Abstract: Objective　To detect the mRNA expression level of the nonessential penicillinbinding protein 4 (PBP4)dacB gene in AmpC beta-lactamase producing Escherichia coli isolates，and investigate the effect of PBP4 in the resistant mechanism of AmpC betalactamase producing gramnegative isolates. Methods　A total of 34 AmpC betalactamase producing Escherichia coli isolates were collected in Shanghai Sixth People′s Hospital from 2003 to 2010.The polymerase chain reaction(PCR) was used for the amplification of dacB gene and ampC gene, and the realtime quantitation PCR was used for the detection of the mRNA expression level of dacB. The isolates were classified into 2 groups: the high expression level group and the low expression level group. Real-time quantitation PCR was used for the detection of the mRNA expression level of ampC. The broth microdilution method was used for the detection of sensitivity. Results　Among the 34 Escherichia coli，26 (76.47%) of 34 isolates had mRNA overexpression level of dacB，and the mRNA of ampC in the high expression level group was higher than that in the low expression level group(P<0.05). Conclusions　The overexpression of PBP4 may be helpful to introduce highlevel resistance by triggering the overexpression of AmpC beta-lactamase among clinical isolates of Escherichia coli.

Key words: Penicillinbinding protein 4, AmpC beta-lactamase, ampC, dacB