Bioscience Evidence 2024, Vol.14, No.5, 238-249 http://bioscipublisher.com/index.php/be 241 been made to reduce ethanol byproducts and increase the yield of other valuable chemicals, such as 2,3-butanediol, by manipulating the carbon flux at key metabolic branching points (Ishii et al., 2018). By understanding these biochemical pathways and the role of specific enzymes, researchers can optimize ethanol fermentation processes for industrial applications, improving efficiency and yield (Straub et al., 2020; Wang et al., 2020; Alharbi et al., 2022). 4 Microbial Organisms in Ethanol Fermentation 4.1 Yeasts: saccharomyces cerevisiae and others Yeasts, particularly Saccharomyces cerevisiae, are the most commonly used microorganisms in ethanol fermentation due to their high ethanol tolerance and efficient fermentation capabilities. S. cerevisiae has been extensively studied and genetically modified to enhance its ethanol production, especially in the context of consolidated bioprocessing (CBP) where it is engineered to express cellulases for direct fermentation of cellulose to ethanol. Additionally, other yeast species have been explored for their potential in ethanol production, often focusing on improving their fermentation efficiency and stress tolerance through genetic modifications (Liu et al., 2018). 4.2 Bacteria: zymomonas mobilis and others Zymomonas mobilis is a highly efficient ethanologenic bacterium that has garnered significant attention for its potential in industrial ethanol production. Unlike S. cerevisiae, Z. mobilis metabolizes sugars through the Entner-Doudoroff pathway, which results in less ATP and biomass production, thereby channeling more sugar towards ethanol production. This bacterium can be engineered for pentose metabolism without cofactor imbalance, making it a robust candidate for cellulosic ethanol production (Xia et al., 2019). Z. mobilis also exhibits several advantageous traits such as high ethanol yield, rapid fermentation rates, and the ability to tolerate high concentrations of ethanol and other inhibitory compounds (Todhanakasem et al., 2020; Fuchino et al., 2021). Other bacteria, such as Lactobacillus amylovorus, have been shown to improve ethanol yields in mixed cultures with yeasts by cross-feeding metabolites like acetaldehyde (Lino et al., 2020). 4.3 Genetic modifications and strain improvement Genetic engineering plays a crucial role in enhancing the ethanol production capabilities of both yeasts and bacteria. For instance, S. cerevisiae has been genetically modified to express recombinant cellulases, enabling it to directly ferment cellulose to ethanol (Liu et al., 2018). Similarly, Z. mobilis has been subjected to various genetic modifications to improve its ethanol yield, stress tolerance, and substrate utilization. Advanced genetic tools, including mutation techniques, genome editing, and metabolic engineering, have been employed to optimize Z. mobilis strains for industrial applications (Todhanakasem et al., 2020). Additionally, the availability of genome sequence information for multiple Z. mobilis strains has facilitated targeted genetic modifications (Xia et al., 2019). 4.4 Comparison of different microbial systems When comparing different microbial systems for ethanol fermentation, several factors need to be considered, including ethanol yield, fermentation rate, substrate range, and tolerance to inhibitory compounds. S. cerevisiae is well-known for its robustness and high ethanol tolerance, making it a preferred choice for many industrial applications. However, Z. mobilis offers several advantages, such as higher ethanol yields and faster fermentation rates due to its unique metabolic pathways (Xia et al., 2019; Todhanakasem et al., 2020). Moreover, Z. mobilis produces fewer by-products and has a lower biomass yield, which can be beneficial for industrial processes (Szambelan et al., 2023). The choice of microbial system often depends on the specific requirements of the fermentation process, including the type of feedstock and the desired product characteristics. Genetic modifications and strain improvements continue to enhance the performance of both yeasts and bacteria, making them increasingly competitive for various industrial applications (Liu et al., 2018; Li et al., 2020a; Todhanakasem et al., 2020).
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