Morphological and enzymatic responses of a recombinant Aspergillus niger to oxidative stressors in chemostat cultures

The phenomenon of strain degeneration in biotechnologically relevant fungi

Fungi are widely exploited for large-scale production in the biotechnological industry to produce a diverse range of substances due to their versatility and relative ease of growing on various substrates. The occurrence of a phenomenon-the so-called fungal strain degeneration-leads to the spontaneous loss or decline of production capacity and results in an economic loss on a tremendous scale. Some of the most commonly applied genera of fungi in the biotechnical industry, such as Aspergillus, Trichoderma, and Penicillium, are threatened by this phenomenon. Although fungal degeneration has been known for almost a century, the phenomenon and its underlying mechanisms still need to be understood. The proposed mechanisms causing fungi to degenerate can be of genetic or epigenetic origin. Other factors, such as culture conditions, stress, or aging, were also reported to have an influence. This mini-review addresses the topic of fungal degeneration by describing examples of productivity losses in biotechnical processes using Aspergillus niger, Aspergillus oryzae, Trichoderma reesei, and Penicillium chrysogenum. Further, potential reasons, circumvention, and prevention methods are discussed. This is the first mini-review which provides a comprehensive overview on this phenomenon in biotechnologically used fungi, and it also includes a collection of strategies that can be useful to minimize economic losses which can arise from strain degeneration. KEY POINTS: • Spontaneous loss of productivity is evident in many fungi used in biotechnology. • The properties and mechanisms underlying this phenomenon are very versatile. • Only studying these underlying mechanisms enables the design of a tailored solution.

Physiological analysis of the improved ε-polylysine production induced by reactive oxygen species

INTRODUCTION

Epsilon-poly-L-lysine (ε-PL) is produced by Streptomyces species in acidic and aerobic conditions, which inevitably induces rapid generation of reactive oxygen species (ROS). The devastating effects of ROS on biomolecules and cell vitality have been well-studied, while the positive effects of ROS are rarely reported.

RESULTS

In this study, we found that a proper dose of intracellular ROS (about 3.3 μmol H₂O₂ /g DCW) could induce a physiological modification to promote the ε-PL production (from 1.2 to 1.5 g/L). It resulted in larger sizes of colony and mycelial pellets as well as vibrant, aggregated, and more robust mycelia, which were of high capability of ROS detoxication. Physiological studies showed that appropriate doses of ROS activated the metabolism of the pentose phosphate pathway at both transcriptional and enzymatic levels, which was beneficial for biomass accumulation. The biosynthesis of lysine was also promoted in terms of transcriptional regulatory overexpression, increased transcription and enzymatic activity of key genes, larger pools of metabolites in the TCA cycle, replenishment pathway, and diaminoheptanedioic acid pathway. In addition, energy provision was ensured by activated metabolism of the TCA cycle, a larger pool of NADH, and higher activity of the electron transport system. Increased transcription of HrdD and pls further accelerated the ε-PL biosynthesis.

SIGNIFICANCE

These results indicated that ROS at proper intracellular dose could act as an inducing signal to activate the ε-PL biosynthesis, which laid a foundation for further process regulation to maintain optimal ROS dose in industrial ε-PL production and was of theoretical and practical significance.

KEY POINTS

• A proper dose of intracellular ROS positively influences the ε-PL production. • Proper dose of ROS enhanced the mycelial activity and antioxidative capability. • ROS increased lysine synthesis metabolism, energy provision and pls expression.

Proteomic analysis on Aspergillus strains that are useful for industrial enzyme production

