Microbial Tolerance Research Articles

Biotechnological recycling of hazardous waste PCBs using Sulfobacillus thermosulfidooxidans through pretreatment of toxicant metals: Process optimization and kinetic studies

The present study dealt with the restricted microbial tolerance for lead and tin during bioleaching of waste printed circuit boards (WPCBs) and lower extraction yields of valuable metals. Pretreatment of WPCBs in 4.0 mol/L HNO3 at 90 °C for 180 min duration prominently dissolved the toxicant metals before the microbial mobilization of valuable metals. Acid pretreatment followed the first-order kinetics that exhibiting an intermediate-controlled mechanism with the apparent activation energy determined to be Ea(Pb), 25.1 kJ/mol and Ea(Sn), 21.9 kJ/mol. Thereafter, the parametric optimization of aeration rate, O2-enrichment, external CO2 supply, temperature, and time for bioleaching of ground WPCBs was examined using Sulfobacillus thermosulfidooxidans (strain RDB). A favourable condition for Cu-bioleaching under higher oxidative environment in comparison to Ni and Zn exhibited the auto-catalytic behaviour of Cu2+ in the biological system. More than 92% of valuable metals were extracted under the optimal condition of aeration rate, 0.5 L/min; O2-enrichement dosage, 30%; external CO2 supply, 0.1%; temperature, 55 °C; and time, 18 days. The bioleaching kinetics followed shrinking core model that exhibiting the shifting of mass transfer from chemically-controlled to the diffusion-controlled mechanism. This process offers two-fold advantages that restoring the valuable metals with low-emission biotechnological route for waste valorization.

Can microorganisms develop resistance against light based anti-infective agents?

Recently, there have been increasing numbers of publications illustrating the potential of light-based antimicrobial therapies to combat antimicrobial resistance. Several modalities, in particular, which have proven antimicrobial efficacy against a wide range of pathogenic microbes include: photodynamic therapy (PDT), ultraviolet light (UVA, UVB and UVC), and antimicrobial blue light (aBL). Using these techniques, microbial cells can be inactivated rapidly, either by inducing reactive oxygen species that are deleterious to the microbial cells (PDT, aBL and UVA) or by causing irreversible DNA damage via direct absorption (UVB and UVC). Given the multi-targeted nature of light-based antimicrobial modalities, it has been hypothesised that resistance development to these approaches is highly unlikely. Furthermore, with the exception of a small number of studies, it has been found that resistance to light based anti-infective agents appears unlikely, irrespective of the modality in question. The concurrent literature however stipulates, that further studies should incorporate standardised microbial tolerance assessments for light-based therapies to better assess the reproducibility of these observations.

Unraveling the bacterial community composition across aquatic sediments in the Southwestern coast of India by employing high-throughput 16S rRNA gene sequencing

In this study, the basic bacterial community composition associated with aquatic sediments of the freshwater, mangrove and marine ecosystems of a coastal region in the Southwest coast of India (Mangalore, Karnataka) was explored using high-throughput 16S rRNA gene sequencing. The V3–V4 hypervariable regions of 16S rRNA gene analysis revealed that freshwater and marine ecosystems had a higher abundance of phyla Planctomyces and Proteobacteria which are known to play a major role in the nitrogen cycle, improving the global nitrogen bioavailability in aquatic ecosystems. Functional prediction of xenobiotic degradation pathways using KEGG database from all three samples suggest the resident microbial tolerance to organic contaminants, xenobiotics and pesticides that play a major role in balancing ecological systems by metabolizing the xenobiotics or contaminants to maintain the resilience of the ecosystem. The study highlights the importance of investigating the bacterial community composition across the aquatic ecosystems as it can serve as an indicator of environmental health as well as provide a valuable gene pool for biotechnological applications.

