Characterization of the RND-Type Multidrug Efflux Pump MexAB-OprM of the Plant Pathogen Pseudomonas syringae

Abstract

ABSTRACT In gram-negative bacteria, transporters belonging to the RND family are the transporters most relevant for resistance to antimicrobial compounds. In Pseudomonas aeruginosa , a clinically important pathogen, the RND-type pump MexAB-OprM has been recognized as one of the major multidrug efflux systems. Here, homologues of MexAB-OprM in the plant pathogens Pseudomonas syringae pv. phaseolicola 1448A, P. syringae pv. syringae B728a, and P. syringae pv. tomato DC3000 were identified, and mexAB-oprM -deficient mutants were generated. Determination of MICs revealed that mutation of MexAB-OprM dramatically reduced the tolerance to a broad range of antimicrobials. Moreover, the ability of the mexAB - oprM -deficient mutants to multiply in planta was reduced. RNA dot blot hybridization revealed growth-dependent regulation of the mexAB-oprM operon in P. syringae ; the expression of this operon was maximal in early exponential phase and decreased gradually during further growth.