Freeze-drying of sol–gel encapsulated recombinant bioluminescent E. coli by using lyo-protectants

Toxicity sensing by live microorganisms has attracted considerable attention during the last decade. These sensors exhibit a non-specific, relatively fast response to changes in water quality or to physiological stress conditions and thus may provide an early warning against intentional or unintentional contamination of water sources. However, maintenance of cell cultures is a demanding task, requiring specialists’ attention and careful control of storage conditions, which seriously limits the practical usefulness of such diagnostics for field analysis and distributed monitoring. Encapsulation in hydrogels is a viable way for the construction of user-friendly solid-state sensors. However, the encapsulated cells should still be kept in wet conditions as drying results in immediate and permanent loss of activity. Particularly, the much-researched cellular silicates, biohybrids, lose activity upon drying. Here we introduce a freeze-drying compatible sol–gel process for the encapsulation of E. coli reporter cells in silicates. We demonstrate that incorporation of optimized concentrations of trehalose and glycerin in the sol–gel silicate precursors prevents the inactivation of E. coli during freeze-drying.