Metabolomic profiling coupled with metabolic network reveals differences in Gluconacetobacter xylinus from static and agitated cultures

Hybrid Silver-Containing Materials Based on Various Forms of Bacterial Cellulose: Synthesis, Structure, and Biological Activity

Sustained interest in the use of renewable resources for the production of medical materials has stimulated research on bacterial cellulose (BC) and nanocomposites based on it. New Ag-containing nanocomposites were obtained by modifying various forms of BC with Ag nanoparticles prepared by metal-vapor synthesis (MVS). Bacterial cellulose was obtained in the form of films (BCF) and spherical BC beads (SBCB) by the Gluconacetobacter hansenii GH-1/2008 strain under static and dynamic conditions. The Ag nanoparticles synthesized in 2-propanol were incorporated into the polymer matrix using metal-containing organosol. MVS is based on the interaction of extremely reactive atomic metals formed by evaporation in vacuum at a pressure of 10^-2 Pa with organic substances during their co-condensation on the cooled walls of a reaction vessel. The composition, structure, and electronic state of the metal in the materials were characterized by transmission and scanning electron microscopy (TEM, SEM), powder X-ray diffraction (XRD), small-angle X-ray scattering (SAXS) and X-ray photoelectron spectroscopy (XPS). Since antimicrobial activity is largely determined by the surface composition, much attention was paid to studying its properties by XPS, a surface-sensitive method, at a sampling depth about 10 nm. C 1s and O 1s spectra were analyzed self-consistently. XPS C 1s spectra of the original and Ag-containing celluloses showed an increase in the intensity of the C-C/C-H groups in the latter, which are associated with carbon shell surrounding metal in Ag nanoparticles (Ag NPs). The size effect observed in Ag 3d spectra evidenced on a large proportion of silver nanoparticles with a size of less than 3 nm in the near-surface region. Ag NPs in the BC films and spherical beads were mainly in the zerovalent state. BC-based nanocomposites with Ag nanoparticles exhibited antimicrobial activity against Bacillus subtilis, Staphylococcus aureus, Escherichia coli bacteria and Candida albicans and Aspergillus niger fungi. It was found that AgNPs/SBCB nanocomposites are more active than Ag NPs/BCF samples, especially against Candida albicans and Aspergillus niger fungi. These results increase the possibility of their medical application.

Enhanced production of fibrous bacterial cellulose in Gluconacetobacter xylinus culture medium containing modified protein of okara waste

In Gluconacetobacter xylinus cultivation for bacterial nanocellulose production, agro-industrial wastes, soybean residual okara, okara extracted protein, and modified okara protein, were used as a protein source. In comparison with homogenized raw okara and protein extracted from raw okara, acetic-acid modified protein provided the higher cellulose yield (2.8 g/l at 3 %w/v protein concentration) due to the improved protein solubility in the culture medium (89 %) and smaller particle size (0.2 μm) leading to facile uptake by the bacteria. Importantly, pH of the culture medium containing the modified protein measured before and after the cultivation was similar, suggesting the buffering capacity of the protein. Nanocellulose fibers were then produced densely in the network of hydrogels with high crystallinity nearly 90 %. Based on the results, economic constraints around nanocellulose production could be alleviated by valorization of okara waste, which provided enhanced sustainability.

Organic Carbon Is Ineffective in Enhancing the Growth of Dunaliella

(1) Background: Mixotrophic growth is commonly associated with higher biomass productivity and lower energy consumption. This paper evaluates the impact of using different carbon sources on growth, protein profile, and nutrient uptake for Dunaliella tertiolecta CCAP 19/30 to assess the potential for mixotrophic growth. (2) Methods: Two experimental sets were conducted. The first assessed the contribution of atmospheric carbon to D. tertiolecta growth and the microalgae capacity to grow heterotrophically with an organic carbon source to provide both carbon and energy. The second set evaluated the impact of using different carbon sources on its growth, protein yield and quality. (3) Results: D. tertiolecta could not grow heterotrophically. Cell and optical density, ash-free dry weight, and essential amino acids index were inferior for all treatments using organic carbon compared to NaHCO3. Neither cell nor optical density presented significant differences among the treatments containing organic carbon, demonstrating that organic carbon does not boost D. tertiolecta growth. All the treatments presented similar nitrogen, phosphorus, sulfur recovery, and relative carbohydrate content. (4) Conclusions: Based on the results of this paper, D. tertiolecta CCAP 19/30 is an obligated autotroph that cannot grow mixotrophically using organic carbon.