Single Stage Reactors for Electrifying Bio-Methanation

As a combined solution for minimizing CO2 emissions and meeting global energy demands, upgrading CO2 to produce renewable natural gas is an emerging technology to utilize and store renewable energy. When this process is performed by efficient and product selective biocatalysts, conventional and energy intense thermal methanation technologies can be replaced by bio-methanation technology. The bio-methanogenesis process is typically performed with hydrogen produced externally in electrolyzers, but is severely limited by poor mass transport and solubility issues of molecular hydrogen. A system where both electrolysis and microbial conversion are combined in a single reactor helps overcome these problems. However, despite the outstanding opportunities of in-situ electro-bio-methanogenesis, there are challenges of electrified bioreactors that must be overcome, including microbial tolerance and enhancing the kinetics of in-situ hydrogen evolution. Thus, developments to achieve the balance between both biochemical and electrochemical requirements will allow these reactors to achieve optimum performance. The developments include maintaining physiological pH, effective separation of anode and cathode, improving charge transfer kinetics and reducing internal resistance.We have used our advanced manufacturing capabilities to design and fabricate components for developing electro-bioreactors from 0.1 W scale in sluggish H-cells to MW scale stacks of flow cell CO2 electrolyzers. This allows us to achieve the balance between both biochemical and electrochemical requirements mentioned above. In this study, we show how utilization of high surface area porous 3D electrodes can support maintaining biocompatible conditions near the vicinity of hydrogen evolving electrode at effective rates of electrolysis, while improving both efficiency and productivity of microbial methanogenesis. We have conducted studies to understand the effect of electrode geometry and surface area on energy efficiency and microbial health. Finally, we demonstrate the ability to produce pipeline-quality methane (>97% CH4) from CO2 for an extended period.

The Molecular Pathophysiology of Psoriatic Arthritis-The Complex Interplay Between Genetic Predisposition, Epigenetics Factors, and the Microbiome.

Psoriasis is a symmetric autoimmune/inflammatory disease that primarily affects the skin. In a significant proportion of cases, it is accompanied by arthritis that can affect any joint, the spine, and/or include enthesitis. Psoriasis and psoriatic arthritis are multifactor disorders characterized by aberrant immune responses in genetically susceptible individuals in the presence of additional (environmental) factors, including changes in microbiota and/or epigenetic marks. Epigenetic changes can be heritable or acquired (e.g., through changes in diet/microbiota or as a response to therapeutics) and, together with genetic factors, contribute to disease expression. In psoriasis, epigenetic alterations are mainly related to cell proliferation, cytokine signaling and microbial tolerance. Understanding the complex interplay between heritable and acquired pathomechanistic factors contributing to the development and maintenance of psoriasis is crucial for the identification and validation of diagnostic and predictive biomarkers, and the introduction of individualized effective and tolerable new treatments. This review summarizes the current understanding of immune activation, genetic, and environmental factors that contribute to the pathogenesis of psoriatic arthritis. Particular focus is on the interactions between these factors to propose a multifactorial disease model.

Evaluation of bacterial strains isolated from Late Quaternary alluvial sediments spanning ~ 28 m in depth for heavy metal tolerance and Cr(VI) removal ability.

Microbial heavy metal tolerance in subsurface samples is indicative of long-term ecotoxicological impact of metals and could also reflect metal contamination of groundwater. However, the heavy metal tolerance characteristics of microbes isolated from subsurface river sediment profiles are still obscure. In the present study, determination of heavy metal tolerance of bacterial strains isolated from two Late Quaternary sediment profiles (~ 28 m and ~25 m deep) located at the Mahi river basin, Western India, was carried out. Identification of bacterial isolates by the 16S rRNA gene sequencing revealed that bacterial isolates affiliated with phyla Actinobacteria, Proteobacteria, Firmicutes, and Bacteroidetes were dominant in both sediment profiles. Heavy metal tolerance of bacterial strains as determined by plate diffusion assay revealed order of metal tolerance as follows: Hg(II)<Cd(II)< Ni(II)<Cu(II)=Cr(VI). Chromate removal study in liquid medium suggested that bacterial strains procured from subsurface possessed the ability to remove Cr(VI) with varied magnitude. A packed bed column experiment indicated that bacterial strains obtained from the subsurface have the potential for Cr(VI) removal from different particle size consistencies of the sediments.

The involvement of the Candida glabrata trehalase enzymes in stress resistance and gut colonization

ABSTRACT Candida glabrata is an opportunistic human fungal pathogen and is frequently present in the human microbiome. It has a high relative resistance to environmental stresses and several antifungal drugs. An important component involved in microbial stress tolerance is trehalose. In this work, we characterized the three C. glabrata trehalase enzymes Ath1, Nth1 and Nth2. Single, double and triple deletion strains were constructed and characterized both in vitro and in vivo to determine the role of these enzymes in virulence. Ath1 was found to be located in the periplasm and was essential for growth on trehalose as sole carbon source, while Nth1 on the other hand was important for oxidative stress resistance, an observation which was consistent by the lower survival rate of the NTH1 deletion strain in human macrophages. No significant phenotype was observed for Nth2. The triple deletion strain was unable to establish a stable colonization of the gastrointestinal (GI) tract in mice indicating the importance of having trehalase activity for colonization in the gut.

