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Natural conjugative plasmids induce bacterial biofilm development


Horizontal gene transfer is a principal source of evolution leading to change in the ecological character of bacterial species1. Bacterial conjugation2, which promotes the horizontal transfer of genetic material between donor and recipient cells by physical contact, is a phenomenon of fundamental evolutionary consequence3. Although conjugation has been studied primarily in liquid, most natural bacterial populations are found associated with environmental surfaces in complex multispecies communities called biofilms4. Biofilms are ideally suited to the exchange of genetic material of various origins, and it has been shown that bacterial conjugation occurs within biofilms5,6. Here I investigate the direct contribution of conjugative plasmids themselves to the capacity of the bacterial host to form a biofilm. Natural conjugative plasmids expressed factors that induced planktonic bacteria to form or enter biofilm communities, which favour the infectious transfer of the plasmid. This general connection between conjugation and biofilms suggests that medically relevant plasmid-bearing strains are more likely to form a biofilm. This may influence both the chances of biofilm-related infection risks and of conjugational spread of virulence factors.

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Figure 1: The F episome induces the formation of a thick biofilm in E. coli.
Figure 2: F pilus synthesis is required for biofilm formation in TG1.
Figure 3: Derepression of conjugation ability induces biofilm development.

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I am grateful to C. Wandersman for support and constant interest during the course of this work. I also thank D. Mazel, G. Gerbaud and P. Courvalin for providing the plasmids and strains used in this study; A. Idja and P. Roux for technical assistance; R. Longin for providing facilities of the Pasteur Institute Laboratory of Fermentations; and D. Mazel, E. Stewart, D. A. Rowe-Magnus and P. Delepelaire for helpful discussions and critical reading of the manuscript. This work was supported by grants from the Programme de Recherche Fondamentale en Microbiologie et Maladie Infectieuses, réseau Infections Nosocomiales (MENRT) and the Pasteur Institute.

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Correspondence to Jean-Marc Ghigo.

Supplementary information

Figure 4

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Supplemental data: A Kinetics of biofilm formation of strain TG1 carrying a GFP-expressing plasmid pGFPmn2. (Top): biofilm at different times after inoculation. Size bars: 10 µM (Bottom) Epifluorescence photomicrographs of the corresponding Pyrex slides. B Scanning laser confocal photomicrographs of TG1 (pGFPmn2). (Top) Schematic indicating the observation point. (Bottom) Z-axis.

Figure 5

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Supplemental data: Kinetics of surface adhesion in the F- and F+ E. coli strains. Compared cell density of strain MG1655 Ftet and MG1655 after 5; 10 and 24 hours of culture in microfermenters. Transmitted light microscopy of the Pyrex slide stained with crystal violet. Size bars: 10 µm

Figure 6

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Supplemental data: Natural conjugative plasmids promote biofilm formation. Representative examples of the biofilm promoting ability of natural conjugative plasmids. (Left) Plasmid-free E. coli F- strain BM21. Incompatibility groups of the presented plasmids are indicated.

Figure 7

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Derepression of conjugation ability of natural conjugative plasmids. Comparison of biofilm formation ability of strain E. coli BM21 carrying natural conjugative plasmids inoculated alone (left) or co-inoculated 24H after the initial inoculation of strain E. coli MG1655. Conjugative plasmids are described in Table 1. 16

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Ghigo, JM. Natural conjugative plasmids induce bacterial biofilm development. Nature 412, 442–445 (2001).

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