A simple intracellular proteomic study was conducted to investigate the biological activities of Aspergillus niger during industrial enzyme production. A strain actively secreting a heterologous enzyme was compared to a reference strain. In total, 1824 spots on 2-D gels were analyzed using MALDI-TOF MS, yielding 343 proteins. The elevated levels of UPR components, BipA, PDI, and calnexin, and proteins related to ERAD and ROS reduction, were observed in the enzyme-producer. The results suggest the occurrence of these responses in the enzyme-producers. Major glycolytic enzymes, Fba1, EnoA, and GpdA, were abundant but at a reduced level relative to the reference, indicating a potential repression of the glycolytic pathway. Interestingly, it was observed that a portion of over-expressed heterologous enzyme accumulated inside the cells and digested during fermentation, suggesting the secretion capacity of the strain was not enough for completing secretion. Newly identified conserved-proteins, likely in signal transduction, and other proteins were also investigated. Abbreviations: 2-D: two-dimensional; UPR: unfolded protein response; ER: endoplasmic reticulum; ERAD: ER-associated protein degradation; PDI: protein disulfide-isomerase; ROS: reactive oxygen species; RESS: Repression under Secretion Stress; CSAP: Conserved Small Abundant Protein; TCTP: translationally controlled tumor protein.

Vital parameters for high gamma-aminobutyric acid (GABA) production by an industrial soy sauce koji Aspergillus oryzae NSK in submerged-liquid fermentation

In submerged-liquid fermentation, seven key parameters were assessed using one-factor-at-a-time to obtain the highest GABA yield using an industrial soy sauce koji Aspergillus oryzae strain NSK (AOSNSK). AOSNSK generated maximum GABA at 30 °C (194 mg/L) and initial pH 5 (231 mg/L), thus was able to utilize sucrose (327 mg/L of GABA) for carbon source. Sucrose at 100 g/L, improved GABA production at 646 mg/L. Single nitrogen sources failed to improve GABA production, however a combination of yeast extract (YE) and glutamic acid (GA) improved GABA at 646.78 mg/L. Carbon-to-nitrogen ratio (C8:N3) produced the highest cell (24.01 g/L) and GABA at a minimal time of 216 h. The key parameters of 30 °C, initial pH 5, 100 g/L of sucrose, combination YE and GA, and C8:N3 generated the highest GABA (3278.31 mg/L) in a koji fermentation. AOSNSK promisingly showed for the development of a new GABA-rich soy sauce.

Carotene production from waste cooking oil by Blakeslea trispora in a bubble column reactor: The role of oxidative stress

The oxidative stress induced by hydroperoxides and reactive oxygen species (ROS) during carotene production from waste cooking oil (WCO) and corn steep liquor (CSL) by the fungus Blakeslea trispora in a bubble column reactor was investigated. The specific activities of the intracellular enzymes superoxide dismutase (SOD) and catalase (CAT) as well as the micromorphology of the fungus were measured in order to study the response of the fungus to oxidative stress. The changes of the morphology of microorganism leaded to pellets formation and documented using a computerized image analysis system. As a consequence of the mild oxidative stress induced by hydroperoxides of WCO and ROS a significant increase in carotene production was obtained. The highest carotene concentration (980.0 mg/l or 51.5 mg/g dry biomass) was achieved in a medium consisted of CSL (80.0 g/L) and WCO (50.0 g/L) at an aeration rate of 5 vvm after 6 days of fermentation. In this case the carotenes produced consisted of β-carotene (71%), γ-carotene (26%), and lycopene (3%). The strong oxidative stress in the fungus caused a significant increase of γ-carotene concentration. Bubble column reactor is a useful fermentation system for carotene production in industrial scale.

The role of oxidative stress on carotene production by Blakeslea trispora in submerged fermentation

In aerobic metabolism, reactive oxygen species (ROS) are formed during the fermentation that can cause oxidative stress in microorganisms. Microbial cells possess both enzymatic and non-enzymatic defensive systems that may protect cells from oxidative damage. The antioxidant enzymes superoxide dismutase and catalase are the two key defensive enzymes to oxidative stress. The factors that induce oxidative stress in microorganisms include butylated hydroxytoluene (BHT), hydrogen peroxide, metal ions, dissolved oxygen tension, elevated temperature, menadione, junglone, paraquat, liquid paraffin, introduction to bioreactors of shake flask inocula and synthetic medium sterilized at initial pH 11.0. Carotenes are highly unsaturated isoprene derivatives. They are used as antioxidants and as coloring agents for food products. In fungi, carotenes are derived via the mevalonate biosynthesis pathway. The key genes in carotene biosynthesis are hmgR, ipi, isoA, carG, carRA and carB. Among microorganisms, Βlakeslea trispora is the main microorganism used for the production of carotenes on the industrial scale. Currently, the synthetic medium is considered the superior substrate for the production of carotenes in a pilot plant scale. The fermentation systems used for the production of carotenes include shake flasks, stirred tank fermentor, bubble column reactor and flat panel photobioreactor. This review summarizes the oxidative stresses in microorganisms and it is focused on the current status of carotene production by B. trispora including oxidative stress induced by BHT, enhanced dissolved oxygen levels, iron ions, liquid paraffin and synthetic medium sterilized at an initial pH 11.0. The oxidative stress induced by the above factors increases significantly the production of carotenes. However, to further reduce the cost of carotene production, new biotechnological methods with higher productivity still need to be explored.

