C-terminal region of Pseudomonas aeruginosa outer membrane porin OprD modulates susceptibility to meropenem

We investigated the unusual susceptibility to meropenem observed for seven imipenem-resistant clinical isolates of Pseudomonas aernginosa. These strai ns were genetically closely related, expressed OprD, as determined by Weste rn blot analyses, and were resistant to imipenem (>5 mug/ml) but susceptibl e to meropenem (<1 mug/ml). The oprD genes from two isolates were entirely sequenced, and their deduced protein sequences showed 93% identity with tha t of OprD of strain PAO1, The major alteration consisted of the replacement of a stretch of 12 amino acids, located in putative external loop L7 of Op rD, by a divergent sequence of 10 amino acid residues, The oprD gene varian ts and the wild-type oprD gene were cloned and expressed in a defined oprD mutant. The meropenem MICs for strains carrying the oprD genes from clinica l isolates were four times lower than that for the strain carrying the wild -type oprD gene. Imipenem activities, however, were comparable for all stra ins. Furthermore, meropenem hypersusceptibility was obtained with a hybrid OprD porin that consisted of the PAO1 oprD gene containing loop L7 from a c linical isolate. These results show that the C-terminal portion of OprD, in particular, loop L7, was responsible for the unusual meropenem hypersuscep tibility, Competition experiments suggested that the observed OprD modifica tions in the clinical isolates did not affect antagonism between imipenem a nd the basic amino acid L-lysine, We further propose that shortening of put ative loop L7 of the OprD porin by 2 amino acid residues sfficiently opens the porin channel to allow optimal penetration of meropenem and increase it s activity. In contrast, this alteration would not affect susceptibility to a smaller carbapenem molecule, such as imipenem.