Abstract: Objective　To investigate the in vitro antibacterial activity of the combined therapy of imipenem with polymyxin B against Pseudomonas aeruginosa with different phenotypes.   Methods　A total of 133 clinically isolated Pseudomonas aeruginosa were collected from Renji Hospital（West Department）and Ruijin Hospital，and the phenotypes were determined by disc diffusion（K-B）method. The minimum inhibitory concentrations（MIC）of imipenem，polymyxin B and imipenem with polymyxin B against Pseudomonas aeruginosa were calculated by agar dilution method，and the fractional inhibitory concentrations（FIC）were calculated. The combined effect was analyzed. There were 2 ratios，1∶0.5 and 1∶1 with imipenem to polymyxin B.  Results　The drug sensitivities of those 2 therapies turned out to be inconsistent when conducting Kappa test. The synergistic rate of 1∶1 with imipenem to polymyxin B was 51.13%，which was much higher than 1∶0.5 with imipenem to polymyxin（19.55%）. Fisher′s exact test demonstrated that 1∶1 with imipenem to polymyxin B maintained high synergistic rate between different phenotypes，and there was no statistically significant difference. However，the difference was significant regarding to 1∶0.5 with imipenem to polymyxin B.  Conclusions　Comparing with monotherapy，the combined therapy of imipenem with polymyxin B significantly increases the sensitivity of Pseudomonas aeruginosa in vitro. The combined therapy applying 1∶1 with imipenem to polymyxin B with a higher synergistic rate than that applying 1∶0.5 with imipenem to polymyxin B and a good stability may be a good choice for the antibiotics resistant Pseudomonas aeruginosa with different phenotypes.

Key words: Pseudomonas aeruginosa, Drug combination, Resistance, Imipenem, Polymyxin B