Lindane removal by pure and mixed cultures of immobilized actinobacteria

Potential and prospects of Actinobacteria in the bioremediation of environmental pollutants: Cellular mechanisms and genetic regulations

Increasing industrialization and anthropogenic activities have resulted in the release of a wide variety of pollutants into the environment including pesticides, polycyclic aromatic hydrocarbons (PAHs), and heavy metals. These pollutants pose a serious threat to human health as well as to the ecosystem. Thus, the removal of these compounds from the environment is highly important. Mitigation of the environmental pollution caused by these pollutants via bioremediation has become a promising approach nowadays. Actinobacteria are a group of eubacteria mostly known for their ability to produce secondary metabolites. The morphological features such as spore formation, filamentous growth, higher surface area to volume ratio, and cellular mechanisms like EPS secretion, and siderophore production in Actinobacteria render higher resistance and biodegradation ability. In addition, these bacteria possess several oxidoreductase systems (oxyR, catR, furA, etc.) which help in bioremediation. Actinobacteria genera including Arthrobacter, Rhodococcus, Streptomyces, Nocardia, Microbacterium, etc. have shown great potential for the bioremediation of various pollutants. In this review, the bioremediation ability of these bacteria has been discussed in detail. The utilization of various genera of Actinobacteria for the biodegradation of organic pollutants, including pesticides and PAHs, and inorganic pollutants like heavy metals has been described. In addition, the cellular mechanisms in these microbes which help to withstand oxidative stress have been discussed. Finally, this review explores the Actinobacteria mediated strategies and recent technologies such as the utilization of mixed cultures, cell immobilization, plant-microbe interaction, utilization of biosurfactants and nanoparticles, etc., to enhance the bioremediation of various environmental pollutants.

The Toxic Effects of Lindane via the Impact of pH and Dissolved Organic Matter on Microcystis aeruginosa

The toxic effects of Lindane (γ-BHC) on Microcystis aeruginosa were studied under lab culture conditions. Total protein levels, as well as malondialdehyde (MDA) levels and superoxide dismutase (SOD) enzyme activity, in algal cells, were determined after exposure to different concentrations of Lindane. The bioaccumulation of Lindane, as well as the influence of pH and dissolved organic matter (DOM) on the toxic effects, was also evaluated in algal cells. The growth of M. aeruginosa was inhibited by the Lindane treatment (96 h), resulting in a 50% maximal effect (EC50) concentration of 442 μg/L. In addition, the lowest observed effect concentration (LOEC) was found to be 120 μg/L; the no observed effect concentration (NOEC) was found to be 60 μg/L, and the maximum acceptable toxicant concentration (MATC) was 85 μg/L. With increasing concentrations of Lindane and exposure time, M. aeruginosa growth was significantly inhibited; in addition, the total protein levels and SOD activity significantly decreased. MDA concentration, however, showed an insignificant increase after 96 h. Lindane has the potential for bioaccumulation in algal cells with a bioconcentration factor (BCF) of 340. Furthermore, the toxic effects of Lindane on M. aeruginosa were influenced by environmental factors, such as pH and DOM. The toxic effects decreased with increasing pH and humic acid concentrations. Ultrastructure cell images were used to depict Lindane-induced apoptosis.

Microbe mediated remediation of dyes, explosive waste and polyaromatic hydrocarbons, pesticides and pharmaceuticals

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Environmental pollutants dyes, pesticides, pharmaceuticals, explosive waste and polyaromatic hydrocarbons.

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Environmental pollutants toxicity.

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Possible microbial biodegradation pathways of environmental pollutants.

Industrialization and human activities have led to serious effects on environment. With the progress taking place in the biodegradation field, it is important to summarize the latest advancement. In this review, we intend to provide insights on the recent progress on the biodegradation of environmental contaminants such as dyes, pesticides, pharmaceuticals, explosive waste and polyaromatic hydrocarbons by microorganisms. Along with the biodegradation of environmental contaminants, toxicity effects have also been discussed.

