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Influence of Quorum Sensing and Iron on Twitching Motility and Biofilm Formation in Pseudomonas aeruginosa

Department of Microbiology and Immunology, Queen's University, Kingston, Ontario, Canada K7L 3N6,¹ The Mina and Everard Goodman Faculty of Life Sciences, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel,² Department of Microbiology, University of Washington School of Medicine, Seattle, Washington 98195-7242³

Received 12 September 2007/ Accepted 2 November 2007

Reducing iron (Fe) levels in a defined minimal medium reduced the growth yields of planktonic and biofilm Pseudomonas aeruginosa, though biofilm biomass was affected to the greatest extent and at FeCl₃ concentrations where planktonic cell growth was not compromised. Highlighting this apparently greater need for Fe, biofilm growth yields were markedly reduced in a mutant unable to produce pyoverdine (and, so, deficient in pyoverdine-mediated Fe acquisition) at concentrations of FeCl₃ that did not adversely affect biofilm yields of a pyoverdine-producing wild-type strain. Concomitant with the reduced biofilm yields at low Fe concentrations, P. aeruginosa showed enhanced twitching motility in Fe-deficient versus Fe-replete minimal media. A mutant deficient in low-Fe-stimulated twitching motility but normal as regards twitching motility on Fe-rich medium was isolated and shown to be disrupted in rhlI, whose product is responsible for synthesis of the N-butanoyl homoserine lactone (C4-HSL) quorum-sensing signal. In contrast to wild-type cells, which formed thin, flat, undeveloped biofilms in Fe-limited medium, the rhlI mutant formed substantially developed though not fully mature biofilms under Fe limitation. C4-HSL production increased markedly in Fe-limited versus Fe-rich P. aeruginosa cultures, and cell-free low-Fe culture supernatants restored the twitching motility of the rhlI mutant on Fe-limited minimal medium and stimulated the twitching motility of rhlI and wild-type P. aeruginosa on Fe-rich minimal medium. Still, addition of exogenous C4-HSL did not stimulate the twitching motility of either strain on Fe-replete medium, indicating that some Fe-regulated and RhlI/C4-HSL-dependent extracellular product(s) was responsible for the enhanced twitching motility (and reduced biofilm formation) seen in response to Fe limitation.

Published ahead of print on 9 November 2007.