Bioenergy and Bioproducts through Bacterial Quorum Sensing
Authors: Jyotsana Prakasha,b, Shikha Koula,b, Subhasree Raya,b, Ravi Kumar a, Vipin Chandra Kaliaa,b
aMicrobial Biotechnology and Genomics, CSIR - Institute of Genomics and Integrative Biology (IGIB), Delhi University Campus, Mall Road, Delhi-110007.
bAcademy of Scientific & Innovative Research (AcSIR), 2, Rafi Marg, Anusandhan Bhawan, New Delhi- 110001.
Fermentation technology has made significant advancements in the field of biofuel production using microbial systems. The most attractive is the bio-waste to energy: bio-hydrogen (Bio-H2), bio-diesel, bio-ethanol, biomethanation, butanediol, etc. using diverse microbes (Porwal et al., 2008; Patel et al., 2015; Kumar et al., 2015; Kalia et al., 2016).
The bio-wastes as feed, however, are limited by the fact that they are accompanied by inherent bacteria as contaminants. In order to retain large number of bacteria of interest within the bioreactor, we need to immobilize them. Biofilms have the capacity to retain large number of bacteria and act as natural immobilizing agents. These biofilms can be formed by the bacteria only under specific conditions of high cell densities (Hema et al., 2015; Kaur et al., 2015). This cell density dependent phenomenon is known as quorum sensing (QS) (Kalia, 2013). These biofilms can be exploited as efficient and economical bacterial support systems for producing bioenergy from biowastes.
Bio-H2 has been recognised as the cleanest fuel of the future. A wide range of microbes have ability to produce H2 from different substrates including bio-wastes. This bio-waste to energy process has attracted the attention of a large number of researchers (Patel et al., 2013). H 2 yields have been found to be quite low and virtually stagnant in a narrow range of 0.3 to 3.8 mole/mole hexose sugars like glucose (Patel et al., 2012). Efforts to retain large bacterial population within the bioreactors have proven effective in enhancing H2 yields. An obviously effective approach has been to immobilize H2-producers on different support materials (Patel et al., 2010). However, a more innovative strategy can be the use of self-flocculating or biofilm forming bacteria. Within the bioreactor, exopolysaccharides (EPS) secreting microbes allow large population of bacteria to be entrapped within the mucilage, which are thus prevented from being washed away. Among a number of potential H2-producers, quite a few of them have an ability to express QS mediated biofilm formation, which include bacterial species belonging to Bacillus, Clostridium, Streptococcus, Sinorhizobium, Enterobacter, Klebsiella, Caldicellulosiruptor and Escherichia (Kalia and Purohit, 2008). Co-cultures of thermophillic bacteria, Caldicellulosiruptor species have been used for biofilm formation to enhance H2 production. The two bacteria together resulted in 2.5 times enhanced H2 yield and 5 times higher H2 productivity to the tune of 20 mmol/L/h at a dilution rate of >1.0h-1 in Up-flow anaerobic reactors, in comparison to individually employed cultures (Pawar et al., 2015).
Biodiesel and other Bioproducts:
Apart from H2, QS has also been reported to be of use in bioethanol and biodiesel production processes. A 50% enhancement in bioethanol production by Zymomonas mobilis was proposed to be under the influence of exogenously added QS signals (AI-2). Induction of biodiesel production process by algal cells was demonstrated by incorporating QS like mechanism into Escherichia coli. This E. coli system in symbiotic association with algae allowed the later to sense their high cell density, which triggered the inhibition of nitrogen fixation genes leading to nitrogen stress. This acted as a switch to induce biodiesel production in algae (Wyss, 2013). QS can be exploited for various other value added compounds such as acetic acid. Acetic acid metabolic pathway in Gluconacetobacter intermedius is under the control of GinI/R QS system (QSS). Blocking the QSS resulted in increased bacterial growth rate in ethanol rich medium, which consequently led to enhanced acetic acid production (Lida et al., 2008). In the work on engineering of the nitrogen flux to convert proteins to biofuels, it was shown that the deletion of QS genes luxS or lsrA resulted in increased isobutanol production. Butanediol is another value added product whose synthesis is controlled by AHLs produced by Aeromonas hydrophila. It prevents acidification of the media, which is sensed by bacteria. As it is likely to inhibit bacterial growth, it shifts its metabolism to butanediol production (Van Houdt et al., 2007).
Researchers are looking for novel energy sources through the use of microbial fuel and electrolysis cells (MFCs and MECs). There is a need to reduce the operational costs and make them more robust. One of the strategies is to target efficient biofilm production, the most important component of the MFC. Generation of bioelectricity by MFCs and H2 and biomethane by MECs require prevention of microbial dispersion from the biofilm. Thus, we need to induce QSS mediated biofilm formation for efficient performance of these fuel cells (Zhou et al., 2013). In MFCs, increase in microbial phenazines or pyocyanin synthesis, which act as electron shuttles has been reported to improve bioelectricity output. This has been achieved by modulation or making direct use of RhlI/R QS circuit in Pseudomonas . Over-production of phenazines in Pseudomonas resulted in ̴1.7 times more output of bioelectricity as compared to wild type strains (Dantas et al., 2013). An AND logic gated MFC has been constructed in Shewanella oneidensis MR-1, amtrA knockout mutant, in which electrons exchange pathway mediated by c-type cytochromes is blocked. The engineered MFC contained IPTG (Isopropyl β-D-1-thiogalactopyranoside), controlled Ptac promoter and Ptac controlledLuxR expression such that electricity output is generated only in the presence of both the signals (AHLs and IPTG) (Hu et al., 2015).
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About Author / Additional Info:
Researchers in Microbial Biotechnology and Genomics at CSIR-IGIB, Delhi.