Date Published: January 24, 2017
Publisher: Public Library of Science
Author(s): Jo Philips, Korneel Rabaey, Derek R. Lovley, Madeline Vargas, Shihui Yang.
The acetogen Clostridium ljungdahlii is capable of syngas fermentation and microbial electrosynthesis. Biofilm formation could benefit both these applications, but was not yet reported for C. ljungdahlii. Biofilm formation does not occur under standard growth conditions, but attachment or aggregation could be induced by different stresses. The strongest biofilm formation was observed with the addition of sodium chloride. After 3 days of incubation, the biomass volume attached to a plastic surface was 20 times higher with than without the addition of 200 mM NaCl to the medium. The addition of NaCl also resulted in biofilm formation on glass, graphite and glassy carbon, the latter two being often used electrode materials for microbial electrosynthesis. Biofilms were composed of extracellular proteins, polysaccharides, as well as DNA, while pilus-like appendages were observed with, but not without, the addition of NaCl. A transcriptome analysis comparing planktonic (no NaCl) and biofilm (NaCl addition) cells showed that C. ljungdahlii coped with the salt stress by the upregulation of the general stress response, Na+ export and osmoprotectant accumulation. A potential role for poly-N-acetylglucosamines and D-alanine in biofilm formation was found. Flagellar motility was downregulated, while putative type IV pili biosynthesis genes were not expressed. Moreover, the gene expression analysis suggested the involvement of the transcriptional regulators LexA, Spo0A and CcpA in stress response and biofilm formation. This study showed that NaCl addition might be a valuable strategy to induce biofilm formation by C. ljungdahlii, which can improve the efficacy of syngas fermentation and microbial electrosynthesis applications.
The acetogen Clostridium ljungdahlii is of high interest for industrial applications, because of its specific metabolic capacities. Firstly, C. ljungdahlii is capable of converting CO2/H2 and CO to acetate and ethanol . Mixtures of these gases are produced during steel production and the gasification of biomass. The fermentation of this syngas by acetogenic bacteria, such as C. ljungdahlii, allows the production of renewable chemicals and biofuels [2–4]. Secondly, C. ljungdahlii is capable of microbial electrosynthesis, i.e. the reduction of CO2 to acetate with electrons derived from an electrode . C. ljungdahlii potentially has a direct electron uptake mechanism , but it could also indirectly derive electrons from an electrode by consuming electrolytically generated H2 . Independent of the mechanism, microbial electrosynthesis is seen as a promising strategy to convert electrical energy into biofuels and other organic commodities [7–9]. Furthermore, a genetic system has recently been developed for C. ljungdahlii, enabling the engineering of strains towards higher-value end-products and increasing the economic feasibility of both syngas fermentations and microbial electrosynthesis [10–13].