A review on pesticide degradation from irrigation water and techno-economic feasibility of treatment technologies

The present study focuses and assures the need for pesticide degradation from various water bodies used for irrigation and the available technologies to treat them effectively. A thorough review of the literature is done on pesticide residues present in various irrigation water sources like rivers, groundwater, river sediments, and soil which signifies the existence of pesticides in the ecosystem. This indicates the severity of water pollution due to various sources around and their adverse effect on the ecosystem. However, several technologies are available to treat these pesticides based on the classification. A Cross comparison between the technologies is done to determine the efficient technology for the treatment of irrigation water.

Evaluation of the sequential coupling of a bacterial treatment with a physicochemical process for the remediation of wastewater containing Cr and organic pollutants

A restoration strategy was developed for the treatment of two artificial liquid systems (Minimal Medium, MM, and Water Carbon Nitrogen, WCN) contaminated with Cr(VI), lindane (γ-HCH), phenanthrene (Phe), and reactive black 5 (RB5), through the use of an actinobacteria consortium, coupled with a physicochemical treatment using a column filled with nano-scale zero valent iron particles immobilized on dried Macrocystis pyrifera algae biomass. The Sequential Treatment A (ST_A: physicochemical followed by biological method) removed the three organic compounds with different effectiveness; however, it was very ineffective for Cr(VI) removal. The Sequential Treatment B (ST_B: biological followed by the physicochemical method) removed the four compounds with variable efficiencies. The removal of γ-HCH, Phe, and RB5 in both effluents did not present significant differences, regardless of the sequential treatment used. The highest removal of Cr(VI) and total Cr was observed in MM and WCN, respectively. Ecotoxicity tests (L. sativa) of the effluents treated with both methodological couplings demonstrated that the toxicity of WCN only decreased at the end of ST_A, while that of MM decreased at all stages of both sequential treatments. Therefore, MM would be more appropriate to perform both treatments.

Bioremediation of lindane contaminated soil: Exploring the potential of actinobacterial strains

Lindane, an organochlorine pesticide, causes detrimental impacts on the environment and human health owing to its high toxicity, low degradation, and bioaccumulation. Its toxic nature can be overcome by biological and eco-friendly approaches involving its degradation and detoxification. The biodegradation of lindane was assessed using actinobacterial species Thermobifida cellulosilytica TB100 (T. cellulosilytica), Thermobifida halotolerans DSM 44931 (T. halotolerans) and Streptomyces coelicolor A3 (S. coelicolor). The degradation conditions of Lindane such as pH, temperature, inoculum volume, glucose concentration and number of days were optimized under broth conditions. Lindane degradation at different concentrations was studied in soil using reverse phase-high performance liquid chromatography over a 30 day period. A bioassay test was performed on seeds of Lactuca sativa (Lettuce) to assess the success of bioremediated soil. Maximum lindane degradation in soil was observed using T. cellulosilytica sp. The degradation trend for different concentrations of lindane using T. halotolerans in sterilized soil was 55 mg kg^-1 (82%) ˃ 155 mg kg^-1 (75%) ˃ 255 mg kg^-1 (70%) after an incubation period of 30 days. Lindane degradation in soil followed the first order reaction kinetics. Phytotoxicity test on seeds of Lactuca sativa showed considerably good vigor index values for the bioremediated sterilized and non-sterilized soil by T. cellulosilytica, T. halotolerans and S. coelicolor in comparison to the contaminated soil without bacteria. This confirms that these actinobacterial species can be implemented in bioaugmentation of contaminated sites to efficiently remediate high lindane concentrations.

