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Many gram-negative bacteria communicate byN -acyl homoserine lactone signals called autoinducers (AIs). InPseudomonas aeruginosa , cell-to-cell signaling controls expression of extracellular virulence factors, the type II secretion apparatus, a stationary-phase sigma factor (ςs ), and biofilm differentiation. The fact that a similar signal,N -(3-oxohexanoyl) homoserine lactone, freely diffuses throughVibrio fischeri andEscherichia coli cells has led to the assumption that all AIs are freely diffusible. In this work, transport of the twoP. aeruginosa AIs,N -(3-oxododecanoyl) homoserine lactone (3OC12 -HSL) (formerly called PAI-1) andN -butyryl homoserine lactone (C4 -HSL) (formerly called PAI-2), was studied by using tritium-labeled signals. When [3 H]C4 -HSL was added to cell suspensions ofP. aeruginosa , the cellular concentration reached a steady state in less than 30 s and was nearly equal to the external concentration, as expected for a freely diffusible compound. In contrast, [3 H]3OC12 -HSL required about 5 min to reach a steady state, and the cellular concentration was 3 times higher than the external level. Addition of inhibitors of the cytoplasmic membrane proton gradient, such as azide, led to a strong increase in cellular accumulation of [3 H]3OC12 -HSL, suggesting the involvement of active efflux. A defined mutant lacking themexA-mexB-oprM -encoded active-efflux pump accumulated [3 H]3OC12 -HSL to levels similar to those in the azide-treated wild-type cells. Efflux experiments confirmed these observations. Our results show that in contrast to the case for C4 -HSL,P. aeruginosa cells are not freely permeable to 3OC12 -HSL. Instead, themexA-mexB-oprM -encoded efflux pump is involved in active efflux of 3OC12 -HSL. Apparently the length and/or degree of substitution of theN -acyl side chain determines whether an AI is freely diffusible or is subject to active efflux byP. aeruginosa .

Active Efflux and Diffusion Are Involved in Transport of Pseudomonas aeruginosa Cell-to-Cell Signals