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Holey Biofilm!

In a recent study published in PNAS, Houry and collaborators used time-lapse microscopy to monitor the biofilms formed by the bacterium Bacillus thuringiensis and noted that a small subset (0.1 to 1%) of all the cells in the biofilm were motile. The rest of the cells were sessile and immobile except for some minor oscillatory motions hampered by the surrounding biofilm matrix. The swimmers infiltrated the biofilms in all directions, creating a landscape of tunnels and holes like in Swiss cheese. By tagging planktonic cells (that is, cells growing free in the surrounding liquid) with the green fluorescent protein (GFP), the authors showed that the biofilm swimmers were in fact planktonic cells. The swimmers infiltrated the biofilms independently of the flow dynamics of the surrounding fluid and their tunneling activity was exclusively dependent on the rotational activity of their flagella. Despite the biofilm barrier, the swimmers had average velocities as high as 7.3 μm/s in young (24 h old) biofilms. For a movie showing these rapid motions, click here. The swimming velocities decreased progressively as the biofilms aged, with the lowest velocities (4.2 μm/s) being measured in the oldest (72h old) biofilms. This is because the biofilm matrix also becomes more dense and rigid over time (and, therefore, more difficult to permeate). Still, these speeds are remarkable for cells that are swimming through a biofilm matrix!

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