The production of bacterial cellulose in Gluconacetobacter xylinus regulated by luxR overexpression of quorum sensing system

Quorum sensing is a mechanism that facilitates cell-to-cell communication. Through signal molecular density for signal recognition, which leads to the regulation of some physiological and biochemical functions. Gluconacetobacter xylinus CGMCC 2955, which produces bacterial cellulose (BC), synthesizes the LuxR protein belonging to the LuxI/LuxR type QS system. Here, a luxR overexpression vector was transformed into G. xylinus CGMCC 2955. The overexpression of luxR increased the yield of BC by 15.6% after 16 days static culture and reduced the cell density by 15.5% after 120-h-agitated culture. The glucose was used up by G. xylinus-pMV24-luxR at 72-h-agitated fermentation, which 12 h earlier than the wild-type (WT). The total N-acylhomoserine lactones (AHL) content of the luxR-overexpressing strain and the WT strain attained 1367.9 ± 57.86 mg/L and 842.9 ± 54.22 mg/L, respectively. The C₁₂-HSL and C₁₄-HSL contents of G. xylinus-pMV24-luxR were 202 ± 21.66 mg/L and 409.6 ± 0.91 mg/L, which were significantly lower than that of WT. In contrast, C₆-HSL showed opposite results. The difference of AHL content proved that overexpression of luxR improved the binding of AHL and showed preference for some specific AHL. The metabolic results demonstrated that upon glucose exhaustion, the consumption of gluconic acid was promoted by luxR overexpression, and the content of D- ( +)-trehalose, an antiretrograde metabolite, increased significantly. KEY POINTS: • The overexpression of luxR increased the yield of bacterial cellulose • The content of signal molecules was significantly different • Differential metabolites were involved in multiple metabolic pathways.

Metabolomic Analysis of Biosynthesis Mechanism of ε-Polylysine Produced by Streptomyces diastatochromogenes

ε-Polylysine (ε-PL), a natural preservative with broad-spectrum antimicrobial activity, has been widely used as a green food additive, and it is now mainly produced by Streptomyces in industry. In the previous study, strain 6#-7 of high-yield ε-PL was obtained from the original strain TUST by mutagenesis. However, the biosynthesis mechanism of ε-PL in 6#-7 is still unclear. In this study, the metabolomic analyses of the biosynthesis mechanism of ε-PL in both strains are investigated. Results show that the difference in metabolisms between TUST and 6#-7 is significant. Based on the results of both metabolomic and enzymatic activities, a metabolic regulation mechanism of the high-yield strain is revealed. The transport and absorption capacity for glucose of 6#-7 is improved. The enzymatic activity benefits ε-PL synthesis, such as pyruvate kinase and aspartokinase, is strengthened. On the contrary, the activity of homoserine dehydrogenase in the branched-chain pathways is decreased. Meanwhile, the increase of trehalose, glutamic acid, etc. makes 6#-7 more resistant to ε-PL. Thus, the ability of the mutagenized strain 6#-7 to synthesize ε-PL is enhanced, and it can produce more ε-PLs compared with the original strain. For the first time, the metabolomic analysis of the biosynthesis mechanism of ε-PL in the high-yield strain 6#-7 is investigated, and a possible mechanism is then revealed. These findings provide a theoretical basis for further improving the production of ε-PL.

Nanocelluloses: Sources, Pretreatment, Isolations, Modification, and Its Application as the Drug Carriers

The 'Back-to-nature' concept has currently been adopted intensively in various industries, especially the pharmaceutical industry. In the past few decades, the overuse of synthetic chemicals has caused severe damage to the environment and ecosystem. One class of natural materials developed to substitute artificial chemicals in the pharmaceutical industries is the natural polymers, including cellulose and its derivatives. The development of nanocelluloses as nanocarriers in drug delivery systems has reached an advanced stage. Cellulose nanofiber (CNF), nanocrystal cellulose (NCC), and bacterial nanocellulose (BC) are the most common nanocellulose used as nanocarriers in drug delivery systems. Modification and functionalization using various processes and chemicals have been carried out to increase the adsorption and drug delivery performance of nanocellulose. Nanocellulose may be attached to the drug by physical interaction or chemical functionalization for covalent drug binding. Current development of nanocarrier formulations such as surfactant nanocellulose, ultra-lightweight porous materials, hydrogel, polyelectrolytes, and inorganic hybridizations has advanced to enable the construction of stimuli-responsive and specific recognition characteristics. Thus, an opportunity has emerged to develop a new generation of nanocellulose-based carriers that can modulate the drug conveyance for diverse drug characteristics. This review provides insights into selecting appropriate nanocellulose-based hybrid materials and the available modification routes to achieve satisfactory carrier performance and briefly discusses the essential criteria to achieve high-quality nanocellulose.

