Aerobic Conditions Research Articles

The Microbiological Quality of Raw Ovine Milk in the Banat Region of Romania with a Focus on Escherichia coli and Its Pathogenic Potential and Antimicrobial Resistance

This study investigated the bacteriological quality of raw ovine milk produced by farms located in the Banat region of Romania. Additionally, the pathogenic potential and antimicrobial resistance of the isolated Escherichia coli strains were evaluated. A total of 95.8% (69/72) of the screened bulk tank milk samples, collected at the farm level immediately after milking, demonstrated appropriate total aerobic mesophilic bacteria (TMB) counts, varying from 3.32 to 6.09 log10 CFU/mL. However, 4.2% (3/72) of the samples were above the regulatory limit of 6.18 log10 CFU/mL. E. coli was identified in 66.6% of the examined samples, and from the total number (n = 48) of isolates, 18.8% harbored the stx2 gene, highlighting pathogenic potential. Antimicrobial susceptibility testing with the Vitek2 system of the isolated E. coli strains revealed resistance against ampicillin (45.8%), gentamicin (20.8%), ticarcillin–clavulanic acid (18.8%), cephalexin (18.8%), amoxicillin–clavulanic acid (8.3%), and trimethoprim–sulfamethoxazole (2.1%). Additionally, 64.6% of the strains showed intermediate resistance against amoxicillin–clavulanic acid, while no resistance was recorded against imipenem. Five (18.5%) strains were multidrug-resistant. This study’s results underline hygienic sanitary deficiencies during the milking phase and indicate that raw ovine milk can be contaminated with pathogenic and multidrug-resistant E. coli strains, highlighting a potential risk to public health. Further studies are encouraged to better understand the risk posed to the consumer via the consumption of ovine milk and derived products.

Characterization of Novel Species of Potassium-Dissolving Purple Nonsulfur Bacteria Isolated from In-Dyked Alluvial Upland Soil for Maize Cultivation

Potassium (K) is immobilized within the clay minerals, making it unavailable for plant use. Therefore, the current study aimed to (i) select isolates of purple nonsulfur bacteria that can dissolve K (K-PNSB) and (ii) evaluate the production of plant-growth-promoting substances by the K-PNSB isolates. The results revealed that from in-dyked alluvial soils in hybrid maize fields, 61 K-PNSB isolates were obtained under the pH 5.50 conditions. The total dissolved K content (Kdis) by the 61 K-PNSB isolates fluctuated from 56.2 to 98.6 mg L−1. Therein, three isolates, including M-Sl-09, M-So-11, and M-So-14 had Kdis of 48.1–48.8 mg L−1 under aerobic dark condition (ADC) and 47.6–49.7 mg L−1 under microaerobic light condition (MLC). Moreover, these three isolates can also fix nitrogen (19.1–21.5 mg L−1 and 2.64–7.24 mg L−1), solubilize Ca-P (44.3–46.8 mg L−1 and 0.737–6.965 mg L−1), produce indole-3-acetic acid (5.34–7.13 and 2.40–3.23 mg L−1), 5-aminolevulinic acid (1.85–2.39 and 1.53–2.47 mg L−1), siderophores (1.06–1.52 and 0.92–1.26 mg L−1), and exopolymeric substances (18.1–18.8 and 52.0–56.0%), respectively, under ADC and MLC. The bacteria were identified according to their 16S rDNA as Cereibacter sphaeroides M-Sl-09, Rhodopseudomonas thermotolerans M-So-11, and Rhodospeudomonas palustris M-So-14. These potential bacteria should be further investigated as a plant-growth-promoting biofertilizer.

Aerobic and anaerobic poise of white swimming muscles of the deep-diving scalloped hammerhead shark: comparison to sympatric coastal and deep-water species

