Department of Food and Drug, University of Parma, Parma, Italy.
Department of Food, Environmental and Nutritional Sciences, University of Milan, Italy.
Int J Food Microbiol. 2019 Oct 2;306:108268. doi: 10.1016/j.ijfoodmicro.2019.108268. Epub 2019 Jul 15.
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.
细菌产生胞外多糖（EPS）越来越受到食品制造商、生物技术行业和营养学家的关注，因为它们具有不同的作用。有几种分析方法可用于从食品中的乳酸菌（LAB）中回收、定量和表征 EPS。然而，EPS 生产的直接筛选方法仍然基于在补充固体生长培养基上发育的菌落的丝状纹理的视觉观察。为了克服许多当前使用的筛选方法的弱点，我们建议采用阻抗微生物学来评估牛奶中 LAB 产生的 EPS。在这项工作中，我们已经证明，通过 BacTrac 4300®系统在牛奶中测量的保加利亚乳杆菌亚种 2214 的电容曲线的特殊形状是由于 EPS 的产生。除了 pH 值测量外，在牛奶中孵育 0、8、13 和 55 小时后评估的 EPS 量与基因表达的评估一致，并通过共聚焦激光扫描显微镜和透射电子显微镜观察得到证实。为了验证所提出方法的适用性，在肉汤培养基和牛奶中评估了 22 种 EPS 产生（EPS+）LAB 菌株和一种阴性（EPS-）对照的电容曲线的下降实体（ΔE%）。在澄清的肉汤培养基中培养的所有菌株都发现了阳性 ΔE%值，这简单地证实了 EPS 的产生，只是观察到菌株依赖于设备测量电极表面上的 EPS 量。当相同的 EPS+菌株在牛奶中培养时，获得的ΔE%值表明，只有少数菌株能够在这种环境中产生 EPS，这支持了它们多样化的利用乳糖进行此目的的能力。这项多学科研究的结果表明，阻抗微生物学是一种合适的方法，可以克服最常用的用于筛选牛奶中 LAB 产生 EPS 的方法的限制。此外，这些结果还为在其他食品和饮料中的应用开辟了道路，其中原位产生的 EPS 可能对食品工业非常感兴趣。