Stimulation of the biosynthesis of carotenes by oxidative stress in Blakeslea trispora induced by elevated dissolved oxygen levels in the culture medium

The adaptive response of the fungus Blakeslea trispora to the oxidative stress induced by elevated dissolved oxygen concentrations during carotene production was investigated by measuring the specific activities of catalase (CAT) and superoxide dismutase (SOD) and the micromorphology of the fungus using a computerized image analysis system. Changes in the ratio of the volume of air (V(a)) over the medium and the volume of medium (V(m)) in the flask caused changes of the morphology of microorganism from clumps to pellets and increases in the specific activities of CAT and SOD. The oxidative stress in B. trispora resulted in a significant increase in carotene production, and a maximum proportion of β-carotene (60%), γ-carotene (50%), and lycopene (10%) (as percentages of total carotenes) was observed at a ratio V(a)/V(m) of 15.7, 4.0 and 1.5, respectively. The highest concentration of carotenes (115.0mg/g dry biomass) was obtained in V(a)/V(m) ratio of 9.0.

The roles of the alternative NADH dehydrogenases during oxidative stress in cultures of the filamentous fungus Aspergillus niger

Despite the importance of filamentous fungi in the biotechnology industry, little is known about their metabolism under the stressful conditions experienced in typical production fermenters. In the present study, oxygen enrichment was used to recreate an industrial batch process, and the effects of the increasing dissolved oxygen tension were studied as regards the cellular metabolism. It was found that elevated dissolved oxygen tension led to an oxidatively stressful environment, as detailed by rapid initial increases in reactive oxygen species (ROS) concentrations and antioxidant enzyme activities. Intracellular protein concentrations also decreased in oxygenated cultures; this appeared to be concomitant with a decrease in the adenosine-5'-triphosphate (ATP) pool in these cultures. Oxygenated cultures showed early senescence and death compared to aerated control cultures. Despite earlier studies proposing various mechanisms for such findings in fungal cultures subjected to oxidative stress, these findings can best be explained by the fact that in such cultures the activity of alternative NADH dehydrogenases was significantly increased, which served to maintain lower ROS concentrations throughout the duration of the process but in doing so also reduced the ability of the organism to create a proton motive force by which to drive ATP synthesis. The findings of the present study help further our understanding of the central roles of these highly conserved enzymes within fungal metabolism under oxidative stress.

Oxidative stress in industrial fungi

Fungi are amongst the most industrially important microorganisms in current use within the biotechnology industry. Most such fungal cultures are highly aerobic in nature, a character that has been frequently referred to in both reactor design and fungal physiology. The most fundamentally significant outcome of the highly aerobic growth environment in fermenter vessels is the need for the fungal culture to effectively combat in the intracellular environment the negative consequences of high oxygen transfer rates. The use of oxygen as the respiratory substrate is frequently reported to lead to the development of oxidative stress, mainly due to oxygen-derived free radicals, which are collectively termed as reactive oxygen species (ROS). Recently, there has been extensive research on the occurrence, extent, and consequences of oxidative stress in microorganisms, and the underlying mechanisms through which cells prevent and repair the damage caused by ROS. In the present study, we critically review the current understanding of oxidative stress events in industrially relevant fungi. The review first describes the current state of knowledge of ROS concisely, and then the various antioxidant strategies employed by fungal cells to counteract the deleterious effects, together with their implications in fungal bioprocessing are also discussed. Finally, some recommendations for further research are made.