Metagenomic analysis for profiling of microbial communities and tolerance in metal-polluted pulp and paper industry wastewater

This work aimed to study the profiling and efficiency of microbial communities and their abundance in the pulp and paper industry wastewater, which contained toxic metals, high biological oxygen demands, chemical oxygen demand, and ions contents. Sequence alignment of the 16S rRNA V3-V4 variable region zone with the Illumina MiSeq framework revealed 25356 operating taxonomical units (OTUs) derived from the wastewater sample. The major phyla identified in wastewater were Proteobacteria, Bacteroidetes, Firmicutes, Chloroflexi, Actinobacteria, Spirochetes, Patesibacteria, Acidobacteria, and others including unknown microbes. The study showed the function of microbial communities essential for the oxidation and detoxifying of complex contaminants and design of effective remediation techniques for the re-use of polluted wastewater. Findings demonstrated that the ability of different classes of microbes to adapt and survive in metal-polluted wastewater irrespective of their relative distribution, as well as further attention can be provided to its use in the bioremediation process.

Can heavy metal pollution induce bacterial resistance to heavy metals and antibiotics in soils from an ancient land-mine?

Microbial resistance to antibiotics is a growing challenge to human health. Recent evidence has indicated that antibiotic resistance can be co-selected for by exposure to heavy metals in agricultural soils. It remains unknown if this is a concern in other environments contaminated by metals. We here investigated soil microbial activities, composition and tolerance to heavy metals and antibiotics in a mining soil survey. We found that microbial respiration, growth, and biomass were affected by available metal concentrations. Most of the variation in microbial PLFA composition was explained by differences in heavy metal and pH. Additionally, pollution-induced bacterial community tolerance to toxicants including Cu, Pb, Zn, tetracycline and vancomycin was determined. Although only bacterial tolerance to Pb increased with higher levels of metals, the links between bacterial metal tolerance and soil metal concentrations were clear when considered together with previously published reports, suggesting that bacterial metal tolerance were universally elevated in the surveyed soils. The induced levels of heavy metal tolerance coincided with elevated levels of tolerance to vancomycin, but not to tetracycline. Our study showed that heavy metals can co-select for resistance to clinically important antibiotics also in ecosystems without manure input or antibiotic pollution.

Identification of a novel strain of fungus Kalmusia italica from untouched marine soil and its heavy metal tolerance activity

Heavy metal pollution is one of the most important environmental problems. Heavy metal discharge to the atmosphere has increased currently because of industrial activity and technological growth and creates serious problems for the climate, soil, and water supplies. Many conventional techniques were used for heavy metals separation and treatment. Micro-organisms play a significant role in the bioremediation of polluted soil and waste products from heavy metals. The most common heavy metal tolerant microbes are fungi and yeast. Fungal isolates are potential microbes that are tolerant of heavy metals. The research aims to isolate and classify the fungal species collected from the soil. The fungal isolates identified were tested for their biosorption property and the tolerance index of stressed strains was investigated by examining the minimum inhibitory concentration (MIC). The ability of the strain to remove heavy metals was investigated. Fungal isolates were molecularly identified using 18S rRNA sequencing. For the isolated fungus, BLAST and the phylogenetic trees were built and were known as Kalmusia italica. The effect of the isolates on the growth in the presence of heavy metals was determined by measuring the colony extension radius against control. The results showed that fungus was tolerant to a higher concentration of heavy metals and was able to absorb the heavy metals. MIC determination is indicative of the fact that the levels of tolerance to individual metals depend on the type of fungus. Biological treatment based on living fungi, which is cheaper and more environmentally friendly, has emerged as a good alternative technique to existing conventional methods for metal removal.

Rhizomicrobiomics of Caesalpinia bonducella, a wonder plant for PCOS treatment.