Scalloped hammerhead sharks (Sphyrna lewini) routinely perform rapid dives to forage on mesopelagic prey. These deep dives consist of intensive swimming followed by recovery periods in the surface mixed layer. Swimming muscle temperature profiles suggest that S. lewini suppresses gill function as a means to reduce convective heat loss during dives into cool water. Such intensive swimming behavior coupled with reduced respiration prompted us to test whether the aerobic and anaerobic metabolic capacities of the white swimming muscle tissue of this species are greater than those of other shark species from the same region. The activities of key enzymes used in aerobic and anaerobic metabolism provide an indirect indicator of the metabolic potential (“poise”) of a tissue. Here we measured the maximal activities [international units (µmol substrate converted to product per min, U) per gram of wet tissue mass at 10°C] of the citric acid cycle enzymes citrate synthase (CS) and malate dehydrogenase (MDH) and glycolytic enzymes pyruvate kinase (PK) and lactate dehydrogenase (LDH) from white swimming muscle of S. lewini. Enzyme activities, and ratios of these enzyme activities that indicate relative indexes of aerobic to anaerobic capacity, were compared to those measured in three sympatric coastal carcharhinid sharks and two deep-dwelling species, Echinorhinus cookei and Hexanchus griseus. This is the first report of swimming-muscle enzyme activity for these deep-dwelling species. In comparison to the other species, S. lewini had significantly higher activities of both LDH and MDH in the white muscle, and a higher MDH/CS ratio. The high LDH activities suggest that the white muscle of S. lewini relies on relatively high rates of anaerobic ATP production, with would result in build up of high lactate levels, during deep foraging dives. High MDH activity in S. lewini white muscle suggests the potential for lactate levels to be rapidly reduced when aerobic conditions are restored while in the surface mixed layer between dives. These biochemical characteristics may enable S. lewini to swim rapidly while suppressing gill function during deep dives and thereby exploit a very different ecological niche from sympatric shark species (e.g., coastal carcharhinids) and hunt more rapidly via faster swimming for deep-water prey compared to species that permanently inhabit deep depths.

Effect of UV-C Treatment on the Quality of Fresh-Cut Potatoes Obtained from Tubers of Different Ages

The physico-chemical properties of potato tubers change with age, which can affect not only the quality of the tubers but also the quality and shelf-life of fresh-cut potatoes (FCP) produced from older tubers. One of the methods that has proven useful for extending the shelf-life of some foods is ultraviolet-C (UV-C) irradiation. This study investigated whether UV-C irradiation can mitigate the negative effects of tuber age. FCP from tubers after storage (1 and 9 months), vacuum-packaged, were irradiated with UV-C (2.70 kJ m−2) and stored (6 °C/15 days). The total aerobic mesophilic bacteria count (TAMBC), total and soluble solids content, pH, firmness, color, chlorogenic acid, reducing sugars, sensory properties of raw FCP and subsequently boiled and fried FCP, and acrylamide content in fried FCP were monitored. UV-C irradiation of FCP does not affect all parameters equally for both tuber ages, but was generally effective in reducing TAMBC, and UV-C-treated samples reached a shelf-life of 15 days. UV-C irradiation improved the sensory properties of raw and thermally processed FCP at both ages. UV-C irradiation caused an increase in acrylamide content in fried FCP, regardless of. the tubers age, but all samples were safe in this respect.

Ozonation as an emerging technique for inactivating microorganisms and mitigating pesticides in apple juice

ABSTRACT Microorganisms and pesticide residues are among the major health concerns associated with food products. Although ozone (O3) is currently being employed in various applications in apple processing, there are few studies investigating the effects of O3 treatments on microorganisms and pesticides in apple juice. The purpose of this study was to investigate the effects of O3 treatment on microorganisms, pesticide residues, and physicochemical properties of apple juice and to compare with that of classical pasteurization. O3 was as effective as pasteurization against E. coli and coliform bacteria; the counts of these microorganisms were below the detectable level after a treatment for 15 min. O3 was also effective in reducing the counts of total aerobic mesophilic bacteria and yeast-mold, but longer treatments were required for total inactivation. All the pesticides investigated were degraded by O3, but at different rates. Reduction rates were quite high (75–100%) for methoxyfenozide, dimethoate, and pyrimethanil, however, acetamiprid and imidacloprid were degraded by only 7.1% and 14.5%, respectively. O3 treatment caused drastic decreases in color and turbidity values, total phenolic content, and antioxidant activity of apple juice. Results revealed that the use of ozone in juice processing has potential as a technological solution to product contamination problem.

