Outer membrane proteins responsible for multiple drug resistance in Pseudomonas aeruginosa

Three types of multiple-drug-resistant mutants which were phenotypically similar to previously described nalB, nfxB, and nfxC mutants were isolated from Pseudomonas aeruginosa PAO1 and two clinical isolates. Type 1 (nalB-type) mutants showed cross-resistance to meropenem, cephems, and quinolones. They overproduced an outer membrane protein with an apparent molecular mass of 50 kDa (OprM). Type 2 (nfxB-type) mutants showed cross-resistance to quinolones and new cephems, i.e., cefpirome and cefozopran, concomitant with overproduction of an outer membrane protein with an apparent molecular mass of 54 kDa (OprJ). Type 3 (nfxC-type) mutants showed cross-resistance to carbapenems and quinolones. They produced decreased amounts of OprD and increased amounts of a 50-kDa protein (OprN), which was almost the same molecular weight as that of OprM, but it was distinguishable from OprM by its heat modifiability on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In the presence of salicylate, the parent strains showed an increased level of resistance to carbapenems and quinolones and produced decreased amounts of OprD and increased amounts of OprN. Salicylate caused the repression of OprJ production and the loss of resistance to cefpirome and cefozopran in two of the three OprJ-overproducing mutants, although salicylate slightly increased the level of resistance in the parent strains. The changes in susceptibilities were transient in the presence of salicylate. These data suggest that at least three different outer membrane proteins, OprM, OprJ, and OprN, are associated with multiple drug resistance in P. aeruginosa.