<p>Biofilm as a living catalysts has been exploited for the production of biofuels and bioelectricity in microbial fuel cells (MFCs) as well as in the synthesis of bulk and fine chemicals. However, the structure and activity of biofilms are highly dynamic and heterogeneous, which makes the control of biofilm-mediated bioprocesses challenging and limits their application in industrial biotechnology. Efficient biofilm-mediated biocatalysis requires the modulation of biofilm formation. In a wide range of bacteria, the development of biofilm is subject to a signal cascade regulated by the intracellular levels of secondary messenger bis-(3'-5') cyclic dimeric guanosine monophosphate (c-di-GMP). Typically, an increase in c-di-GMP level enhances biofilm formation while a decrease in c-di-GMP level leads to biofilm dispersal. Intracellular levels of c-di-GMP are modulated in response to internal and environmental cues, which is achieved by the activities of diguanylate cyclases (DGCs, c-di-GMP synthases) and phosphodiesterases (PDEs, c-di-GMP hydrolases). Hence, biofilm formation can be modulated by controlling the expression of genes encoding DGCs or PDEs using chemical inducers.</p>
<p>For biofilm-mediated biocatalysis for chemicals synthesis, it would be desirable if chemical inducers could be avoided for modulating biofilm formation. Light as a non-invasive stimulus is such an attractive inducer, which can offer a higher spatiotemporal resolution in the regulation of biological activities. Several light-responsive regulatory modules have been reported for protein degradation, protein&#8211;protein interactions and gene expression. In particular, the near infrared (NIR) light (680&#8211;880 nm) is an attractive inducer for the photocontrol of biological activities because of its deep penetration and non-toxicity. In this study, we engineered a near infrared (NIR) light-responsive, tryptophan-producing <em>Escherichia coli</em> biofilm by introducing a NIR light-responsive DGC and the genes encoding tryptophan synthase (TrpAB). Biofilm formation of the engineered <em>E. coli</em> could be modulated by NIR light through manipulation of the intracellular c-di-GMP concentration and the biotransformation of indole and serine into tryptophan was catalyzed by TrpAB. The engineered catalytic biofilm overcame the inhibition of indole on biofilm formation by E. coli and reduced the reverse reaction from tryptophan to indole. By applying the engineered biofilm to catalyze the biotransformation of indole into tryptophan in submerged biofilm reactors, we showed that NIR light enhanced biofilm formation to result in 30% increase in tryptophan yield. This demonstrates the feasibility of applying light to modulate the catalytic biofilm formation and performance for chemical production in bioreactors. The optogenetic approach for modulating catalytic biofilm we have demonstrated here would allow the wide application for further biofilm-mediated biocatalysis.</p>
The Mechanism of Moxibustion: Ancient Theory and Modern Research