Characterization of a Cd2+-Resistant Strain of Ochrobactrum sp. Isolated from Slag Disposal Site of an Iron and Steel Factory

Exopolysaccharides and indole-3-acetic acid producing Bacillus safensis strain FN13 potential candidate for phytostabilization of heavy metals

Microbial population of soils irrigated with industrial wastewater may contain certain exopolysaccharides (EPS) and indole-3-acetic acid (IAA) producing bacterial strains having the ability to tolerate heavy metals along with plant growth-promoting (PGP) traits. As cadmium is one of the most toxic heavy metals for soils, plants, animals, and human beings, the present study was planned to isolate and characterize EPS- and IAA-producing, Cd-tolerant bacterial strains having tolerance against heavy metals along with plant growth-promoting traits. A total of 30 rhizobacterial strains (FN1-FN30) were isolated from rhizosphere soil collected from fields around industrial areas and roadsides irrigated with industrial wastewater. Out of these, eight isolates with the combined ability of IAA production and EPS production were characterized for PGP traits. On the basis of multifarious PGP traits and the results of root colonization assay, three most efficient EPS- and IAA-producing, Cd-tolerant plant growth-promoting strains, i.e., FN13, FN14, and FN16, were selected for multiple metal (Cd, Pb, Ni, and Cu) tolerance test along with quantification of growth, and IAA and EPS production abilities under Cd stress. Increasing levels of Cd stress negatively affected the tested characteristics of these strains, but FN13 showed more stability in growth, IAA production (18.24 μg mL^-1), and EPS production (148.99 μg mL^-1) compared to other strains under Cd stress. The morphological and biochemical analysis confirmed FN13 as Gram-positive, rod-shaped bacteria with smooth colonies of yellow appearance. The strain FN13 has strong root colonization (3.36 × 10⁶ CFU g^-1) ability for mustard seedlings and can solubilize Zn and phosphate along with the production of HCN, ammonia, and siderophores. The 16S rRNA sequencing confirmed it as the Bacillus safensis strain FN13. It can be explored as potential phytostabilizing biofertilizer for heavy metal-contaminated soils.

Perspectives regarding the use of metallurgical slags as secondary metal resources - A review of bioleaching approaches

Smelting activity by its very nature produces large amounts of metal-bearing waste, often called metallurgical slag(s). In the past, industry used to dispose of these waste products at dumping sites without the appropriate environmental oversight. Once there, ongoing biogeochemical processes affect the stability of the slags and cause the release of metallic contaminants. Rather than viewing metallurgical slags as waste, however, such deposits should be viewed as secondary metal resources. Metal bioleaching is a "green" treatment route for metallurgical slags, currently being studied under laboratory conditions. Metal-laden leachates obtained at the bioleaching stage have to be subjected to further recovery operations in order to obtain metal(s) of interest to achieve the highest levels of purity possible. This perspective paper considers the feasibility of the reuse of base-metal slags as secondary metal resources. Special focus is given to current laboratory bioleaching approaches and associated processing obstacles. Further directions of research for development of more efficient methods for waste slag treatment are also highlighted. The optimized procedure for slag treatment is defined as the result of this review and should include following steps: i) slag characterization (chemical and phase composition and buffering capacity) following the choice of initial pH, ii) the choice of particle size, iii) the choice of the liquid-to-solid ratio, iv) the choice of microorganisms, v) the choice of optimal nutrient supply (growth medium composition). An optimal combination of all these parameters will lead to efficient extraction and generation of metal-free solid residue.

Bioaccumulation of cadmium by Enterobacter sp. and enhancement of rice seedling growth under cadmium stress

Bacteria-mediated plant growth promotion and bioremediation of heavy metal containing soil is a widely accepted eco-friendly method. The present study is aimed to screen out cadmium resistant bacterial strain from metal contaminated rice rhizosphere and evaluate its effects on the growth of rice seedlings under cadmium stress. Among four different isolates (designated as S1, S2, S3 and S5), the S2 isolate was screened on the basis of different PGP traits and multi heavy metal resistance (minimum inhibitory concentration for cadmium, lead and arsenic were 3500, 2500 and 1050 µg/ml respectively). The selected S2 strain has ability to produce ACC deaminase (236.11 ng α-keto-butyrate/mg protein/h), IAA (726 µg/ml), solubilize phosphate (73.56 ppm) and fix nitrogen (4.4 µg of nitrogen fixed/h/mg protein). The selected strain was identified as Enterobacter sp. on the basis of phenotypic characterization, MALDI-TOF MS analysis of ribosomal proteins, FAME analysis and 16 S rDNA sequence homology. The high cadmium removal efficiency (> 95%) of this strain from the growth medium was measured by Atomic Absorption Spectrophotometer and it was due to intracellular cadmium accumulation evidenced by SEM-EDX-TEM-EDX study. SEM analysis also revealed no distortion of surface morphology of this strain even grown in the presence of high cadmium concentration (3000 µg/ml). Inoculation of this strain with rice seedlings significantly enhanced various morphological, biochemical characters of seedling growth compared with un-inoculated seedlings under Cd stress. The strain also exhibited alleviation of cadmium-induced oxidative stress, reduction of stress ethylene and decreased the accumulation of cadmium in seedlings as well that conferred cadmium tolerance to the plant. Thus the S2 strain could be considered as a potent heavy metal resistant PGPR applicable in heavy metal contaminated agricultural soil for bioremediation and plant growth promotion as well.