The role of genetic manipulation and in situ modifications on production of bacterial nanocellulose: A review

Natural polysaccharides are well-known biomaterials because of their availability and low-cost, with applications in diverse fields. Cellulose, a renowned polysaccharide, can be obtained from different sources including plants, algae, and bacteria, but recently much attention has been paid to the microorganisms due to their potential of producing renewable compounds. In this regard, bacterial nanocellulose (BNC) is a novel type of nanocellulose material that is commercially synthesized mainly by Komagataeibacter spp. Characteristics such as purity, porosity, and remarkable mechanical properties made BNC a superior green biopolymer with applications in pharmacology, biomedicine, bioprocessing, and food. Genetic manipulation of BNC-producing strains and in situ modifications of the culturing conditions can lead to BNC with enhanced yield/productivity and properties. This review mainly highlights the role of genetic engineering of Komagataeibacter strains and co-culturing of bacterial strains with additives such as microorganisms and nanomaterials to synthesize BNC with improved functionality and productivity rate.

Design, construction and optimization a flexible bench-scale rotating biological contactor (RBC) for enhanced production of bacterial cellulose by Acetobacter Xylinium

In this research a bench scale rotating biological contactor (RBC) was designed and constructed to produce BC. The effects of variables including rotation speed of the disk, distance between disks, disk type and external aeration on BC productivity were investigated. Results showed that the highest weight of BC produced on the surface of integrated polyethylene discs which rotated at 13 rpm. It was also found that the highest amount of BC was obtained when the space between two adjacent discs was adjusted to 1 cm and the disk number was 16. An aquarium pump was used to investigate the impact of aeration on RBC made of 12 integrated polyethylene discs and operated at optimal rotation speed of 13 rpm. Disk spacing distance was adjusted to 1.5 cm to consider the possible increasing of the thickness of BC film by aeration. Wet weight and dry weight of BC resulted from aerated fermentation increased more than 64 and 47%, respectively as compared to non-aerated RBC. In comparison with static culture, wet weight and dry weight of BC produced in aerated RBC fermentation increased more than 90.7 and 71%, respectively. Nanoscale structure of produced bacterial cellulose was confirmed by SEM analysis.

A Lambda Red and FLP/FRT-Mediated Site-Specific Recombination System in Komagataeibacter xylinus and Its Application to Enhance the Productivity of Bacterial Cellulose

Komagataeibacter xylinus has received increasing attention as an important microorganism for the conversion of several carbon sources to bacterial cellulose (BC). However, BC productivity has been impeded by the lack of efficient genetic engineering techniques. In this study, a lambda Red and FLP/FRT-mediated site-specific recombination system was successfully established in Komagataeibacter xylinus. Using this system, the membrane bound gene gcd, a gene that encodes glucose dehydrogenase, was knocked out to reduce the modification of glucose to gluconic acid. The engineered strain could not produce any gluconic acid and presented a decreased bacterial cellulose (BC) production due to its restricted glucose utilization. To address this problem, the gene of glucose facilitator protein (glf; ZMO0366) was introduced into the knockout strain coupled with the overexpression of the endogenous glucokinase gene (glk). The BC yield of the resultant strain increased by 63.63-173.68%, thus reducing the production cost.

A novel strategy to alleviate medium acidosis for simultaneously yielding more bacterial cellulose and electricity

Bacterial cellulose (BC), a fascinating and renewable polymer, can be applied widely in various bio-based materials. However, its synthesis is generally limited by medium acidosis. Herein, we demonstrated a built-in galvanic cell within the BC fermentation medium to alleviate the acidosis, by which BC yield was promoted by 191%, and simultaneously the yield of electrical power of 0.68 W to 8.10 W during the incubation.

