Influence of the tonB energy-coupling protein on efflux-mediated multidrug resistance in Pseudomonas aeruginosa

TonB couples the energized state of the cytoplasmic membrane to the operation of outer membrane receptors responsible for Fe(III) siderophore uptake across the outer membrane of gram-negative bacteria. A tonB mutant of Pseudomonas aeruginosa deficient in iron siderophore uptake was shown in the present study to be hypersusceptible to a wide variety of antibiotics, reminiscent of the phenotype of mutants defective in the mexAB-oprM antibiotic efflux operon. This was not related to influences of a tonB mutation on the iron status of the cell, and indeed, intrinsic antibiotic susceptibility and mexAB-oprM expression were unaffected by iron levels in the growth medium. The presence of tonB on a multicopy plasmid increased the level of resistance of a MexAB-OprM(+) strain but not that of a MexAB-OprM(-) strain to a variety of antimicrobial agents, mexAB-oprM expression was not, however, altered in a tonB deletion mutant, indicating that any influence of TonB on MexAB-OprM-mediated multidrug resistance was at the level of pump activity. Consistent with this, drug accumulation assays revealed that the tonB deletion mutant exhibited decreased levels of drug efflux. Still, the multidrug resistance of a nalB strain was not wholly abrogated by a tonB mutation, indicating that it is likely not an essential component of the efflux apparatus. Similarly, elimination of tonB from an nfxB strain only partially compromised MexCD-OprJ-mediated multidrug resistance. Intriguingly, the drug susceptibility of a mexAB-oprM deletion strain was increased following deletion of tonB, suggesting that TonB may also influence antibiotic resistance mediated by determinants other than MexAB-OprM (and MexCD-OprJ). Thus, TonB plays an important role in both intrinsic and acquired antibiotic resistance in P. aeruginosa.