Plasmid transfer in biofilms: a perspective on limitations and opportunities

Abstract

Biofilms dominate microbial life, and their importance for human health and the environment can no longer be dismissed. Nevertheless many of the processes governing this form of microbial growth are still poorly understood. This includes the horizontal exchange of genetic information, which is a major driver in bacterial evolution and rapid adaptation, exemplified by the alarming spread of multi-drug resistance among pathogens mediated by plasmids. Biofilms are often considered hot spot for horizontal gene transfer, yet several studies have shown that plasmid transfer is limited to the outer layers. On the basis of results from decades of research we analyse this paradox and discuss the mechanisms by which biofilm growth can promote the initial transfer of some plasmids, but also limit further plasmid invasion into the population or community. If we want to adequately promote or combat horizontal gene spread in biofilms, we need to gain better insight into the physicochemical and biological mechanisms that control this process.

Figure 1

Plasmid transfer and loss in bacterial populations grown on agar. (a, b) Confocal laser scanning microscopy photographs of populations of donors of plasmid pB10 marked with dsRed (pB10::rfp in P. putida SM1443) and recipient cells (P. putida KT2442::gfp, green) grown at 30 °C for 4 days. Because the plasmid-borne dsRed expression is chromosomally repressed in the donor, the donor cells are black in the photograph. The bacteria were either streaked on lysogeny broth (LB) agar (a) in a cross, allowing initial mixing, or (b) next to each other (see schematics in the inserts for set-up); (c, d) photographs of two bacterial species forming colonies with sectors. The bacterial inoculum contained pB10::rfp and was allowed to grow for many days without selective pressure for plasmid retention. Loss of plasmid is visible as white sectors. All images support the notion that self-transmissible plasmids do not readily invade existing populations of plasmid-free cells.

Figure 2

Confocal laser scanning microscopy photographs of a biofilm formed in a continuous flow chamber by a mixture of donor cells carrying plasmid pB10 marked with dsRed (pB10::rfp in E. coli MG1655, red) and recipient cells marked with gfp (P. putida KT2244::gfp, green), generating yellow/orange transconjugants at a low frequency. The mixture (ratio 1/1) of the two bacteria was grown for 27 h in a flow cell continuously fed with 1/3 M9.

Figure 3

Theoretical representation of a self-transmissible plasmid that completely invades an unstructured population over time (A, dashed line) yet only partially invades a structured population (B, dotted line). The drawing on the right represents a population of plasmid-free (white) and plasmid-containing (dark) bacteria growing in a well-mixed environment (top) or on a surface (bottom). As discussed in the text plasmid invasion can occur by vertical and horizontal transfer.