Control and promotion of probiotic cells' aggregation and viability using DC electric field and standing acoustic waves

Abstract

Bacterial cells' long-term stability can be induced through cell aggregation. This study aimed to stimulate probiotic cells' aggregation, suspended in 10 mM KH2PO4 at 18 ± 2 °C, utilizing DC electric fields (10–100 mA) and combined electric field (40 mA) with standing acoustic waves (at a frequency range of 26 kHz to 296 kHz). It was demonstrated that the sole application of acoustic waves on the rod-shaped Bifidobacterium animalis subsp. lactis (BIFIDO) did not trigger cell aggregation. However, the application of DC electric field to BIFIDO, induced polarization and aggregation of the BIFIDO cells, as indicated by spectrophotometric measurements (OD600) and visualized by fluorescence microscopy. The aggregation of BIFIDO cells was accelerated (∼6 times) when applying simultaneously DC electric field and standing acoustic waves, due to the accumulation of the cells at the acoustic pressure nodes, and due to the polarization of the cells by dielectrophoresis (DEP) phenomenon. The synergistic effect of DC electric field and standing acoustic waves not only facilitated the aggregation of the cells, but also considerably enhanced the hydrophobicity of BIFIDO cells, without compromising their viability or altering their surface charges. Enhanced viability and stability of the aggregated probiotic cells, in comparison to the non-aggregated, was also observed. Industrial relevanceThe present study points out the application of DC electric field and Standing Acoustic waves to aggregate probiotic cells. An important attribute of aggregated cells is their greater tolerance to environmental stresses. This technology could be applied for large-scale manipulation and aggregation of cells, as for instance during fermentation, prior to drying or encapsulation of cells, or during other industrial cell treatment technologies.