Modified zeolite-supported biofilm in service of pesticide biodegradation

The development of biofilms on modified natural zeolites was investigated with purpose to obtain biocomposites with biodegradation activity towards pesticides MCPA (2-methyl-4-chlorophenoxyacetic acid) and glyphosate (N-(phosphonomethyl)glycine) for potential application in bioaugmentation of polluted agricultural soils. Microbial communities were selected from agricultural pesticide-contaminated soil/water samples and enriched on the basis of their ability to biodegrade the pesticides. In order to enhance affinity of microbial communities to the support material, the natural mineral zeolite was modified by nontoxic environmentally friendly cations (Li⁺, Na⁺, K⁺, NH₄⁺, H⁺, Mg²⁺, Ca²⁺, Fe³⁺) by methods preserving its structure and characterised using powder XRD, surface area measurement and chemical composition analysis. Kinetics of pesticide degradation by the biocomposites was studied in liquid media. Results showed that according to zeolite modifications, the microbial activity and biodiversity changed. The best biodegradation rate of MCPA and glyphosate reached 0.12-0.13 mg/h with half-life of 16-18 h, which is considerably quicker than observed in natural environment. However, in some cases, biodegradation activity towards pesticides was lost which was connected to unfavourable zeolite modification and accumulation of toxic metabolites. High-throughput sequencing on the 16S rRNA genes of the biofilm communities highlighted the selection of bacteria genera known to metabolise MCPA (Aminobacter, Cupriavidus, Novosphingobium, Pseudomonas, Rhodococcus, Sphingobium and Sphingopyxis) and glyphosate (Pseudomonas). Altogether, results suggested that zeolites do not only have a passive role of biofilm support but also have protective and nutrient-supportive functions that consequently increase biodiversity of the pesticide degraders growing in the biofilm and influence the pesticide biodegradation rate.

Performance of a continuous stirred tank bioreactor employing an immobilized actinobacteria mixed culture for the removal of organophosphorus pesticides

In this study, we evaluated polyurethane foam (PF), volcanic rock (VR), and a modified plastic cap (MPC) as supports for the immobilization of organophosphorus (OP) pesticide-degrading actinobacterial strains. The colonization and activity of four streptomycetes were favoured by PF, which was selected as the carrier to use in a continuous stirred tank bioreactor (CSTR) that can be operated at increasing inflows of a pesticide mixture that contains the insecticides chlorpyrifos (CP) and diazinon (DZ). Our results demonstrate that the CSTR can be operated at flow rates of 10 and 40 mL h^-1 with greater than 85% removal of the pesticides in the short term. A significant decrease in the efficiency of CP removal was observed at the highest inflows into the reactor. The CP and DZ loading rates in the bioreactor ranged from 0.44 to 1.68 mg L^-1 h^-1 and from 0.50 to 2.17 mg L^-1 h^-1, respectively. Although the treated wastewater exhibited moderate toxicity for Raphanus sativus, a bioreactor inoculated with a mixed culture formed by Streptomyces spp. strains AC5, AC9, GA11 and ISP13 may provide an effective biotechnological strategy for the reduction of OP pesticide residues produced during agronomic and manufacturing practices and therefore prevent environmental pesticidal pollution.

Removal of oxyfluorfen from polluted effluents by combined bio-electro processes

In this work, the combination of biological and electrochemical processes to mineralize oxyfluorfen has been studied. First, an acclimatized mixed-culture biological treatment was used to degrade the biodegradable fraction of the pesticide, reaching up to 90% removal. After that, the non-biodegraded fraction was oxidised by electrolysis using boron-doped diamond as the anode. The results showed that the electrochemical technique was able to completely mineralize the residual pollutants. The study of the influence of the supporting electrolyte on the electrochemical process showed that the trace mineral solution used in the biological treatment was enough to completely mineralize the oxyfluorfen, resulting in total organic carbon removal rates that were well-fitted by a first-order model with a kinetic constant of 0.91 h^-1. However, the first-order degradation rate increased approximately 20% when Na₂SO₄ was added as supporting electrolyte, reaching a degradation rate of 1.16 h^-1 with a power consumption that was approximately 70% lower.