Towards control of cellulose biosynthesis by Komagataeibacter using systems-level and strain engineering strategies: current progress and perspectives

The strains of the Komagataeibacter genus have been shown to be the most efficient bacterial nanocellulose producers. Although exploited for many decades, the studies of these species focused mainly on the optimisation of cellulose synthesis process through modification of culturing conditions in the industrially relevant settings. Molecular physiology of Komagataeibacter was poorly understood and only a few studies explored genetic engineering as a strategy for strain improvement. Only since recently the systemic information of the Komagataeibacter species has been accumulating in the form of omics datasets representing sequenced genomes, transcriptomes, proteomes and metabolomes. Genetic analyses of the mutants generated in the untargeted strain modification studies have drawn attention to other important proteins, beyond those of the core catalytic machinery of the cellulose synthase complex. Recently, modern molecular and synthetic biology tools have been developed which showed the potential for improving targeted strain engineering. Taking the advantage of the gathered knowledge should allow for better understanding of the genotype–phenotype relationship which is necessary for robust modelling of metabolism as well as selection and testing of new molecular engineering targets. In this review, we discuss the current progress in the area of Komagataeibacter systems biology and its impact on the research aimed at scaled-up cellulose synthesis as well as BNC functionalisation.

Key points • The accumulated omics datasets advanced the systemic understanding of Komagataeibacter physiology at the molecular level. • Untargeted and targeted strain modification approaches have been applied to improve nanocellulose yield and properties. • The development of modern molecular and synthetic biology tools presents a potential for enhancing targeted strain engineering. • The accumulating omic information should improve modelling of Komagataeibacter’s metabolism as well as selection and testing of new molecular engineering targets.

Physiological and Metabolic Responses of a Novel Dunaliella salina Strain to Myo-inositol1

Dunaliella salina is well known for its ability to accumulate large amounts of β-carotene. Myo-inositol (MI) enhances the biomass production of D. salina, but the underlying mechanisms were unclear. The present study showed that the concentration of exogenous MI decreased gradually and reached a constant level at the 4th day of cultivation. MI enhanced the contents of total colored carotenoids and the activity of photosystem II. Metabolic profiles were significantly changed after the addition of exogenous MI, as revealed by multivariate statistical analysis. The metabolites could be categorized into four groups based on the relative levels in different samples. Exogenous MI increased the levels of most detected sugars, amino acids, and total saturated and unsaturated fatty acids. Based on the physiological and metabolic analyses, a hypothetical growth-promoting model that MI promotes the growth of D. salina TG by increasing the levels of key metabolites and possibly enhancing photosynthesis, was proposed. This study provides valuable information for understanding the growth-promoting mechanisms of MI in D. salina from the metabolic perspective.

Tailoring bacterial cellulose structure through CRISPR interference-mediated downregulation of galU in Komagataeibacter xylinus CGMCC 2955

Diverse applications of bacterial cellulose (BC) have different requirements in terms of its structural characteristics. culturing Komagataeibacter xylinus CGMCC 2955, BC structure changes with alterations in oxygen tension. Here, the K. xylinus CGMCC 2955 transcriptome was analyzed under different oxygen tensions. Transcriptome and genome analysis indicated that BC structure is related to the rate of BC synthesis and cell growth, and galU is an essential gene that controls the carbon metabolic flux between the BC synthesis pathway and the pentose phosphate (PP) pathway. The CRISPR interference (CRISPRi) system was utilized in K. xylinus CGMCC 2955 to control the expression levels of galU. By overexpressing galU and interfering with different sites of galU sequences using CRISPRi, we obtained strains with varying expression levels of galU (3.20-3014.84%). By testing the characteristics of BC, we found that the porosity of BC (range: 62.99-90.66%) was negative with galU expression levels. However, the crystallinity of BC (range: 56.25-85.99%) was positive with galU expression levels; galU expression levels in engineered strains were lower than those in the control strains. Herein, we propose a new method for regulating the structure of BC to provide a theoretical basis for its application in different fields.

