Biofilms formed by the archaeon Haloferax volcaniiexhibit cellular differentiation and social motility, and facilitate horizontal gene transfer

Biofilms formed by the archaeon Haloferax volcaniiexhibit cellular differentiation and social motility, and facilitate horizontal gene transfer

BMC Biology, August 2014

25124934

Scott Chimileski, Michael J Franklin, R Thane Papke

BackgroundArchaea share a similar microbial lifestyle with bacteria, and not surprisingly then, also exist within matrix-enclosed communities known as biofilms. Advances in biofilm biology have been made over decades in model bacterial species, and include characterizations of social behaviors and cellular differentiation during biofilm development. Like bacteria, archaea impact ecological and biogeochemical systems. However, the biology of archaeal biofilms is only now being explored. Here, we investigated the development, composition and dynamics of biofilms formed by the haloarchaeon Haloferax volcanii DS2.ResultsBiofilms were cultured in static liquid and visualized with fluorescent cell membrane dyes and by engineering cells to express green fluorescent protein (GFP). Analysis by confocal scanning laser microscopy showed that Hfx. volcanii cells formed microcolonies within 24 hours that developed into larger clusters by 48 hours and matured into flake-like towers often greater than 100 microns in height after seven days. To visualize extracellular matrix, biofilms formed by GFP-expressing cells were stained with concanavalin A, DAPI, Congo red, and thioflavin T. Stains co-localized with larger cellular structures and indicated that the extracellular matrix may contain a combination of polysaccharides, extracellular DNA, and amyloid protein. Following a switch to biofilm growth conditions, a sub-population of cells differentiated into chains of long rods sometimes exceeding 25 microns in length, compared to their planktonic disc-shaped morphology. Time-lapse photography of static liquid biofilms also revealed wave-like social motility. Finally, we quantified gene exchange between biofilm cells, and found that it was equivalent to the mating frequency of a classic filter-based experimental method.ConclusionsThe developmental processes, functional properties and dynamics of Hfx. volcanii biofilms provide insight on how haloarchaeal species might persist, interact, and exchange DNA in natural communities. Haloferax volcanii demonstrates some biofilm phenotypes similar to bacterial biofilms, but also has interesting phenotypes that may be unique to this organism or to this class of organisms, including changes in cellular morphology and an unusual form of social motility. Because Hfx. volcanii has one of the most advanced genetic systems for any archaeon, the phenotypes reported here may promote the study of genetic and developmental processes in archaeal biofilms.