Reduction of silver nanoparticle toxicity affecting ammonia oxidation using cell entrapment technique

Occurrence of silver nanoparticles (AgNPs) in wastewater treatment systems could impact the ammonia oxidation (AO). This study investigated the reduction of AgNPs and dissociated silver ion (Ag⁺) toxicity on nitrifying sludge using cell entrapment technique. Three entrapment materials, including barium alginate (BA), polyvinyl alcohol (PVA), and a mixture of polyvinyl alcohol and barium alginate (PVA-BA), were applied. The BA beads provided the highest reduction of silver toxicity (up to 90%) and durability. Live/dead assays showed fatality of entrapped cells after exposure to AgNPs and Ag⁺. The maximum AO rate of the BA-entrapped cells was 5.6 mg-N/g-MLSS/h. The AO kinetics under the presence of silver followed an uncompetitive inhibition kinetic model. The experiments with AgNPs and Ag⁺ gave the apparent maximum AO rates of 4.2 and 4.8 mg-N/g-MLSS/h, respectively. The apparent half-saturation constants of the BA-entrapped cells under the presence of silver were 10.5 to 13.4 mg/L. Scanning electron microscopic observation coupled with energy-dispersive X-ray spectroscopy indicated no silver inside the beads. This elucidates that the silver toxicity can be reduced by preventing silver penetration through the porous material, leading to less microbial cell damage. This study revealed the potential of the entrapment technology for mitigating the effect of silver species on nitrification.

Combined treatment of contaminated soil with a bacterial Stenotrophomonas strain DXZ9 and ryegrass (Lolium perenne) enhances DDT and DDE remediation

Bioremediation of contaminated soils by a combinational approach using specific bacterial species together with ryegrass is a promising strategy, resulting in potentially highly efficient degradation of organic contaminants. The present study tested the combination of strain DXZ9 of Stenotrophomonas sp. with ryegrass to remove DDT and DDE contaminants from soil under natural conditions in a pot experiment. The strain DXZ9 was successfully colonized in the natural soil, resulting in removal rates of approximately 77% for DDT, 52% for DDE, and 65% for the two pollutants combined after 210 days. Treatment with ryegrass alone resulted in slightly lower removal rates (72 and 48%, respectively, 61% for both combined), while the combination of strain DXZ9 and ryegrass significantly (p < 0.05) improved the removal rates to 81% for DDT and 55% for DDE (69% for both). The half-life of the contaminants was significantly shorter in combined treatment with DXZ9 and ryegrass compared to the control. The remediation was mostly due to degradation of the contaminants, as the net uptake of DDT and DDE by the ryegrass accounted for less than 3% of the total amount in the soil. DDT is reductively dechlorinated to DDD and dehydrochlorinated to DDE in the soil; the metabolites of DDE and DDD were multiple undefined substances. The toxicity of the soil was significantly reduced as a result of the treatment. The present study demonstrates that the bioremediation of soil contaminated with DDT and DDE by means of specific bacteria combined with ryegrass is feasible.

Alginate immobilized enrichment culture for atrazine degradation in soil and water system

An atrazine degrading enrichment culture, a consortium of bacteria of genus Bacillus along with Pseudomonas and Burkholderia, was immobilized in sodium alginate and was used to study atrazine degradation in mineral salts medium (MSM), soil and wastewater effluent. Sodium alginate immobilized consortium, when stored at room temperature (24 ± 5°C), was effective in degrading atrazine in MSM up to 90 days of storage. The survival of bacteria in alginate beads, based on colony formation unit (CFU) counts, suggested survival up to 90 days and population counts decreased to 1/5^th on 120 days. Comparison of atrazine degrading ability of the freely suspended enrichment culture and immobilized culture suggested that the immobilized culture took longer time for complete degradation of atrazine as a lag phase of 2 days was observed in the MSM inoculated with alginate immobilized culture. The free cells resulted in complete degradation of atrazine within 6 days, while immobilized cells took 10 days for 100% atrazine degradation. Further, immobilized cultures were able to degrade atrazine in soil and wastewater effluent. Alginate beads were stable and effective in degrading atrazine till 3rd transfer and disintegrated thereafter. The study suggested that immobilized enrichment culture, due to its better storage and application, can be used to degrade atrazine in soil water system.