Assessment of hygienic-sanitary status and implementation of an integral risk management program in butcher shops from Tafí Viejo city, Tucumán, Argentina

Meat and meat products promote the growth of pathogens such as Shiga toxin-producing Escherichia coli and Salmonella spp. In this work, an evaluation was carried out through risk quantification and microbiological analysis on ground beef and environmental surfaces in butcher shops, of Tafí Viejo city, Tucumán province. The study was carried out in three periods: period 1 (description of the health-status), period 2 (implementation of improvement actions) and period 3 (verification of success). Ground beef samples and environmental surfaces were analyzed in the previously described periods. The improvement actions in each butcher shop were implemented on the deviations detected in the first period. An increase of 35.7 % in low-risk butchers was observed. Microbial indicators in ground beef were significantly lower for total aerobic mesophilic microorganisms and Stapylococcus aureus, compared to the first period. In ground beef samples and environmental surfaces, the detection/isolation of STEC non-O157, presented a 100 % decrease in the verification period. The detection of Salmonella spp. showed a reduction of 14.3 % only on ground beef samples. This study confirms that the implementation of a risk management program in butcher shops would reduce contamination of ground beef and the environment with pathogens.

Zic-HILIC MS/MS Method for NADomics Provides Novel Insights into Redox Homeostasis in Escherichia coli BL21 Under Microaerobic and Anaerobic Conditions

Background: Nicotinamide adenine dinucleotide (NAD+), its precursors, and its derivatives (collectively NADome) play a crucial role in cellular processes and maintain redox homeostasis. Understanding the dynamics of these metabolic pools and redox reactions can provide valuable insights into metabolic functions, especially cellular regulation and stress response mechanisms. The accurate quantification of these metabolites is challenging due to the interconversion between the redox forms. Methods: Our laboratory previously developed a zwitterionic hydrophilic interaction liquid chromatography (zic-HILIC)–tandem mass spectrometry method for the quantification of five essential pyridine nucleotides, including NAD+ derivatives and it’s reduced forms, with 13C isotope dilution and matrix-matched calibration. In this study, we have improved the performance of the chromatographic method and expanded its scope to twelve analytes for a comprehensive view of NAD+ biosynthesis and utilization. The analytical method was validated and applied to investigate Escherichia coli BL21 under varying oxygen supplies including aerobic, microaerobic, and anaerobic conditions. Conclusions: The intracellular absolute metabolite concentrations ranged over four orders of magnitude with NAD+ as the highest abundant, while its precursors were much less abundant. The composition of the NADome at oxygen-limited conditions aligned more with that in the anaerobic conditions rather than in the aerobic phase. Overall, the NADome was quite homeostatic and E. coli rapidly, but in a minor way, adapted the metabolic activity to the challenging shift in the growth conditions and achieved redox balance. Our findings demonstrate that the zic-HILIC-MS/MS method is sensitive, accurate, robust, and high-throughput, providing valuable insights into NAD+ metabolism and the potential significance of these metabolites in various biological contexts.

Growth substrate limitation enhances anaerobic arsenic methylation by Paraclostridium bifermentans strain EML.

Microbial arsenic methylation is established as a detoxification process under aerobic conditions (converting arsenite to monomethylated arsenate) but is proposed to be a microbial warfare strategy under anoxic conditions due to the toxicity of its main product, monomethylarsonous acid (MMAs(III)). Here we leveraged a paddy soil-derived anaerobic arsenic methylator, Paraclostridium bifermentans strain EML, to gain insights into this process. Strain EML was inoculated into a series of media involving systematic dilutions of Reinforced Clostridial Broth (RCB) with 25 µM arsenite to assess the impact of growth substrate concentration on arsenic methylation. Growth curves evidenced the sensitivity of strain EML to arsenite, and arsenic speciation analysis revealed the production of MMAs(III). Concentrations of MMAs(III) and arsenic methylation gene (arsM) transcription were found to be positively correlated with RCB dilution, suggesting that substrate limitation enhances arsM gene expression and associated anaerobic arsenic methylation. We propose that growth substrate competition among microorganisms may also contribute to an increase in anaerobic arsenic methylation. This hypothesis was further evaluated in an anaerobic co-culture system involving strain EML and either wild-type Escherichia coli K-12 MG1655 (WT) or E. coli expressing the MMAs(III)-resistance gene (arsP) (ArsP E. coli). We observed increased MMAs(III) production in the presence of E. coli than its absence and growth inhibition of WT E. coli to a greater extent than ArsP E. coli, presumably due to the MMAs(III) produced by strain EML. Collectively, our findings suggest an ecological role for anaerobic arsenic methylation, highlighting the significance of microbe-microbe competition and interaction in this process.IMPORTANCEMicrobial arsenic methylation is highly active in rice paddy fields under flooded conditions, leading to increased accumulation of methylated arsenic in rice grains. In contrast to the known detoxification process for aerobic arsenic methylation, the ecological role of anaerobic arsenic methylation remains elusive and is proposed to be an antibiotic-producing process involved in microbial warfare. In this study, we interrogated a rice paddy soil-derived anaerobic arsenic-methylating bacterium, Paraclostridium bifermentans strain EML, to explore the effect of growth substrate limitation on arsenic methylation in the context of the microbial warfare hypothesis. We provide direct evidence for the role of growth substrate competition in anaerobic arsenic methylation via anaerobic prey-predator co-culture experiments. Moreover, we demonstrate a feedback loop, in which a bacterium resistant to MMAs(III) enhances its production, presumably through enhanced expression of arsM resulting from substrate limitation. Our work uncovers the complex interactions between an anaerobic arsenic methylator and its potential competitors.