Introduction to bioreactors of shake-flask inocula leads to development of oxidative stress in Aspergillus niger

Inoculation of bioreactors with shake-flask cultures present the organism with an immediate shift from an environment with little O2 to one in which O2 is typically at 100% saturation. The inoculation of such shake-flasks cultures into bioreactors sparged with 1 vvm air or 1 vvm air/O2 mix i.e. 50% O2 enrichment is an oxidatively stressful event, as judged by immediate increases in the intracellular concentrations of superoxide anion radical (O2*-) (from 4,600 to 11,600 RLU mg DCW(-1) and 5,500 to 23,000 RLU mg DCW(-1) respectively) and changes in the activities of the major antioxidant enzymes superoxide dismutase and catalase in all cultures. There are further effects on metabolic indices, particularly decreased nutrient consumption in oxygenated cultures (from 0.16 to 0.12 g starch g DCW h(-1)) and decreased protein production, indicating that inoculation of the bioreactor exerts a global burden on the cellular metabolic networks.

Role of hydrolytic enzymes and oxidative stress in autolysis and morphology of Blakeslea trispora during β-carotene production in submerged fermentation

The role of hydrolytic enzymes (proteases and chitinase) and oxidative stress in the autolysis and morphology of Blakeslea trispora during beta-carotene production from a chemically defined medium in shake flask culture was investigated. The process of cellular autolysis was studied by measuring the changes in biomass dry weight, pH, concentration of beta-carotene, specific activity of the hydrolytic enzymes and micromorphology of the fungus using a computerized image analysis system. In addition, the phenomenon of autolysis was associated with high concentrations of reactive oxygen species (ROS). The accumulation of ROS produced during fermentation causes oxidative stress in B. trispora. Oxidative stress was examined in terms of the activities of two key defensive enzymes: catalase (CAT) and superoxide dismutase (SOD). The profile of the specific activities of the above enzymes appeared to correlate with the oxidative stress of the fungus. The high activities of CAT and SOD showed that B. trispora is found under oxidative stress during beta-carotene production. The culture began to show signs of autolysis nearly in the growth phase and autolysis increased significantly during the production phase. The morphological differentiation of the fungus was a result of the degradation of the cell membrane by hydrolytic enzymes and oxidative stress. Increased beta-carotene production is correlated with intense autolysis of clumps, which has as a consequence the increase of the freely dispersed mycelia.

Effects ofN,N-bis(3-aminopropyl)dodecylamine on antioxidant enzyme activities, mitochondrial morphology and metabolism inAspergillus niger

The activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSP) as well as of succinate dehydrogenase (SDG), NADH dehydrogenase (NDG) and fumarate hydratase (FHT) were examined in relation to mitochondrial ultrastructure changes in Aspergillus niger exposed to N,N-bis(3-aminopropyl)dodecylamine (Apd) that was shown to exhibit fungicidal activity. There was a progressive increase in SOD, CAT and GSP activities 1 and 4 h after 0.05 and 0.1 % Apd application. However, this was followed by a pronounced activity decrease when 0.05 % Apd treatment was prolonged by 1 d. The destructive effect on fungal morphology was observed when this fungicidal agent was applied at the concentration of 0.1 % for 1 d. In the treated hyphae mitochondria degenerated after all organelles. The morphological malformations of mitochondria had an impact on their metabolic state; however, the activities of SDG, NDG and FHT were affected to a different extent. In A. niger the fungicidal effect of Apd could be mediated by oxidative stress impairing the vital mitochondria-related cellular functions.

Role of antioxidant enzymes in survival of conidiospores of Aspergillus niger 26 under conditions of temperature stress

AIMS

A better understanding of the role of antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT) in the protection of Aspergillus niger spores against thermal stress.

METHODS AND RESULTS

Conidiospores from A. niger 26 were subjected to wide range of temperatures (30, 50, 60 and 80 degrees C). The stress response was investigated by the determination of spore germination and mycelial growth of survivors under submerged cultivation. Exposure to any temperature above the optimal value induced an increase in SOD and CAT activities. PAGE demonstrated enhanced level of Cu/ZnSOD under stress conditions. We compared the influence of heat shock and superoxide-generating agent paraquat on growth and antioxidant enzyme defence and found different response to the both type of stresses.