Bioaugmentation as a strategy for the remediation of pesticide-polluted soil: A review

Bioaugmentation, a green technology, is defined as the improvement of the degradative capacity of contaminated areas by introducing specific microorganisms, has emerged as the most advantageous method for cleaning-up soil contaminated with pesticides. The present review discusses the selection of pesticide-utilising microorganisms from various sources, their potential for the degradation of pesticides from different chemical classes in liquid media as well as soil-related case studies in a laboratory, a greenhouse and field conditions. The paper is focused on the microbial degradation of the most common pesticides that have been used for many years such as organochlorinated and organophosphorus pesticides, triazines, pyrethroids, carbamate, chloroacetamide, benzimidazole and derivatives of phenoxyacetic acid. Special attention is paid to bacterial strains from the genera Alcaligenes, Arthrobacter, Bacillus, Brucella, Burkholderia, Catellibacterium, Pichia, Pseudomonas, Rhodococcus, Serratia, Sphingomonas, Stenotrophomonas, Streptomyces and Verticillum, which have potential applications in the bioremediation of pesticide-contaminated soils using bioaugmentation technology. Since many factors strongly influence the success of bioaugmentation, selected abiotic and biotic factors such as pH, temperature, type of soil, pesticide concentration, content of water and organic matter, additional carbon and nitrogen sources, inoculum size, interactions between the introduced strains and autochthonous microorganisms as well as the survival of inoculants were presented.

Removal of a mixture of pesticides by a Streptomyces consortium: Influence of different soil systems

Although the use of organochlorine pesticides (OPs) is restricted or banned in most countries, they continue posing environmental and health concerns, so it is imperative to develop methods for removing them from the environment. This work is aimed to investigate the simultaneous removal of three OPs (lindane, chlordane and methoxychlor) from diverse types of systems by employing a native Streptomyces consortium. In liquid systems, a satisfactory microbial growth was observed accompanied by removal of lindane (40.4%), methoxychlor (99.5%) and chlordane (99.8%). In sterile soil microcosms, the consortium was able to grow without significant differences in the different textured soils (clay silty loam, sandy and loam), both contaminated or not contaminated with the OPs-mixture. The Streptomyces consortium was able to remove all the OPs in sterile soil microcosm (removal order: clay silty loam > loam > sandy). So, clay silty loam soil (CSLS) was selected for next assays. In non-sterile CSLS microcosms, chlordane removal was only about 5%, nonetheless, higher rates was observed for lindane (11%) and methoxychlor (20%). In CSLS slurries, the consortium exhibited similar growth levels, in the presence of or in the absence of the OPs-mixture. Not all pesticides were removed in the same way; the order of pesticide dissipation was: methoxychlor (26%)>lindane (12.5%)>chlordane (10%). The outlines of microbial growth and pesticides removal provide information about using actinobacteria consortium as strategies for bioremediation of OPs-mixture in diverse soil systems. Texture of soils and assay conditions (sterility, slurry formulation) were determining factors influencing the removal of each pesticide of the mixture.

Actinobacteria: Current research and perspectives for bioremediation of pesticides and heavy metals

Actinobacteria exhibit cosmopolitan distribution since their members are widely distributed in aquatic and terrestrial ecosystems. In the environment they play relevant ecological roles including recycling of substances, degradation of complex polymers, and production of bioactive molecules. Biotechnological potential of actinobacteria in the environment was demonstrated by their ability to remove organic and inorganic pollutants. This ability is the reason why actinobacteria have received special attention as candidates for bioremediation, which has gained importance because of the widespread release of contaminants into the environment. Among organic contaminants, pesticides are widely used for pest control, although the negative impact of these chemicals in the environmental balance is increasingly becoming apparent. Similarly, the extensive application of heavy metals in industrial processes lead to highly contaminated areas worldwide. Several studies focused in the use of actinobacteria for cleaning up the environment were performed in the last 15 years. Strategies such as bioaugmentation, biostimulation, cell immobilization, production of biosurfactants, design of defined mixed cultures and the use of plant-microbe systems were developed to enhance the capabilities of actinobacteria in bioremediation. In this review, we compiled and discussed works focused in the study of different bioremediation strategies using actinobacteria and how they contributed to the improvement of the already existing strategies. In addition, we discuss the importance of omic studies to elucidate mechanisms and regulations that bacteria use to cope with pollutant toxicity, since they are still little known in actinobacteria. A brief account of sources and harmful effects of pesticides and heavy metals is also given.