Determination and Optimization of Aerobic and Anaerobic Decomposition of Paper Sludge

The processing of paper sludge is currently an important environmental topic due to its high global production. The aim of this study is to monitor the biodegradation of paper sludge when the initial conditions change. Biodegradability tests 301F and OECD 311 were used to determine biodegradation. The data obtained from the tests were subsequently obtained for the simulation in MATLAB R2023b. The highest aerobic decomposition was approximately 80% after 28 days at an initial concentration of paper sludge leachate of 76 g/L. By simulating 3D modelling, we can predict that with a retention time of 1 day with degradation under aerobic conditions at the level of 70%, the ideal initial concentration of organic substances will be 157.55 g/L. Based on this model, it is possible to estimate that with a biogas production of 554 m3/tVS and a decomposition time of 20 days, it is necessary to set a concentration of approximately 128 g/L. Based on biodegradability tests, paper sludge was evaluated as suitable for aerobic or anaerobic biological decomposition.

Mathematical Modeling and Simulation of 1,3-Propanediol Production by Klebsiella pneumoniae BLh-1 in a Batch Bioreactor Using Bayesian Statistics.

Mathematical modeling and computer simulation are fundamental for optimizing biotechnological processes, enabling cost reduction and scalability, thereby driving advancements in the bioindustry. In this work, mathematical modeling and estimation of fermentative kinetic parameters were carried out to produce 1,3-propanediol (1,3-PDO) from residual glycerol and Klebsiella pneumoniae BLh-1. The Markov chain Monte Carlo method, using the Metropolis-Hastings algorithm, was applied to experimental data from a batch bioreactor under aerobic and anaerobic conditions. Sensitivity analysis and parameter evolution studies were conducted. The root-mean-square error (rRMSE) was chosen as the validation and calibration metric for the developed mathematical model. The results indicated that the average tolerance of glycerol was 174.68 and 44.85 g L-1, the inhibitory products was 150.95 g L-1 for ethanol and 35.56 g L-1 for 1,3-PDO, and the maximum specific rate of cell growth was 0.189 and 0.275 h-1, for aerobic and anaerobic cultures, respectively. The model presented excellent fits in both crops, with rRMSE values between 0.09 - 33.74% and 3.58 - 31.82%, for the aerobic and anaerobic environment, respectively. With this, it was possible to evaluate and extract relevant information for a better understanding and control of the bioprocess.

Recreational Water Safety in Hotels: Lessons from the COVID-19 Pandemic and the Way Forward for a Safe Aquatic Environment

The recreational waters of hotels are widely used by locals and travelers all around the world, and this impacts public health. In this regard, the WHO frequently offers recommendations—especially during the COVID-19 era—that operators and managers of public pools, hot tubs, or splash pads should continue to follow. This study aims to evaluate the microbiological results of COVID-19 sanitation protocols in recreational waters in hotels during the summer periods post-COVID-19 (2020–2022) and compare them with those pre-COVID-19 (2019). Of the samples tested during the pre-pandemic period, less than 1/5 were positive for total coliforms, common aerobic bacteria, and P. aeruginosa, while less than 3% of the samples tested were positive for E. coli. During the pandemic years, the percentages dropped by half for total coliforms, common aerobic bacteria, and E. coli, while only the percentage for P. aeruginosa remained the same. The relative risk (RR) for the presence of P. aeruginosa in recreational waters was greatly affected by chlorine levels, pH values, and water temperature. Our results elucidate the impact of sanitation protocols on the safety of recreational waters in hotels during public health crises. Moreover, the challenges operators face in maintaining long-term implementation of these protocols are highlighted, which in turn dictates the appropriate choice of preventive and applicable measures to ensure health and safety.

