Introduction

         The demeanors of microbial community have been widely exploited by the scientific population for industrial development, ameliorative and therapeutic researches. In order to achieve these scientists greatly rely on genetic engineering, an exemplary tool to design microbial communities that suits the human needs. Such altered communities manifest improved functions that can be employed for a wide range of applications. These developments have exalted the community design approach which includes selective proliferation, excerption of suitable communities, combinatorial evaluation, and in silico modeling of microbiome. Combining the community design with computational approach will greatly advantage the development of synthetic community. The present reveiw herein discusses the role of these synthetic microbial community with respect to human needs, effective techniques in designing synthetic communities and also highlights the need for sophisticated techniques to overcome the problems involved in evaluating the designed communities.

Role of synthetic microbial communities

         Primary sectors such as health care, agriculture and industry are positively influenced by the microbial community under various aspects. Conspicuously, in the health sector the role of human microbiome in the human physiology, immunity, development and nutrition has gained much introspection. For instance, the human gut health can be ameliorated by means of microbially synthesized butyrate which contributes as a major energy source (Donohoe et al., 2011) and anti- inflammatory agent (Canani et al., 2011). The microbiome also plays a major role in the human immune system by its effect on lymphoid development and T cell differentiation (Hooper et al., 2012; Kabat et al., 2014). Whereas, in the industrial sector the microbiome endows its influence in waste water treatment (Pitt et al., 1981; Daims et al., 2001) and biofuels production (Abate et al., 1996; Mamma et al., 1996). In the agricultural sector ingression of nitrogen (Franche et al., 2009; Rovira, 1991) and phosphorus (Afzal, 2008) by the plants have been convalesced by the environmental microbiomes. All these multitude function renders microbial communities as an effective contrivance in the primary and health sectors with respect to microbiome-based technologies and therapeutics (Mimee et al., 2016; Marchesi et al., 2016). Specially engineered microbial communities can be effectively maneuvered for clinical and industrial applications. These engineered microbes can be developed by various techniques one of which includes manipulating environmental aspects to improve functional abilities. This approach has widely been used in fermentation technologies.

Designing synthetic microbial communities

         Previous reports in this field have proclaimed that substrate composition (Cui et al., 2009; Pholchan et al., 2010; Bassin et al., 2012), aeration (Simova et al., 2003), pH (Cui et al., 2009; Fang and Liu 2002) and temperature (Zoetemeyer et al., 1982) are the major environmental factors that can be manipulated to design microbial community that suits the human needs. Another approach to develop engineered microbes involves excerption of suitable communities by removing undesired communities. Most recently, antibiotics follow this approach to eliminate harmful pathogens from the human body and in contrary to this approach, probiotics exerts beneficial microbes to improve the human gut health (Walsh et al., 2014; Derrien et al., 2015). However, to unveil the propensity of the engineered communities more sophisticated techniques must be utilized to have complete control over the engineered microbes and this is a must, especially while using combinations of microbes. Moreover, complete control over the engineered organisms can effectively modulate the human microbiome and surpass the side effect accompanying the antibiotics (Bartlett et al., 2006; Theriot et al., 2014) and also overcome uncertainties in the probiotic engraftment (Zmora et al., 2018; Maldonado-Go et al., 2016).

A Combinatorial Approach

         These synthetic microbial communities can be highly effective while using combinatorial approach where merged microbial metabolism can improve the production and degradation of target compounds in an industrial setup (Bader et al., 2010). Overall, the synthetic microbial community caters a malleable and influential approach that bolster the ability of researchers to design the microbial community as a whole rather than focusing only on the existing communities. However, the major impediment underlying this approach includes selection of suitable microbial combinations that can effectively alter the targeted function. In order to completely exploit this rational design to the benefit of human needs, more sophisticated techniques must be employed and various researches are underway that underlines these needs.