Bioaugmentation potential of free and formulated 2,6-dichlorobenzamide (BAM) degrading Aminobacter sp. MSH1 in soil, sand and water

Pesticides are used extensively worldwide, which has led to the unwanted contamination of soil and water resources. Former use of the herbicide 2,6-dichlorobenzonitrile (dichlobenil) has caused pollution of ground and surface water resources by the stable degradation product 2,6-dichlorobenzamide (BAM) in several parts of Europe, which has resulted in the costly closure of several drinking water wells. One strategy for preventing this in future is bioaugmentation using bacterial degraders. BAM-degrading Aminobacter sp. MSH1 was therefore formulated into dried beads and tests undertaken to establish their potential for use in the remediation of polluted soil, sand and water. The formulation procedure included freeze drying, combined with trehalose addition for cell wall protection, thus ensuring a high amount of viable cells following prolonged storage at room temperature. The beads were round-shaped pellets with a diameter of about 1.25 mm, a dry matter content of approximately 95 % and an average viable cell content of 4.4 × 10(9) cells/g bead. Formulated MSH1 cells led to a similar, and frequently even faster, BAM mineralisation (20-65 % (14)CO2 produced from (14)C-labelled BAM) in batch tests conducted with sand, water and different soil moisture contents compared to adding free cells. Furthermore, the beads were easy to handle and had a shelf life of several months.

Versatility of Streptomyces sp. M7 to bioremediate soils co-contaminated with Cr(VI) and lindane

The aim of this work was to study the impact of environmental factors on the bioremediation of Cr(VI) and lindane contaminated soil, by an actinobacterium, Streptomyces sp. M7, in order to optimize the process. Soil samples were contaminated with 25 µg kg(-1) of lindane and 50 mg kg(-1) of Cr(VI) and inoculated with Streptomyces sp. M7. The lowest inoculum concentration which simultaneously produced highest removal of Cr(VI) and lindane was 1 g kg(-1). The influence of physical and chemical parameters was assessed using a full factorial design. The factors and levels tested were: Temperature: 25, 30, 35°C; Humidity: 10%, 20%, 30%; Initial Cr(VI) concentration: 20, 50, 80 mg kg(-1); Initial lindane concentration: 10, 25, 40 µg kg(-1). Streptomyces sp. M7 exhibited strong versatility, showing the ability to bioremediate co-contaminated soil samples at several physicochemical conditions. Streptomyces sp. M7 inoculum size was optimized. Also, it was fitted a model to study this process, and it was possible to predict the system performance, knowing the initial conditions. Moreover, optimum temperature and humidity conditions for the bioremediation of soil with different concentrations of Cr(VI) and lindane were determined. Lettuce seedlings were a suitable biomarker to evaluate the contaminants mixture toxicity. Streptomyces sp. M7 carried out a successful bioremediation, which was demonstrated through ecotoxicity test with Lactuca sativa.

Mixed culture fermentation and media optimization by response surface model: Streptomyces and Brachybacterium species in bioflocculant production

The biofloculant production potential of a consortium of Streptomyces and Brachybacterium species were evaluated. Optimum bioflocculant yields (g/L) and flocculation activities (%) were observed for the following preferred nutritional sources: glucose (56%; 2.78 ± 0.15 g/L), (NH₄)₂NO₃ (53%; 2.81 ± 0.37 g/L) and CaSO₄·H₂O (47%; 2.19 ± 0.13 g/L). A Plackett-Burman design revealed the critical fermentation media components. The concentrations of these components were optimized [glucose; 16.0, (NH₄)₂NO₃; 0.5 and CaSO₄·H₂O; 1.2 (g/L)] through a central composite design with optimum bioflocculant yield of 3.02 g/L and flocculation activity of 63.7%. The regression coefficient (R² = 0.6569) indicates a weak estimation of the model’s adequacy and a high lack-of-fit value (34.1%). Lack of synergy in the consortium may have been responsible for the model inadequacy observed. FTIR spectrometry showed the bioflocculant to be a heteropolysaccharide, while SEM imaging revealed an amorphous loosely arranged fluffy structure with interstial spacing of less than 1 µm.