A Mucous Permeable Local Delivery Strategy Based on Manganese-Enhanced Bacterial Cuproptosis-like Death for Bacterial Pneumonia Treatment.

Bacterial pneumonia is one of the most challenging global infectious diseases with high morbidity and mortality. Considering the antibiotic abuse and resistance of bacterial biofilms, a variety of metal-based materials have been developed. However, due to the high oxygen environment of the lungs, some aerobic infection bacteria have high tolerance to oxygen and ROS, and most of the metal-based materials based on ROS may not achieve good therapeutic effects. Inspired by the sensitivity of cuproptosis to aerobic respiratory cells, we designed a copper composite antibacterial nanoparticle and found that it can effectively induce cuproptosis-like death in the aerobic bacteria of the lungs. To address the challenge of in vivo application of cuproptosis, manganese dioxide was first incorporated to deplete protective glutathione, which can interact with copper and thus hinder the interaction of copper with proteins and assist in antibacterial action through immune enhancement. Cuproptosis-like death also requires a large number of copper ions. To meet this demand, we deliver positively hydrophilic modified composite nanoparticles that effectively penetrate the lung mucus layer directly to the lungs through local administration, and the copper ions are further released rapidly by the acidic environment at the infected site, which can further destroy bacterial biofilms in synergy with manganese. This drug-delivery system can effectively treat pneumonia caused by aerobic bacteria and avoid systemic toxicity that can be caused by large doses of copper.

Control of bacterial contamination in three types of fish selected from the coastal and inland regions of Tipaza (Algeria)

This study addresses the problem of microbiological contamination in fish sold in the region of Tipaza, where food safety is a growing concern. It aims to assess the contamination levels in three fish species: sardine, mackerel saurale, and bogue. To achieve this, samples were collected and analyzed for total aerobic mesophilic bacteria, thermotolerant coliforms, coagulase-positive Staphylococcus aureus, and Salmonella. The results revealed that all samples contained total aerobic mesophilic bacteria and thermotolerant coliforms within acceptable limits. However, high levels of coagulase-positive Staphylococcus aureus were detected in bogue and saurale samples, indicating unsatisfactory bacteriological quality, particularly in samples from the city of Tipaza. No Salmonella contamination was observed. These findings highlight significant hygiene issues during fish transport and sale, contributing to microbial contamination. The study concludes that improving handling practices is crucial to ensuring fish safety and reducing health risks to consumers.

Dynamic succession of the quantity and composition of epiphytic microorganisms at different growth stages on rice surface

The quality of silage is uneven, which is due to the difference of epiphytic microorganisms of raw materials. To improve the quality of fermentation, the quantity and composition of epiphytic microorganisms are usually analyzed to better prepare silage. In this research, plate coating method and 16S high-throughput sequencing were used to analyze the differences in the quantity and composition of rice epiphytic microorganisms during different growth stages. The Lactic acid bacteria (LAB) and aerobic bacteria were the highest at the flowering stage, the yeast was the highest at the milk ripening stage, and the mould was the highest at the full ripening stage. And the growth stage also had a great influence on the composition of epiphytic bacterial community, at the phylum level, it was mainly composed of Proteobacteria. And at the genus level, Pantoea, Acinetobacter, Pseudomonas and Chryseobacterium were dominanted at the flowering stage; Pantoea, Stenotrophomonas and Sphingobacterium were dominanted at the milk ripening stage; Acinetobacter, Pantoea, Chryseobacterium and Lactococcus were dominanted at the dough stage; Acinetobacter and Klebsiella were dominated at the full ripening stage. Overall, the growth stage significantly affected the quantity and composition of rice epiphytic microorganisms. Therefore, rice silage can be modulated reasonably according to the number and composition of epiphytic microorganisms in different growth stages.

