Lyophilized, non-viable, recombinant E. coli cells for cadmium bioprecipitation and recovery

The phoN gene, encoding a non-specific acid phosphatase from Salmonella enterica sv. Typhi, was cloned and overexpressed into Escherichia coli. The E. coli cells bearing phoN showed high acid phosphatase activity and removed 83% of cadmium from a 1 mM solution in 3 h, when provided with 5 mM β-glycerophosphate as a source of phosphate. Such cells, when lyophilized without any lyo/cryoprotectant, were rendered non-viable but fully retained cadmium precipitation ability. Lyophilized recombinant cells could be stored at room temperature, without significant loss of activity for up to 6 months, and removed upto 21 g cadmium/g dry weight. The precipitated cadmium could be easily recovered from the cells by dilute acid wash, following which the cells retained their cadmium precipitation ability, facilitating their reuse. The use of genetically engineered, non-viable E. coli cells offers an environmentally safe biotechnology for bioremediation of cadmium from contaminated sites.