If you want a thicker biofilm you need to get your wiring sorted
A study in Nature Communications has found elusive genetic evidence for why Geobacter produces nanowires.
We have know for a long time that Geobacter produced nanowires, we just weren't sure why. As the authors of this new paper state, the reason is that most Geobacter strains that have mutations in the pili that form nanowires, also have mutations in the cytochrome genes making it very difficult to tell which mutation brings about changes in the phenotype.
In this study, a team from Michigan State University used a pili mutant with intact cytochrome genes to try and isolate the effect of the nanowires on phenotype. They found that the the pili mutants were able to form biofilms up to 10 μm thick on an anode surface but they could not produce any thicker. Conversly, strains with an intact pili gene were able to form thicker biofilms on the electrode surface. The working hypothesis is that the presence of nanowires allows bacterial cells to discharge respiratory electrons further away from the anode surface.
Electricity generation by Geobacter sulfurreducens biofilms grown on electrodes involves matrix-associated electron carriers, such as c-type cytochromes. Yet, the contribution of the biofilm’s conductive pili remains uncertain, largely because pili-defective mutants also have cytochrome defects. Here we report that a pili-deficient mutant carrying an inactivating mutation in the pilus assembly motor PilB has no measurable defects in cytochrome expression, yet forms anode biofilms with reduced electroactivity and is unable to grow beyond a threshold distance (~10 μm) from the underlying electrode. The defects are similar to those of a Tyr3 mutant, which produces poorly conductive pili. The results support a model in which the conductive pili permeate the biofilms to wire the cells to the conductive biofilm matrix and the underlying electrode, operating coordinately with cytochromes until the biofilm reaches a threshold thickness that limits the efficiency of the cytochrome pathway but not the functioning of the conductive pili network.
Read the full article here:
Mechanistic stratification in electroactive biofilms of Geobacter sulfurreducens mediated by pilus nanowires
Rebecca J. Steidl, Sanela Lampa-Pastirk & Gemma Reguera
Nature Communications 7, Article number: 12217 doi:10.1038/ncomms12217
Received 03 April 2015 Accepted 09 June 2016 Published 02 August 2016