Novel function of single-target regulator NorR involved in swarming motility and biofilm formation revealed in Vibrio alginolyticus

NorR, as a single-target regulator, has been demonstrated to be involved in NO detoxification in bacteria under anaerobic conditions. Here, the norR gene was identified and deleted in the genome of Vibrio alginolyticus. The results showed that deletion of norR in Vibrio alginolyticus led to lower swarming motility and more biofilm formation on aerobic condition. Moreover, we proved that NorR from E. coli had a similar function in controlling motility. NorR overexpression led to increased resistance to oxidative stress and tetracycline. We also observed a reduced ability of the NorR-overexpressing strain to adapt to iron limitation condition. Transcriptome analysis showed that the genes responsible for bacterial motility and biofilm formation were affected by NorR. The expressions of several sigma factors (RpoS, RpoN, and RpoH) and response regulators (LuxR and MarR) were also controlled by NorR. Furthermore, Chip-qPCR showed that there is a direct binding between NorR and the promoter of rpoS. Based on these results, NorR appears to be a central regulator involved in biofilm formation and swarming motility in Vibrio alginolyticus.

The Impact of Triclosan-Containing Toothpaste on the Overall and Cariogenic Salivary Microbiota Among Primary School Children

Objectives: Triclosan-diphenyl ether derivate is an agent, which belongs to the Non-Ionic antimicrobial agents group. This study aims to estimate the salivary levels of Streptococcus mutans and Lactobacillus species in the saliva before and after teeth brushing with a dentifrice containing triclosan/copolymer and to compare the number of whole salivary flora by saliva analysis before and after teeth brushing. Methods: To accomplish our objective, we used a Colgate Total Plus Whitening®, Colgate-Palmolive dentifrice. The group consisted of 24 healthy schoolchildren aged 9 – 13 of both sexes. The counts of Streptococcus mutans and Lactobacillus species were determined with commercial CRT bacteria strips produced by Ivoclar-Vivadent, Liechtenstein. The total count of the saliva microbial was determined by standard microbiological methods. Results: A significant reduction in salivary MS and LB levels was observed in all samples as well as a decrease in the total count of aerobe and anaerobe bacteria and yeast. Conclusion: The effectiveness of Colgate Total Plus Whitening® toothpaste in reducing salivary aerobes is statistically significant, but its impact on reducing salivary anaerobic microbes is statistically negligible. The bacteria Streptococcus mutans experiences a decrease that is greater than tenfold, while the bacterium Lactobacillus species experiences a reduction of around tenfold.

Characterization of 'borde': A traditional ethiopian cereal-based fermented beverage - Microbial analysis, rheological profiling, and functional attributes

The study focused on the characterization of an Ethiopian traditional beverage, borde, produced by lactic acid bacteria (LAB) and yeast through natural fermentation. Samples were collected from three distinct locations in Ethiopia and subjected to analyses encompassing microbiological, antioxidant, enzyme inhibition, rheological and physicochemical aspects. The early stages of fermentation exhibited prevalence of Enterobacteriaceae and aerobic mesophilic bacteria. Towards the end of fermentation, yeast (6.07–6.72) and LAB (6.52–7.12) (log CFU mL−1) dominated the microbiological profile of all borde samples. Significant difference (P < 0.05) in the number of microorganisms were noted between the samples. Antioxidant activities, assessed through DPPH and ABTS values, ranged between 51.5 and 64.6 % and 47.4–62.9.1 %, respectively. The average value of α-glucosidase and pancreatic lipase inhibition for borde samples were ranged from 36.56 to 43.23 % and 31.08–40.19 %, respectively. Rheological investigations indicated pseudoplastic fluid behavior, as evidenced by the decreasing apparent viscosity values with shear rate. Moreover, the samples displayed solid-like structures, with lower loss modulus (G″) values compared to storage modulus (G′). The shared physicochemical characteristics and the dominance of specific microbial groups suggest the potential for developing a direct fermentation system for this traditional cereal-based beverage. Overall, these findings contribute valuable insights into the multifaceted nature of borde, paving the way for further advancements in its production and quality control.

Cross-Coupling of Mo- and V-Nitrogenases Permits Protein-Mediated Protection from Oxygen Deactivation.

