Objectives: To provide information about the genetic relationships and mechanism underlying carbapenem resistance in Pseudomonas aeruginosa  clinical isolates of a hospital in China.
Materials and methods: One hundred and sixty P. aeruginosa  strains were isolated from a hospital in China. Susceptibility to 14 antimicrobial agents was determined by antimicrobial susceptibility testing. Multilocus sequence typing was used to characterize the genetic backgrounds of these clinical isolates. Forty-five strains were randomly selected for further evaluation of their carbapenem resistance mechanism. Their oprD  gene was compared with the PAO1 sequence.
Results: Multilocus sequence typing analysis demonstrated that these isolates were highly diverse; 68 sequence types were identified, of which 28 were novel sequence types. Polygenic and eBURST analysis demonstrated genetically similar clones with dissimilar resistance profiles. Among the 45 randomly selected strains associated with carbapenem resistance, 2 were metallo β-lactamase producers; all the 45 strains were not AmpC overproducers. Sequence analysis revealed a high diversity in the oprD  sequences among isolates. Strains susceptible to imipenem and meropenem with shortened L7 and L8 loops in oprD  were the major strain types observed in this hospital.
Conclusion: This study indicated that oprD  provided the main mechanism for carbapenem resistance. The shortened L7 and L8 loops are responsible for carbapenem susceptibility.
Keywords: Pseudomonas aeruginosa , multilocus sequence typing, carbapenem resistance, L7 and L8 loops in OprD