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Metabolic Engineering of Yeast, Zymomonas mobilis, and Clostridium thermocellum to Increase Yield of Bioethanol
Published in Ayerim Y. Hernández Almanza, Nagamani Balagurusamy, Héctor Ruiz Leza, Cristóbal N. Aguilar, Bioethanol, 2023
S. Sánchez-Muñoz, M. J. Castro-Alonso, F. G. Barbosa, E. Mier-Alba, T. R. Balbino, D. Rubio-Ribeaux, I. O. Hernández-De Lira, J. C. Santos, C. N. Aguilar, S. S. Da Silva
As well documented, high concentration of ethanol can also influence the growth rate and cause inhibition of intracellular metabolites, damage of peptidoglycan cell wall, and consequently reducing the potential of pumping protons across membrane [183, 184]. Regarding this problem, Carreón-Rodríguez et al. [185] obtained and characterized two ethanol-tolerant mutant strains, through the cultivation of Z. mobilis ZM4 in medium by increasing ethanol concentrations in consecutive steps. They observed that overexpression of the spoT/relA gene increased the synthesis of (p) ppGpp alarmone, that influenced the increase expression of genes related to amino acid synthesis mutation. Despite the improvement in ethanol tolerance, most of the ethanol-tolerant mutant strains did not increase ethanol production. However, Tan et al. [186] obtains ethanol-tolerant mutants and enhance ethanol production yield by utilizing random mutagenesis of the sigma factor RpoD protein (σ70).
Interconnection between PHA and Stress Robustness of Bacteria
Published in Martin Koller, The Handbook of Polyhydroxyalkanoates, 2020
Stanislav Obruca, Petr Sedlacek, Iva Pernicova, Adriana Kovalcik, Ivana Novackova, Eva Slaninova, Ivana Marova
Lower sensitivity of several PHA-positive strains against cold environments has recently been heavily reported. Recent study of the behavior of bacteria producing PHA and their PHA deficient mutants showed higher capability of bacteria with PHA inclusions to endure unfavorable low temperatures or even freezing when compared to PHA – deficient mutant strain [26]. The explanation can be the relationship between the PHA metabolism and ppGpp briefly described in the part dedicated to heat shock. It was found that the presence of PHA in bacteria may be related to the synthesis of nucleotides, including ppGpp and alarmone which activates the expression of the rpoS gene encoding for alternative sigma factor of RNA polymerase. RpoS mediates the formation of protective proteins, and thus increases the resistance of bacteria against various stresses including low temperatures [27,47]. It was observed that PHA as carbon storage materials alleviates the oxidative stress induced in bacteria by cold environments. Principally, the PHA metabolism is a dynamic process which combines simultaneous biosynthesis and degradation. Hence, it was experimentally confirmed that after cold shock PHA is degraded and the NADH/NAD+ ratio is maintained to keep the redox state in balance and to adapt bacteria to cold environments [3]. The investigation of some Antarctic strains (e.g. Pseudomonas sp. 14-3 also classified as Pseudomonas extremaustralis DSM 17835T) showed that the PHA synthesis and degradation significantly contribute to the increased motility and survival of these bacteria under icy conditions and high pressure [3,48]. Also, in another psychrophilic bacterium Sphingopyxis chilensis PHA plays a crucial role when cells are exposed to freezing [49]. Nevertheless, cold protective efficiency of PHA against low temperatures has also been detected in the case of mesophiles such as model strain of PHA metabolism – Cupriavidus necator (formerly Ralstonia eutropha, Wautersia eutropha and Alcaligenes eutrophus). Nowroth et al. reported that PHA reservoirs in C. necator wild-type strain protected cells from entering the viable but not cultivable physiological state when exposed to 5°C since the PHA-deficient strain was much more sensitive to the exposition to low temperature [4]. Furthermore, the survival of C. necator was also studied when exposed to sub-zero temperatures. Again, it was observed that wild-type strains of C. necator revealed higher tolerance to repeated freezing-thawing cycles than the PHA-deficient mutant. This might be a consequence of higher intracellular concentration of PHA monomers in wild-type strains since 3HB demonstrated strong cryoprotective properties. In addition, it was demonstrated that the PHA granules maintain high flexibility even under extremely low temperatures, which suggests that the PHA granules might protect bacterial cells against injury from extracellular ice. Further, the presence of PHA granules modifies the adhesive forces between water and cellular components and thus protects bacterial cells at low temperatures and inhibits the formation of cytoplasmatic ice [5].
Formation mechanisms of viable but nonculturable bacteria through induction by light-based disinfection and their antibiotic resistance gene transfer risk: A review
Published in Critical Reviews in Environmental Science and Technology, 2021
Yiwei Cai, Jianying Liu, Guiying Li, Po Keung Wong, Taicheng An
Formation mechanisms of VBNC bacteria are still the focus of current research. The stringent response mechanism occurs after bacteria or plants receive an amino acid starvation signal mediated by the alarmone, guanosine tetraphosphate and guanosine pentaphosphate (collectively referred to as (p)ppGpp). There is evidence that E. coli mutants that cannot produce ppGpp are less likely to be induced into VBNC bacteria, while bacteria that overproduce ppGpp are more likely to be induced to the VBNC state (Boaretti et al., 2003). This means that the stringent response plays an important role in the formation of VBNC bacteria (Figure 2). VBNC V. cholerae O1, was found to have highly up-regulated expression of the relA gene, which is involved in the stringent response signaling pathway. RelA catalyzes the synthesis of ppGpp and causes it to accumulate in the cell, which in turn affects the synthesis of DNA, RNA and proteins, leading to growth arrest (Mishra et al., 2012). The general stress response system, mainly controlled by RNA polymerase σS (RpoS) and LysR transcription regulator (OxyR), also plays an important role in the induction and formation of VBNC bacteria (Boaretti et al., 2003; Liao et al., 2019). The rpoS gene is important for activating the formation of VBNC bacteria and acts as the main signal to regulate the stress response factor. RpoS endows bacteria with tolerance to a variety of environment stressors, while OxyR is mainly involved in the process that allows bacteria to deal with oxidative stress. Evidence suggests that OxyR is an important regulatory protein, and its absence causes the VBNC state in many bacteria such as S. typhimurium (Liao et al., 2019). The transcription and translation of rpoS is regulated by ppGpp, and downstream rpoS further encodes the sigma factor (Figure 2) (Boaretti et al., 2003).