Plant and rhizobacterial interactions contribute partly to a plant's medicinal properties and are well studied through metagenomics. In this study, 16S rDNA, 18S rDNA, and ITS meta-sequencing were performed using the genomic DNA obtained from the rhizosphere of Caesalpinia bonducella-a medicinal shrub widely used to treat polycystic ovary syndrome (PCOS). Of the 665Operational Taxonomic Units (OTUs) obtained from 16S rDNA sequencing, 23.9% comprised of microbes that increase the therapeutic value of plants (Bacillus, Paenibacillus), 6.4% belonged to stress and drought tolerant microbes (Pseudomonas, Rhizobium, Serratia), 8% belonged to plant-growth promoting rhizobacteria-predominantly Proteobacteria, and Firmicutes and the remaining were the microbes performing various other functions. Alpha diversity indexing by GAIA-metagenomics tool revealed the presence of a highly diverse group of microbes in the rhizosphere of C. bonducella; Chao.1 index (665), Shannon Weiner index (3.53), Simpson index (0.83) and Fisher index (106.13). The highly diverse microbes lingering around the roots of C. bonducella could possibly be due to a strong symbiotic association with the plant; root exudates nourish the microbes and the microbes in turn enrich the medicinal value of the plant.

Engineering prokaryotic regulator IrrE to enhance stress tolerance in budding yeast

BackgroundStress tolerance is one of the important desired microbial traits for industrial bioprocesses, and global regulatory protein engineering is an efficient approach to improve strain tolerance. In our study, IrrE, a global regulatory protein from the prokaryotic organism Deinococcus radiodurans, was engineered to confer yeast improved tolerance to the inhibitors in lignocellulose hydrolysates or high temperatures.ResultsThree IrrE mutations were developed through directed evolution, and the expression of these mutants could improve the yeast fermentation rate by threefold or more in the presence of multiple inhibitors. Subsequently, the tolerance to multiple inhibitors of single-site mutants based on the mutations from the variants were then evaluated, and 11 mutants, including L65P, I103T, E119V, L160F, P162S, M169V, V204A, R244G, Base 824 Deletion, V299A, and A300V were identified to be critical for the improved representative inhibitors, i.e., furfural, acetic acid and phenol (FAP) tolerance. Further studies indicated that IrrE caused genome-wide transcriptional perturbation in yeast, and the mutant I24 led to the rapid growth of Saccharomyces cerevisiae by primarily regulating the transcription level of transcription activators/factors, protecting the intracellular environment and enhancing the antioxidant capacity under inhibitor environments, which reflected IrrE plasticity. Meanwhile, we observed that the expression of the wild-type or mutant IrrE could also protect Saccharomyces cerevisiae from the damage caused by thermal stress. The recombinant yeast strains were able to grow with glucose at 42 ℃.ConclusionsIrrE from Deinococcus radiodurans can be engineered as a tolerance-enhancer for Saccharomyces cerevisiae. Systematic research on the regulatory model and mechanism of a prokaryotic global regulatory factor IrrE to increase yeast tolerance provided valuable insights for the improvements in microbial tolerance to complex industrial stress conditions.

Drought legacy affects microbial community trait distributions related to moisture along a savannah grassland precipitation gradient

Abstract Ecosystem models commonly use stable‐state assumptions to predict responses of soil microbial functions to environmental change. However, past climatic conditions can shape microbial functional responses resulting in a ‘legacy effect’. For instance, exposure to drier conditions in the field may shape how soil microbial communities respond to subsequent drought and drying and rewetting (DRW) events. We investigated microbial tolerance to low moisture levels (‘resistance’) and ability to recover after a DRW perturbation (‘resilience’) across a steep precipitation gradient in Texas, USA. Although differences in precipitation regime did not result in differences in resistance and resilience of soil microbes, microbial communities appeared to be generally resilient and resistant across the gradient, suggesting that frequent exposure to drought had characterised the trait distributions of microbial communities. Moreover, microbial communities from historically drier sites used carbon more efficiently during a DRW perturbation suggesting that long‐term drought history leaves a legacy effect on microbial functions. This may have been due to an indirect effect of drought caused via precipitation‐induced differences in primary productivity, influencing the availability of soil organic matter to microbes. Alternatively, different exposures to drought might have shaped the microbial ‘readiness’ to cope with the DRW disturbance. Microbial community composition was also linked to drought history, but was unrelated to variation in function. Synthesis. Exposure to drought can have both direct and indirect effects on soil microbial communities, which can result in lasting legacy effects on the functions they control.

