Impedance microbiology to speed up the screening of lactic acid bacteria exopolysaccharide production

Abstract

Bacterial production of exopolysaccharides (EPS) is of increasing interest near food manufacturers, biotechnology industries and nutritionists because of their different roles. Several analytical methods are available for recovery, quantification and characterization of EPS from lactic acid bacteria (LAB) in food. However, direct screening method for production of EPS is still based on the visual observation of filamentous texture of the colonies developed on supplemented solid growth media. To overcome weaknesses of many currently used screening methods, we propose adopting impedance microbiology to evaluate the EPS production from LAB in milk. In this work we have proven that the peculiar shape of capacitance curve of Lactobacillus delbrueckii subsp. bulgaricus 2214, measured in milk by means of a BacTrac 4300® system, is due to production of EPS. Besides the pH measurement, the amounts of EPS evaluated after 0, 8, 13 and 55 h of incubation in milk, were in agreement with the evaluation of gene expression and confirmed by the observations by confocal laser scanning microscopy and by transmission electron microscopy. With the aim to verify the applicability of the proposed method, the drop entity of the capacitance curve (ΔE%) of 22 EPS-producing (EPS+) LAB strains and one negative (EPS-) control was evaluated both in broth medium and in milk. The positive ΔE% value found for all of the strains cultivated in the clear broth medium allowed to confirm the EPS production, simply observing a strain-dependent amount of EPS on surface of the measurement electrodes of the device. When the same EPS+ strains were cultivated in milk, the obtained ΔE% values showed that only a few of them were able to produce EPS in this environment, supporting their diversified ability to utilize lactose for this purpose. Results obtained by this multidisciplinary study demonstrate that impedance microbiology represents a suitable method to overcome the limits of the most commonly used methods to screen LAB for EPS production in milk. Moreover, these results also open a door to the application to other food and beverages, in which the EPS produced in situ could be of great interest for food industry.

Keywords: Conductance; Confocal laser microscopy; Milk; Transmission electron microscopy.