Applications

         Various advantages evinced by the synthetic communities also include its potential to co-exist with communities that can never be achieved in the natural environment. Such combinatorial benefits enhance the metabolic activity there by leading to a better or entirely new function that cannot be achieved by naturally prevailing communities. This pragmatic approach is highly effective in biodiesel production and synthesis of bio active compounds (Bader et al., 2010). The enhanced metabolic activities of the combinatorial synthetic communities have been successfully exploited for the production of various resources that include hydrogen (Asada et al., 2006), acetic acid (Collet et al., 2005; Kondo and Kondo 1996; Talabardon et al., 2000) and lactic acid (Taniguchi et al., 2004; Roble et al., 2003). Degradation of polycyclic aromatic hydrocarbons (Boonchan et al., 2000) and cellulose (Haruta et al., 2002; Poszytek et al., 2016) has also been successfully carried out by this approach. Treatment of textile effluent has been made easy by this combinatorial approach whereby the researchers have combined three different species which can successfully degrade the textile effluents thereby improving the water quality (Ayed et al., 2010).

Techniques to overcome complications

         Increasing the number of species in combinatorial approach also allows exponential increase in number of complications which makes the combinatorial evaluation difficult. Fractional Factorial Design (FFD) is an effective technique that can overcome this problem and drastically reduce the number of combinatorial evaluations. FFD effectively evaluates impacts of external factors such as substrate composition (Prakasham et al., 2007; Skonieczny and Yargeau 2009; Jime´nez et al., 2014; Molina-Barahona et al., 2004), pH (Skonieczny and Yargeau 2009; Kikot et al., 2010), temperature (Prakasham et al., 2007) and heavy metal toxicity (Kikot et al., 2010) on the community functions. Apart from combinatorial evaluation, performance of individual species can also be evaluated using FFD or similar techniques. In a recent study, scientists have successfully evaluated the individual performance of every single community present in a random gut community subset (Faith et al., 2014). Though the study was based on random community subset, it is well evident from results that FFD can be effectively utilized in a similar manner on synthetic combinatorial communities. Effectual use of FFD in textile effluent treatment has been reported already (Chen et al., 2009; Chen et al., 2011). The individual and interactive inter species impacts on total organic carbon (TOC) degradation along with substrate utilization have been appraised in the above mentioned studies. These studies ushered the scientists to develop synthetic combinations to improve the degradation capacity. The intriguing results suggested that combination of less number of species (3 or 4) rendered the combination more effective than a baseline mixture (6 or more).

         On contrary, treating the microbial consortia on the whole as a single unit has also been proven effective. This technique is specifically used for consortium with proven emergent functions. The marine consortium used above-mentioned approach to develop synthetic combinatorial community that can effectively fix CO₂ in the marine environment (Hu et al., 2014; 91. Hu et al., 2016). This approach has also successfully improved the lignocelluloytic enzyme activity by designing a synthetic consortium from a preexisting consortium with cellulolytic activity (Poszytek et al., 2016). Apart from reducing the number of community members in the consortium this technique also allows to include new species in the consortium that cannot be isolated. These techniques allow the research community to effectively evaluate and improve the functional abilities of a pre-existing synthetic consortium without drastically altering the combinatorial evaluations.

Conclusion

         The fast growing population makes it mandatory to synthesize compounds of human interest rapidly. Exploiting synthetic communities seems to be the most efficient approach to effectively synthesize these compounds and these are considered advantageous over the natural communities due to their efficiency and rapidity. Combining two or more synthetic communities makes it easy to perform complicated synthesis in highly robust environmental fluctuations. However, the rate of success highly depends upon selection and evaluation of suitable communities that can be combined to improve the target functions. Hence, there arises a need to overcome the difficulties that accompanies the combinatorial approach. In order to overcome these complications development of sophisticated computational techniques have become the need of the hour. The present review underlines the importance of these techniques that can assist the research community to chose the suitable consortium and also have complete control over the selected communities thereby minimizing side effects and improve the efficiency of synthesis.

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