Effect of microaeration on cell growth and glucose/xylose fermentation of Kluyveromyces marxianus from the imitate lignocellulosic-derived hydrolysate

The inhibitory effect of toxic byproducts mainly derived from the pretreatment of lignocellulosic biomass is one of the limiting factors hindering industrial application. In this work, a simple yet practicable aeration strategy was proposed to enhance microbial tolerance to inhibitors, including formic acid, acetic acid, furfural, and 5-hydroxymethyl furfural (5-HMF). Batch fermentation by Kluyveromyces marxianus CICC 1727−5 using xylose and glucose alone and in combination under different aeration conditions coupled with multiple inhibitors was performed. The fermentation time and residual xylose significantly decreased with high xylitol production (0.7 g/g) and high ethanol production (0.42 g/g) at an aeration rate of 0.5 vvm. The postglucose effect was effectively relieved during co-fermentation by the microaeration strategy. Transcriptomics analysis was performed to determine the mechanism of inhibitor resistance. Transcription of genes related to DNA replication, protein processing, carbon metabolism, biosynthesis of the ribosome, amino acids, peroxidase, sterols, and unsaturated fatty acids were significantly upregulated accompanied by downregulated expression of genes related to autophagy under aeration conditions in the presence of multiple inhibitors. These results provide a valuable reference for future studies on biofuel fermentation from lignocellulosic biomass by a facultative fermenting microbe.

Efficient recovery of metal tolerant fungi from the soil of industrial area and determination of their biosorption capacity

Due to various anthropogenic activities, different heavy metals like arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), mercury (Hg) and nickel (Ni) accumulate in the soils of industrial areas. In the current study, contaminated soil was collected from Yuepu industrial area of Shanghai, China. Soil analysis revealed the presence of Pb and Cu in high concentrations, ranging from 125.9–128.0 mg/kg and 98.4–100.2 mg/kg, respectively. These soil samples were further used to isolate metal tolerant microbes. Subsequent screening resulted in the isolation of four major fungal species including Fusarium fujikuroi, Fusarium solani, Trichoderma citronoviridae and Trichoderma reesei. Molecular characterization of isolated fungi was performed and amplified sequences were deposited in GenBank NCBI database. Metal tolerance and biosorption capacity of these fungal strains towards lead and copper were tested. Fungal strains were exposed to increasing concentrations (100–1000 ppm) of chlorides of lead and copper and the tolerance of the selected fungi was evaluated by measuring the minimum inhibitory concentrations (MIC). The range of MIC values was found to be 400 ppm to 1000 ppm. The tolerance index of all the tested fungi was calculated and T. citronoviridae was observed to be tolerant at 1 mM concentration of lead while F. solani was the most tolerant species at 1 mM concentration of copper. The highest biosorption capacity of Pb was exhibited by T. citronoviridae, while T. reesei showed the best absorption capacity of Cu followed by F. solani. Scanning electron microscopy (SEM) showed visible adsorption of metal on fungal mycelia and suggested it to be the mechanism of metal removal. On the basis of these findings, it could be concluded that T. citronoviridae and F. solani are the potential mycoremediation microbes in Pb and Cu contaminated soils, respectively.

Survival and growth of soil microbial communities under influence of sodium perchlorates

AbstractPreviously conducted space missions revealed the presence of perchlorates, which are known to have a high oxidizing potential in Martian regolith, at the level of 0.5%. Due to hygroscopic properties and crystallization features of perchlorate-containing solutions, assumptions leading to the possibility of the existence of liquid water in the form of brines, which can contribute to the vital activity of microorganisms, have been made. At the same time, high concentrations of perchlorates can inhibit the growth of microorganisms and cause their death. Previously performed studies have discovered the presence of highly diverse microbial communities in terrestrial perchlorate-containing soils and have also demonstrated the stability and activity of some prokaryotes cultured on highly concentrated perchlorates media (over 10%). Nevertheless, the limits of microbial tolerance to perchlorates and whether microbial communities are able to withstand the effects of high concentrations of perchlorates remain uncertain. The aim of this research was to study the reaction of microbial communities of hot-arid and cryo-arid soils and sedimentary rocks to the adding of a highly concentrated solution of sodium perchlorate (5%) in situ. An increase in the total number of prokaryotes, the number of metabolically active Bacteria and Archaea, and the variety of the consumed substrates were revealed. It was observed that in samples incubated with sodium perchlorate, a high taxonomic diversity of the microbial community is preserved at a level comparable to control sample. The study shows that the presence of high concentrations of sodium perchlorate (5%) in the soil does not lead to the death or significant inhibition of microbial communities.

