Abstract: Objective To investigate the changes of virulence of Escherichia coli and Klebsiella pneumoniae after space mutation. Methods Four bacterial strains including the ground control and the space mutation strain of Escherichia coli and Klebsiella pneumoniae were selected, and the median lethal dose (LD50) of them were determined after the mice (with the age of 30 d) were intraperitoneally injected with 0.5 ml of bacterial suspension at different dilution concentrations. Results The LD50 of the space mutation and ground control strain of Escherichia coli were 10^7.25CFU/ml and 10^7.5CFU/ml respectively, and those of Klebsiella pneumonia were 10^7.25CFU/ml and 10^7.75CFU/ml.The median lethal dose of the space mutation strain was lower than that of the ground control strain in both Escherichia coli and Klebsiella pneumonia. Conclusion Space mutation can enhance the virulence of Escherichia coli and Klebsiella pneumoniae.