CONCLUSIONS

Heat stress elicits the enhanced synthesis of enzymes whose functions are to scavenge reactive oxygen species. These results suggested an association between thermal and oxidative stress.

SIGNIFICANCE AND IMPACT OF THE STUDY

Evidence is provided for the possibility that oxidative stress plays a major role in the effect of heat in low eucaryotes such as A. niger. This knowledge may be of importance in controlling both fermentation and pathogenicity.

Oxidative stress response of filamentous fungi induced by hydrogen peroxide and paraquat

Although, oxidative stress response, which protects organisms from deleterious effects of reactive oxygen species (ROS), has been extensively studied in pro- and eukaryotes, the information about filamentous fungi is fragmentary. We investigated the effect of two ROS-generating agents (paraquat, PQ, and H2O2) on cellular growth and antioxidant enzyme induction in 12 fungal species. Our results indicate that exposure of fungal spores or mycelia to PQ and H2O2 promoted oxidative stress, as evidenced by remarkable inhibition of spore germination and biomass production; stimulation of cyanide-resistant respiration; accumulation of oxidative modified proteins. Cell responses against both superoxide and peroxide stresses include enhanced expression of superoxide dismutase (SOD) and catalase, however, the extent was different: treatment with PQ increased mainly SOD, whereas exogenous H2O2 led to enhanced catalase. We also found that G6PD has a role in the mechanism of protection against superoxide and peroxide stresses. The activation of antioxidant enzyme defence was blocked by the translation inhibitor, cycloheximide, suggesting that there was de novo enzyme synthesis.

Glutathione, Altruistic Metabolite in Fungi

Glutathione (GSH; gamma-L-glutamyl-L-cysteinyl-glycine), a non-protein thiol with a very low redox potential (E'0 = 240 mV for thiol-disulfide exchange), is present in high concentration up to 10 mM in yeasts and filamentous fungi. GSH is concerned with basic cellular functions as well as the maintenance of mitochondrial structure, membrane integrity, and in cell differentiation and development. GSH plays key roles in the response to several stress situations in fungi. For example, GSH is an important antioxidant molecule, which reacts non-enzymatically with a series of reactive oxygen species. In addition, the response to oxidative stress also involves GSH biosynthesis enzymes, NADPH-dependent GSH-regenerating reductase, glutathione S-transferase along with peroxide-eliminating glutathione peroxidase and glutaredoxins. Some components of the GSH-dependent antioxidative defence system confer resistance against heat shock and osmotic stress. Formation of protein-SSG mixed disulfides results in protection against desiccation-induced oxidative injuries in lichens. Intracellular GSH and GSH-derived phytochelatins hinder the progression of heavy metal-initiated cell injuries by chelating and sequestering the metal ions themselves and/or by eliminating reactive oxygen species. In fungi, GSH is mobilized to ensure cellular maintenance under sulfur or nitrogen starvation. Moreover, adaptation to carbon deprivation stress results in an increased tolerance to oxidative stress, which involves the induction of GSH-dependent elements of the antioxidant defence system. GSH-dependent detoxification processes concern the elimination of toxic endogenous metabolites, such as excess formaldehyde produced during the growth of the methylotrophic yeasts, by formaldehyde dehydrogenase and methylglyoxal, a by-product of glycolysis, by the glyoxalase pathway. Detoxification of xenobiotics, such as halogenated aromatic and alkylating agents, relies on glutathione S-transferases. In yeast, these enzymes may participate in the elimination of toxic intermediates that accumulate in stationary phase and/or act in a similar fashion as heat shock proteins. GSH S-conjugates may also form in a glutathione S-transferases-independent way, e.g. through chemical reaction between GSH and the antifugal agent Thiram. GSH-dependent detoxification of penicillin side-chain precursors was shown in Penicillium sp. GSH controls aging and autolysis in several fungal species, and possesses an anti-apoptotic feature.