Reconstruction, verification and in-silico analysis of a genome-scale metabolic model of bacterial cellulose producing Komagataeibacter xylinus

In this study, a comprehensive genome-scale metabolic network of Komagataeibacter xylinus as the model microorganism was reconstructed based on genome annotation, for better understanding of metabolic growth and biosynthesis of bacterial cellulose (BC). The reconstructed network included 640 genes, 783 metabolic reactions and 865 metabolites. The model was completely successful to predict the lack of growth under anaerobic conditions. Model validation by the data for the growth of acetic acid bacteria with ethanol-limited chemostat cultures showed that there is a good agreement for the O₂ and CO₂ fluxes with actual growth conditions. Then the model was used to forecast the simultaneous production of BC and by-products. The obtained data showed that the rate of BC production is consistent with experimental data with an accuracy of 93.7%. Finally, the study of flux balance analysis (FBA) data showed that the pentose phosphate pathway and the TCA cycle play an important role in growth-promoting metabolism in K. xylinus and have a close relationship with BC biosynthesis. By integrating this model with various metabolic engineering and systems biology tools in the future, it is possible to overcome the common challenges in the large-scale BC production, such as low yield and productivity.

Current progress on the production, modification, and applications of bacterial cellulose

Adoption of biomass for the development of biobased products has become a routine agenda in evolutionary metabolic engineering. Cellulose produced by bacteria is a "rising star" for this sustainable development. Unlike plant cellulose, bacterial cellulose (BC) shows several unique properties like a high degree of crystallinity, high purity, high water retention, high mechanical strength, and enhanced biocompatibility. Favored with those extraordinary properties, BC could serve as ideal biomass for the development of various industrial products. However, a low yield and the requirement for large growth media have been a persistent challenge in mass production of BC. A significant number of techniques has been developed in achieving efficient BC production. This includes the modification of bioreactors, fermentation parameters, and growth media. In this article, we summarize progress in metabolic engineering in order to solve BC growth limitation. This article emphasizes current engineered BC production by using various bioreactors, as well as highlighting the structure of BC fermented by different types of engineered-bioreactors. The comprehensive overview of the future applications of BC, aims to provide readers with insight into new economic opportunities of BC and their modifiable properties for various industrial applications. Modifications in chemical composition, structure, and genetic regulation, which preceded the advancement of BC applications, were also emphasized.

Complete genome analysis of Gluconacetobacter xylinus CGMCC 2955 for elucidating bacterial cellulose biosynthesis and metabolic regulation

Complete genome sequence of Gluconacetobacter xylinus CGMCC 2955 for fine control of bacterial cellulose (BC) synthesis is presented here. The genome, at 3,563,314 bp, was found to contain 3,193 predicted genes without gaps. There are four BC synthase operons (bcs), among which only bcsI is structurally complete, comprising bcsA, bcsB, bcsC, and bcsD. Genes encoding key enzymes in glycolytic, pentose phosphate, and BC biosynthetic pathways and in the tricarboxylic acid cycle were identified. G. xylinus CGMCC 2955 has a complete glycolytic pathway because sequence data analysis revealed that this strain possesses a phosphofructokinase (pfk)-encoding gene, which is absent in most BC-producing strains. Furthermore, combined with our previous results, the data on metabolism of various carbon sources (monosaccharide, ethanol, and acetate) and their regulatory mechanism of action on BC production were explained. Regulation of BC synthase (Bcs) is another effective method for precise control of BC biosynthesis, and cyclic diguanylate (c-di-GMP) is the key activator of BcsA–BcsB subunit of Bcs. The quorum sensing (QS) system was found to positively regulate phosphodiesterase, which decomposed c-di-GMP. Thus, in this study, we demonstrated the presence of QS in G. xylinus CGMCC 2955 and proposed a possible regulatory mechanism of QS action on BC production.

Enhanced bacterial cellulose production by Gluconacetobacter xylinus via expression of Vitreoscilla hemoglobin and oxygen tension regulation

