Some observations in freeze-drying of recombinant bioluminescent Escherichia coli for toxicity monitoring

A recombinant bioluminescent bacteria, containing a fabA::luxCDABE fusion gene, has been used to characterize freeze-drying methods, which may be conveniently used as a tool for the development of a portable biosensor. Through residual water, viability, biosensing activity and scanning electron microscopy analyses, the characteristics that four cryoprotectants, trehalose, sucrose, sorbitol, and mannitol, conferred on freeze-dried samples were elucidated, including the morphology, water content and activity of the cells. It was found that trehalose showed the best freeze-drying efficiency among the tested cryoprotectants and it might have a specific capacity limitation in protection of the cells during the freeze step. Humidity might result in damage to the cells, according to the viability, when exposed to air during storage, while the water remaining post freeze-drying showed good correlation with damage to the freeze-dried cells when under air-tight storage conditions. The results with other recombinant bioluminescent bacteria indicated that these findings might be general features of the freeze-drying processes.