Nitrogenases catalyze dinitrogen (N2) fixation to ammonia (NH3). While these enzymes are highly sensitive to deactivation by molecular oxygen (O2) they can be produced by obligate aerobes for diazotrophy, necessitating a mechanism by which nitrogenase can be protected from deactivation. In the bacterium Azotobacter vinelandii, one mode of such protection involves an O2-responsive ferredoxin-type protein ("Shethna protein II", or "FeSII") which is thought to bind with Mo-dependent nitrogenase's two component proteins (NifH and NifDK) to form a catalytically stalled yet O2-tolerant tripartite protein complex. This protection mechanism has been reported for Mo-nitrogenase, however, in vitro assays with V-nitrogenase suggest that this mechanism is not universal to the three known nitrogenase isoforms. Here we report that the reductase of the V-nitrogenase (VnfH) can engage in this FeSII-mediated protection mechanism when cross-coupled with Mo-nitrogenase NifDK. Interestingly, the cross-coupling of the Mo-nitrogenase reductase NifH with the V-nitrogenase VnfDGK protein does not yield such protection.

Regulation mechanism of nitrite degradation in Lactobacillus plantarum WU14 mediated by Fnr.

Fumarate and nitrate reduction regulatory protein (Fnr)-a global transcriptional regulator-can directly or indirectly regulate many genes in different metabolic pathways at the top of the bacterial transcription regulation network. The present study explored the regulatory mechanism of Fnr-mediated nitrite degradation in Lactobacillus plantarum WU14 through gene transcription and expression analysis of oxygen sensing and nir operon expression regulation by Fnr. The interaction and the mechanism of transcriptional regulation between Fnr and GlnR were also examined under nitrite stress. After Fnr and GlnR purification by glutathione S-transferase tags, they were successfully expressed in Escherichia coli by constructing an expression vector. The results of electrophoresis mobility shift assay and qRT-PCR indicated that Fnr specifically bound to the PglnR and Pnir promoters and regulated the expression of nitrite reductase (Nir) and GlnR. After 6-12h of culture, the expressions of fnr and nir under anaerobic conditions were higher than under aerobic conditions; the expression of these two genes increased with sodium nitrite (NaNO2) addition during aerobic culture. Overall, the present study indicated that Fnr not only directly participated in the expression of Nir and GlnR but also indirectly regulated the expression of Nir through GlnR regulation.

Aerobic–anaerobic DHS reactor for enhancing denitrification in municipal wastewater treatment

Providing anaerobic environments can enhance the denitrification capacity of wastewater treatment systems. This study developed an aerobic–anaerobic downflow hanging sponge (DHS) reactor to increase denitrification. A siphon tube was integrated with a conventional DHS to create anaerobic conditions by controlling a water column inside the reactor. The siphon heights were set in the middle of the reactor, representing 2/5 of the total volume. Anaerobic conditions were maintained at the bottom (1/5), whereas aerobic conditions were maintained at the top (2/5). The middle part experienced alternating aerobic–anaerobic conditions within a siphon cycle. The hydraulic retention time (HRT) was 3 h, and the temperature was maintained at 30 °C. The study was divided into the low-loading and standard phases based on the influent soluble chemical oxygen demand (sCOD) concentration. During the low-loading phase, the aerobic–anaerobic DHS exhibited a higher sCOD removal rate (53.6 ± 26.6 %) than the control aerobic DHS (35.6 ± 31.9). However, the removal efficiencies for NH₄+–N and total nitrogen (TN) were lower, averaging 37.1 ± 24.7 % and 31.5 ± 21 %, respectively. In the standard phase, the aerobic–anaerobic DHS achieved an sCOD removal rate of 69.3 ± 14.8 % and an NH₄+–N removal rate of 17.2 ± 11.4 %. The control aerobic DHS showed higher sCOD and NH₄+–N removal at 84.7 ± 10.8 % and 54.1 ± 12.8 %, respectively, possibly due to the reduced aerobic zone volume in the aerobic–anaerobic DHS. The TN removal rates were also lower in the aerobic–anaerobic DHS (17.1 ± 11.4 %) than in the control DHS (25 ± 12.3 %). However, based on the ratio of nitrification to TN removal, the aerobic–anaerobic DHS showed a high denitrification rate of 85 %–100 %. The highest number of denitrifiers was in the middle section of the aerobic–anaerobic DHS, with Rhodocyclaceae the most prominent, followed by Comamonadaceae. Maintaining an anaerobic zone improved the denitrification capacity, indicating the potential of the aerobic–anaerobic DHS for efficient municipal wastewater treatment.