and early stationary phase cells during acid stress and storage at 75% RH. The treatment improved viability during storage at 37°C for cells grown with pH control. Protein analysis using iTRAQ (isobaric tags for relative and absolute quantitation) method revealed proteins in L. fermentum PC1 that were up-regulated or down-regulated during stationary phase and by heat-shock treatment. General responses, protein synthesis, lipid metabolism, carbohydrate and pyruvate metabolism, nucleotide metabolism and regulation of intracellular pH were associated with these up-regulated and down-regulated proteins in L. fermentum PC1, which led to the cells having improved stress tolerance. In conclusion, pH control during growth as well as the growth phase for harvesting the L. fermentum PC1 cells from the fermenter influenced the subsequent susceptibility to stresses, and heat-shock treatment improved tolerance to some stresses. Proteomics confirmed that the stress tolerance was linked to altered metabolic and cellular properties