Removal of lindane from an aqueous solution by using aminopropyl silica gel-immobilized calix[6]arene

An aminopropyl silica gel-immobilized calix[6]arene (C[6]APS) has been used for the removal of lindane from an aqueous solution in batch sorption technique. The C[6]APS was synthesized with p-tert-butylcalix[6]arene hexacarboxylate derivative and aminopropyl silica gel in the presence of N,N'-diisopropyl carbodiimide coupling reagent. The sorption study was carried out as functions of solution pH, contact time, initial lindane concentration, C[6]APS dosage and ionic strength of solution. The matrix effect of natural water samples on the sorption efficiency of C[6]APS was also investigated. Maximum lindane removal was obtained at a wide pH range of 2-8 and sorption equilibrium was achieved in 2h. The isotherm analysis indicated that the sorption data can be represented by both Langmuir and Freundlich isotherm models. Increasing ionic strength of the solutions increased the sorption efficiency and matrix of natural water samples had no effect on the sorption of lindane. By using multilinear regression model, regression equation was also developed to explain the effects of the experimental variables.

Enhanced removal of a pesticides mixture by single cultures and consortia of free and immobilized Streptomyces strains

Pesticides are normally used to control specific pests and to increase the productivity in crops; as a result, soils are contaminated with mixtures of pesticides. In this work, the ability of Streptomyces strains (either as pure or mixed cultures) to remove pentachlorophenol and chlorpyrifos was studied. The antagonism among the strains and their tolerance to the toxic mixture was evaluated. Results revealed that the strains did not have any antagonistic effects and showed tolerance against the pesticides mixture. In fact, the growth of mixed cultures was significantly higher than in pure cultures. Moreover, a pure culture (Streptomyces sp. A5) and a quadruple culture had the highest pentachlorophenol removal percentages (10.6% and 10.1%, resp.), while Streptomyces sp. M7 presented the best chlorpyrifos removal (99.2%). Mixed culture of all Streptomyces spp. when assayed either as free or immobilized cells showed chlorpyrifos removal percentages of 40.17% and 71.05%, respectively, and for pentachlorophenol 5.24% and 14.72%, respectively, suggesting better removal of both pesticides by using immobilized cells. These results reveal that environments contaminated with mixtures of xenobiotics could be successfully cleaned up by using either free or immobilized cultures of Streptomyces, through in situ or ex situ remediation techniques.

Bacterial bio-resources for remediation of hexachlorocyclohexane

In the last few decades, highly toxic organic compounds like the organochlorine pesticide (OP) hexachlorocyclohexane (HCH) have been released into the environment. All HCH isomers are acutely toxic to mammals. Although nowadays its use is restricted or completely banned in most countries, it continues posing serious environmental and health concerns. Since HCH toxicity is well known, it is imperative to develop methods to remove it from the environment. Bioremediation technologies, which use microorganisms and/or plants to degrade toxic contaminants, have become the focus of interest. Microorganisms play a significant role in the transformation and degradation of xenobiotic compounds. Many Gram-negative bacteria have been reported to have metabolic abilities to attack HCH. For instance, several Sphingomonas strains have been reported to degrade the pesticide. On the other hand, among Gram-positive microorganisms, actinobacteria have a great potential for biodegradation of organic and inorganic toxic compounds. This review compiles and updates the information available on bacterial removal of HCH, particularly by Streptomyces strains, a prolific genus of actinobacteria. A brief account on the persistence and deleterious effects of these pollutant chemical is also given.