Enhancing n-Butanol Tolerance of Escherichia coli by Overexpressing of Stress-Responsive Molecular Chaperones.

Microbial tolerance to organic solvents is critical for efficient production of biofuels. In this study, n-butanol tolerance of Escherichia coli JM109 was improved by overexpressing of genes encoding stress-responsive small RNA-regulator, RNA chaperone, and molecular chaperone. Gene rpoS, coding for sigma S subunit of RNA polymerase, was the most efficient in improving n-butanol tolerance of E. coli. The highest OD600 and the specific growth rate of JM109/pQE80L-rpoS reached 1.692 and 0.144h-1 respectively at 1.0% (v/v) n-butanol. Double and triple expression of molecular chaperones rpoS, secB, and groS were conducted and optimized. Recombinant strains JM109/pQE80L-secB-rpoS and JM109/pQE80L-groS-secB-rpoS exhibited the highest n-butanol tolerance, with specific growth rates of 0.164 and 0.165h-1, respectively. Membrane integrity, potentials, and cell morphology analyses demonstrated the high viability of JM109/pQE80L-groS-secB-rpoS. This study provides guidance on employing various molecular chaperones for enhancing the tolerance of E. coli against n-butanol.

ATP regulation strategy and its application in the synthesis of microbial metabolites

Cofactor engineering, as a new branch of metabolic engineering, mainly involves ATP/ADP, NADH/NAD⁺, NADPH/NADP⁺ and other cofactors. Cofactor engineering can maximize metabolic flow by directly regulating the concentration and form of the cofactor of key enzymes in cells, and quickly direct carbon flow to target metabolites. ATP, as an important cofactor, is involved in many enzyme-catalyzed reactions in microbial cells, and leads to the restriction of the distribution of metabolic pathways by connecting or linking them into a complex network. Therefore, ATP regulation strategy is expected to be a favorable tool for industrial strain modification, to improve the concentration and production capacity of target metabolites, strengthen microbial tolerance to the environment and promote substrate utilization rate. The present review focuses on the recently used effective ATP regulation strategies and the effects of ATP regulation on cell metabolism in order to provide references for the efficient construction of microbial cell factories.

Energy Generation in Single Chamber Microbial Fuel Cell from Pure and Mixed Culture Bacteria by Copper Reduction

In this study, a single chamber microbial fuel cell (SCMFC) with nano-polypyrrole modified graphite felt as anode- and platinum-coated carbon cloth as cathode was employed to reduce the amount of copper, Cu2+, in synthetic wastewater. The performance of the SCMFC was studied for two inocula, Shewanella putrefaciens and mixed culture. Maximum tolerable concentrations of 50 mg/L and 70 mg/L of copper and power densities of 0.33 W/m2 and 0.304 W/m2 were achieved, respectively, for the pure and mixed cultures. The study shows that microbial tolerance level towards the toxicity of metal strongly influences the MFC performance and the pure culture showed better performance than mixed culture over the Cu toxicity. This study exhibits the possibility of using single chamber microbial fuel cell for treating wastewater containing copper.

Dependence of arsenic resistance and reduction capacity of Aeromonas hydrophila on carbon substrate

The high toxicity and prevalence of arsenic in the environment have aroused increasing research interest in understanding the mechanisms of microbial arsenic resistance. A wide spectrum of arsenic resistant microbes with ability of arsenic bio-transformation has been isolated from arsenic-contaminated environments. However, arsenic resistance processes and reduction abilities of microbes under various growth conditions remain poorly understood. In this work, a high correlation between the arsenic resistance and reduction ability of Aeromonas hydrophila and the carbon substrate was identified. Genome analysis suggests that the arsenic resistance system is widely present in Aeromonas genus, and the arsenic resistance was associated with the ars operon. The sensitivity of A. hydrophila to As(V) and As(III) depended heavily on the type of carbon substrate. The upregulated expression of arsA, arsB, arsD and/or downregulated expression of glpF might be responsible for the increased microbial tolerance to As(III). The As(V) reduction rate was also affected by the type of carbon substrate. Our results provide new insights into the impacts of carbon substrate on the arsenic biotoxicity as well as arsenic biotransformation processes.