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Transcriptional profiling of Legionella pneumophila biofilm cells and the influence of iron on biofilm formation

Thomas Hindré^1,

¹ Laboratoire de Chimie de l'Eau et de l'Environnement, UMR 6008, Université de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
² Unité de Génomique des Microorganismes Pathogènes and CNRS URA 2171, Institut Pasteur, 28 Rue du Dr Roux, 75724 Paris, France

Correspondence
Yann Héchard
yann.hechard{at}univ-poitiers.fr

In aquatic environments, biofilms constitute an ecological niche where Legionella pneumophila persists as sessile cells. However, very little information on the sessile mode of life of L. pneumophila is currently available. We report here the development of a model biofilm of L. pneumophila strain Lens and the first transcriptome analysis of L. pneumophila biofilm cells. Global gene expression analysis of sessile cells as compared to two distinct populations of planktonic cells revealed that a substantial proportion of L. pneumophila genes is differentially expressed, as 2.3 % of the 2932 predicted genes exhibited at least a twofold change in gene expression. Comparison with previous results defining the gene expression profile of replicative- and transmissive-phase Legionella suggests that sessile cells resemble bacteria in the replicative phase. Further analysis of the most strongly regulated genes in sessile cells identified two induced gene clusters. One contains genes that encode alkyl hydroperoxide reductases known to act against oxidative stress. The second encodes proteins similar to PvcA and PvcB that are involved in siderophore biosynthesis in Pseudomonas aeruginosa. Since iron has been reported to modify biofilm formation in other species, we further focused on iron control of gene expression and biofilm formation. Among the genes showing the greatest differences in expression between planktonic cells and biofilm, only pvcA and pvcB were regulated by iron concentration. A pvcA L. pneumophila mutant showed no changes in biofilm formation compared to the wild-type, suggesting that the pvcA product is not mandatory for biofilm formation. However, biofilm formation by L. pneumophila wild-type and a pvcA strain was clearly inhibited in iron-rich conditions.

Present address: Institut National de Recherches Agronomiques, Laboratoire de Microbiologie, Centre de Recherche INRA de Clermont Ferrand Theix, 63122 Saint-Genes Champanelle, France.

The microarray results for these experiments are accessible on the website http://genoscript.pasteur.fr/ (see text for details).

Two supplementary tables showing genes with significant differences in expression in sessile and planktonic cells and the expression fold change of genes associated with iron homeostasis are available with the online version of this paper.

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