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

Bacterial production of exopolysaccharides (EPS) is of increasing interest near food manufacturers, bio-technology industries and nutritionists because of their different roles. Several analytical methods are availablefor recovery, quantification and characterization of EPS from lactic acid bacteria (LAB) in food. However, directscreening method for production of EPS is still based on the visual observation offilamentous texture of thecolonies developed on supplemented solid growth media. To overcome weaknesses of many currently usedscreening methods, we propose adopting impedance microbiology to evaluate the EPS production from LAB inmilk. In this work we have proven that the peculiar shape of capacitance curve ofLactobacillus delbrueckiisubsp.bulgaricus2214, measured in milk by means of a BacTrac 4300®system, is due to production of EPS. Besides thepH measurement, the amounts of EPS evaluated after 0, 8, 13 and 55 h of incubation in milk, were in agreementwith the evaluation of gene expression and confirmed by the observations by confocal laser scanning microscopyand 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 brothmedium and in milk. The positiveΔE% value found for all of the strains cultivated in the clear broth mediumallowed to confirm the EPS production, simply observing a strain-dependent amount of EPS on surface of themeasurement electrodes of the device. When the same EPS+ strains were cultivated in milk, the obtainedΔE%values showed that only a few of them were able to produce EPS in this environment, supporting their diversifiedability to utilize lactose for this purpose.Results obtained by this multidisciplinary study demonstrate that impedance microbiology represents asuitable method to overcome the limits of the most commonly used methods to screen LAB for EPS production inmilk. Moreover, these results also open a door to the application to other food and beverages, in which the EPSproduced in situ could be of great interest for food industry