The effects of pH oscillation on Lactobacillus rhamnosus batch cultivation

Inhomogeneous mixing in industrial-sized fermentation processes causes oscillations in process parameters such as temperature or pH value in the cultivation medium, which causes stress to the bacteria being cultivated. In this work, the impact of extracellular pH oscillations on the production of Lactobacillus rhamnosus, a well-studied probiotic bacteria, were investigated by means of a scale-down batch process, simulating inhomogeneous pH values by controlling the pH value of the medium on sinusoidal trajectories. Effects of pH stimulation on the bacteria were assessed by testing storage and freeze-drying stability of harvested cells, two factors relevant for the industrial process. Furthermore, gene expressions of six selected genes, i.e. atpA, fat, cfa, groEL, hrcA, and pstS, known to be related to stress response were monitored. Although storage stability is only slightly negatively affected by pH stimulation of the bacteria, gene expression of four of the studied genes, i.e. fat, hrcA, groEL, and pstS show to correlate with amplitude and frequency of the oscillation.