Oxygen plays a key role during bacterial cellulose (BC) biosynthesis by Gluconacetobacter xylinus. In this study, the Vitreoscilla hemoglobin (VHb)-encoding gene vgb, which has been widely applied to improve cell survival during hypoxia, was heterologously expressed in G. xylinus via the pBla-VHb-122 plasmid. G. xylinus and G. xylinus-vgb ⁺ were statically cultured under hypoxic (10 and 15% oxygen tension in the gaseous phase), atmospheric (21%), and oxygen-enriched conditions (40 and 80%) to investigate the effect of oxygen on cell growth and BC production. Irrespective of vgb expression, we found that cell density increased with oxygen tension (10-80%) during the exponential growth phase but plateaued to the same value in the stationary phase. In contrast, BC production was found to significantly increase at lower oxygen tensions. In addition, we found that BC production at oxygen tensions of 10 and 15% was 26.5 and 58.6% higher, respectively, in G. xylinus-vgb ⁺ than that in G. xylinus. The maximum BC yield and glucose conversion rate, of 4.3 g/L and 184.7 mg/g, respectively, were observed in G. xylinus-vgb ⁺ at an oxygen tension of 15%. Finally, BC characterization suggested that hypoxic conditions enhance BC's mass density, Young's modulus, and thermostability, with G. xylinus-vgb ⁺ synthesizing softer BC than G. xylinus under hypoxia as a result of a decreased Young's modulus. These results will facilitate the use of static culture for the production of BC.

Reconstruction of a Genome-scale Metabolic Network of Komagataeibacter nataicola RZS01 for Cellulose Production

Bacterial cellulose (BC) is widely used in industries owing to its high purity and strength. Although Komagataeibacter nataicola is a representative species for BC production, its intracellular metabolism leading to BC secretion is unclear. In the present study, a genome-scale metabolic network of cellulose-producing K. nataicola strain RZS01 was reconstructed to understand its metabolic behavior. This model iHZ771 comprised 771 genes, 2035 metabolites, and 2014 reactions. Constraint-based analysis was used to characterize and evaluate the critical intracellular pathways. The analysis revealed that a total of 71 and 30 genes are necessary for cellular growth in a minimal medium and complex medium, respectively. Glycerol was identified as the optimal carbon source for the highest BC production. The minimization of metabolic adjustment algorithm identified 8 genes as potential targets for over-production of BC. Overall, model iHZ771 proved to be a useful platform for understanding the physiology and BC production of K. nataicola.

Complete genome sequence of the cellulose-producing strain Komagataeibacter nataicola RZS01

Komagataeibacter nataicola is an acetic acid bacterium (AAB) that can produce abundant bacterial cellulose and tolerate high concentrations of acetic acid. To globally understand its fermentation characteristics, we present a high-quality complete genome sequence of K. nataicola RZS01. The genome consists of a 3,485,191-bp chromosome and 6 plasmids, which encode 3,514 proteins and bear three cellulose synthase operons. Phylogenetic analysis at the genome level provides convincing evidence of the evolutionary position of K. nataicola with respect to related taxa. Genomic comparisons with other AAB revealed that RZS01 shares 36.1%~75.1% of sequence similarity with other AAB. The sequence data was also used for metabolic analysis of biotechnological substrates. Analysis of the resistance to acetic acid at the genomic level indicated a synergistic mechanism responsible for acetic acid tolerance. The genomic data provide a viable platform that can be used to understand and manipulate the phenotype of K. nataicola RZS01 to further improve bacterial cellulose production.

Metabolic Investigation in Gluconacetobacter xylinus and Its Bacterial Cellulose Production under a Direct Current Electric Field

The effects of a direct current (DC) electric field on the growth and metabolism of Gluconacetobacter xylinus were investigated in static culture. When a DC electric field at 10 mA was applied using platinum electrodes to the culture broth, bacterial cellulose (BC) production was promoted in 12 h but was inhibited in the last 12 h as compared to the control (without DC electric field). At the cathode, the presence of the hydrogen generated a strong reductive environment that is beneficial to cell growth. As compared to the control, the activities of glycolysis and tricarboxylic acid cycle, as well as BC productivity were observed to be slightly higher in the first 12 h. However, due to the absence of sufficient oxygen, lactic acid was accumulated from pyruvic acid at 18 h, which was not in favor of BC production. At the anode, DC inhibited cell growth in 6 h when compared to the control. The metabolic activity in G. xylinus was inhibited through the suppression of the tricarboxylic acid cycle and glycolysis. At 18-24 h, cell density was observed to decrease, which might be due to the electrolysis of water that significantly dropped the pH of cultural broth far beyond the optimal range. Meanwhile, metabolites for self-protection were accumulated, for instance proline, glutamic acid, gluconic acid, and fatty acids. Notably, the accumulation of gluconic acid and lactic acid made it a really tough acid stress to cells at the anode and finally led to depression of cell growth.