Abstract
The rapid assessment of cell viability is crucial for process optimization, e.g., during media selection, determination of optimal environmental growth conditions and for quality control. In the present study, the cells' electric anisotropy of polarizability (AP) as well as the mean cell length in Lactobacillus plantarum batch and fed-batch fermentations were monitored with electrooptical measurements coupled to fully automated sample preparation. It was examined, whether this measurement can be related to the cells' metabolic activity, and thus represents a suitable process analytical technology. It is demonstrated that the AP is an early indicator to distinguish between suitable and unsuitable growth conditions in case of a poor energy regeneration or cell membrane defects in L. plantarum batch and fed-batch cultivations. It was shown that the applied method allowed the monitoring of physiological and morphological changes of cells in various growth phases in response to a low pH-value, substrate concentration changes, temperature alterations, exposure to air and nutrient limitation. An optimal range for growth in batch mode was achieved, if the AP remained above 25·10−28 F·m2 and the mean cell length at ~2.5 μm. It was further investigated, in which way the AP develops after freeze-drying of samples, which were taken in different cultivation phases. It was found that the AP increased most rapidly in resuspended samples from the retardation and late stationary phases, while samples from the early stationary phase recovered slowly. Electrooptical measurements provide valuable information about the physiologic and morphologic state of L. plantarum cells, e.g., when applied as starter cultures or as probiotic compounds.
Highlights
Lactic acid bacteria are applied for food preservation, but more importantly for yogurt and probiotics production
The aim of this study is the investigation of the suitability of electrooptical polarizability measurements to determine uncomfortable growth conditions and potential losses of cell viability and metabolic activity
This encouraged further investigation whether different cultivation conditions will lead to a distinct change of the anisotropy of polarizability (AP), and whether this change can be correlated with growth and lactic acid formation
Summary
Lactic acid bacteria are applied for food preservation, but more importantly for yogurt and probiotics production. Polarizability of Lactobacillus plantarum Cultures physiologic key parameters in L. plantarum cultivations like multi-parameter flow cytometry (Arnold et al, 2002; Schenk et al, 2008; Bensch et al, 2014; Tropcheva et al, 2015), quantitative real-time PCR, e.g., (Clementschitsch et al, 2005; Davis, 2014; Sohier et al, 2014; Pega et al, 2016; Emerson et al, 2017), and viable cell counting for viability analysis (Savini et al, 2010; Perdana et al, 2012; Glušac et al, 2015) All these systems provide information on the metabolic state and physiology of the cell, they require manual off-line sample pre-treatment. The electrooptical analysis of the anisotropy of polarizability (AP), in contrast, represents a fully automated method that has been developed to monitor the viability of rod-shaped microorganisms, since they orientate under the effect of an electrical field This orientation is measured by the change of extinction from two orthogonal light sources. The method provides a spectrum of the AP, since various frequencies (kHz to MHz) can be applied in one measurement
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