Long-term preservation of silica gel-encapsulated bacterial biocatalysts by desiccation

Whole cells encapsulated in silica gels are used in a wide variety of applications in biomedicine, biotechnology and bioremediation. Drying after encapsulation is desirable to enhance the strength of the gel and to make it lighter, facilitating its use, storage and transportation. However, preserving biological activity of the cells in a desiccated state remains a formidable challenge. In this study, different drying conditions for a silica gel-encapsulated bacterial biocatalyst (atrazine biodegrading Escherichia coli) were studied to enhance mechanical properties while sustaining long-term biocatalytic activity of the bacteria. Effects of lyoprotectant solutions containing 0.4 M sucrose, 0.4 M trehalose or 30 % (wt/wt) glycerol on the activity of the encapsulated bacteria during drying were investigated. It was determined that two orders of magnitude increase in the elastic modulus (E) and the compressive stress at failure (σ) of the gel could be achieved by drying, independent of the drying rate. It was shown that partially desiccated silica gels preserved and enhanced the biocatalytic activity of the encapsulated bacteria up to a critical drying level. Atrazine biodegradation activity of encapsulated bacteria suspended with 0.4 M sucrose and PBS increased with increasing water removal, reaching a maximum at 68 % water loss. This enhanced activity was sustained for 3 months, when the gels were stored at 4 °C.