Cell length alternations as a stress indicator for Lactobacillus johnsonii NCC 533

•Cell length changes of Lactobacillus johnsonii NCC 533 were monitored to investigate post-stress growth behavior.•We report significant cell length changes linked to cell stress.•The impact of heat or PEF-stress was clearly distinguishable by the measured cell length.•The optical density can be influenced by the cell lengthening.

The aim of this work was to evaluate whether cell length alternations of Lactobacillus johnsonii NCC 533 can be used to monitor the response of this bacterium towards various stress factors during its growth. The selected stressors were heat shock (46 to 55 °C for 15, 25 min) and pulsed electric fields (PEF; at 15, 20 kJ kg− 1 with 7 to 20 kV cm− 1). These stress factors were applied either at the beginning or at the end of the log-phase in MRS broth. The cell length changes were also monitored during normal growth conditions. It was found, that the cell lengths of bacteria changed in the course of growth; with clear differences between lag, log and stationary growth phases. Heat shock led to significant cell elongation, whereas PEF-stress resulted in cell shrinkage. The stress treatments by heat and PEF were more influential on cell length changes at the beginning than at the end of the log-phase. The magnitude of cell length changes correlated with the intensity of the applied stresses.Industrial relevanceMicrobial fermentations require a reliable online monitoring, which rapidly gives information about occurring stresses, their extent and nature as well as the cell damage, the capability of cell recovery and the possibility of reaching the desired cell count. The present investigations and method developments could improve the understanding of these mechanisms to lead to better controlled industrial fermentations.