MULTIPLE ANTIBIOTIC-RESISTANCE IN PSEUDOMONAS-AERUGINOSA - EVIDENCE FOR INVOLVEMENT OF AN EFFLUX OPERON

An outer membrane protein of 50 kDa (OprK) was overproduced in a sider ophore-deficient mutant of Pseudomonas aeruginosa capable of growth on iron-deficient minimal medium containing 2,2'-dipyridyl (0.5 mM). The expression of OprK in the mutant (strain K385) was associated with en hanced resistance to a number of antimicrobial agents, including cipro floxacin, nalidixic acid, tetracycline, chloramphenicol, and streptoni grin. OprK was inducible in the parent strain by growth under severe i ron limitation, as provided, for example, by the addition of dipyridyl or ZnSO4 to the growth medium. The gene encoding OprK (previously ide ntified as ORFC) forms part of an operon composed of three genes (ORFA BC) implicated in the secretion of the siderophore pyoverdine. Mutants defective in ORFA, ORFB, or ORFC exhibited enhanced susceptibility to tetracycline, chloramphenicol, ciprofloxacin, streptonigrin, and dipy ridyl, consistent with a role for the ORFABC operon in multiple antibi otic resistance in P. aeruginosa. Sequence analysis of ORFC (oprK) rev ealed that its product is homologous to a class of outer membrane prot eins involved in export. Similarly, the products of ORFA and ORFB exhi bit homology to previously described bacterial export proteins located in the cytoplasmic membrane. These data suggest that ORFA-ORFB-oprK ( ORFC)-dependent drug efflux contributes to multiple antibiotic resista nce in P. aeruginosa. We propose, therefore, the designation mexAB (mu ltiple efflux) for ORFAB.