MAIN FINDING

A cadmium resistant plant growth promoting Enterobacter sp. was isolated that accumulated cadmium evidenced by SEM-TEM-EDX study. It reduced Cd uptake and enhanced growth in rice seedlings.

Characterization of Cd-resistant Klebsiella michiganensis MCC3089 and its potential for rice seedling growth promotion under Cd stress

Application of heavy metal resistant plant growth promoting rhizobacteria has an important role as they help to evade metal-induced toxicity in plants on one hand and enhance plant growth on the other. The present study is therefore focused on the characterization of a cadmium resistant bacterial strain isolated from heavy metal contaminated rhizospheric soil designated as S8. This S8 strain was selected in terms of cadmium resistance and plant growth promoting traits. Moreover, it also showed resistance to lead and arsenic to a considerable extent. The selected strain S8 was identified as Klebsiella michiganensis by modern approaches of bacterial taxonomy. The plant growth promoting traits exhibited by the strain include 1-aminocyclopropane-1-carboxylic acid deaminase activity (58.33 ng α-keto butyrate/mg protein/h), Indole-3-acetic acid production (671 μg/ml), phosphate solubilization (71.98 ppm), nitrogen fixation (3.72 μg of nitrogen fixed/h/mg protein) etc. Besides, the strain also exhibited high cadmium removal efficiency (73-97%) from the medium and intracellular accumulation as well. Its efficiency to alleviate cadmium-induced toxicity was determined against a rice cultivar in terms of morphological and biochemical changes. Enhanced growth and reduced oxidative stress were detected in presence of the bacterium. On the basis of these results, it can be concluded that K. michiganensis strain S8 is cadmium accumulating plant growth promoting rhizobacterium that can be applied in cadmium contaminated agricultural soil to achieve better productivity of rice.

Characterization of cadmium-resistant Klebsiella pneumoniae MCC 3091 promoted rice seedling growth by alleviating phytotoxicity of cadmium

Cadmium (Cd) phytotoxicity in agricultural land is a major global concern now-a-days resulting in very poor yield. Plant growth-promoting rhizobacteria (PGPR)-mediated bioremediation is one of the convenient strategies for detoxification of Cd from the soil and for plant growth promotion under Cd stress. The selected strain K5 was identified as Klebsiella pneumoniae based on MALDI-TOF MS ribosomal protein and 16S rDNA sequence-based homology. The strain possessed several PGP traits viz. IAA production (3413 μg/mL), phosphate solubilization (80.25 ppm), ACC deaminase activity (40 ng α-ketobutyrate/mg protein/h), N₂ fixation ability (1.84 μg N₂ fixed/h), etc. and has the highest Cd resistance (4000 μg/mL) among Cd-resistant PGPR so far reported. This strain efficiently accumulated Cd and remained viable under Cd stress as confirmed by AAS-TEM-EDX analysis and viability test, respectively. The significant (p < 0.05) positive effect of the strain was reflected in various plant growth parameters like increased seed germination (50 to 90%), root length (5-fold), shoot length (about 2-fold), root fresh weight (> 2-fold), and shoot fresh weight (1.23-fold) under Cd stress compared with uninoculated set. Moreover, the positive impact of this strain on antioxidant enzyme activity (CAT, MDA, SOD) and several other biochemical parameters (proline, α-amylase, protease, total sugar, total protein, chlorophyll content) were also measured that favors plant growth promotion under Cd stress. Besides, the K5 strain also decreased stress-ethylene level under Cd stress and reduction of Cd accumulation in seedling (> 1.5-fold) was conducive to alleviate Cd phytotoxicity. Hence, K. pneumoniae strain K5 can be used as a phytostimulating and Cd-bioremediating biofertilizer for sustainable agriculture in heavy metal-contaminated sites.

Niches, population structure and genome reduction in Ochrobactrum intermedium: clues to technology-driven emergence of pathogens

Ochrobactrum intermedium is considered as an emerging human environmental opportunistic pathogen with mild virulence. The distribution of isolates and sequences described in literature and databases showed frequent association with human beings and polluted environments. As population structures are related to bacterial lifestyles, we investigated by multi-locus approach the genetic structure of a population of 65 isolates representative of the known natural distribution of O. intermedium. The population was further surveyed for genome dynamics using pulsed-field gel electrophoresis and genomics. The population displayed a clonal epidemic structure with events of recombination that occurred mainly in clonal complexes. Concerning biogeography, clones were shared by human and environments and were both cosmopolitan and local. The main cosmopolitan clone was genetically and genomically stable, and grouped isolates that all harbored an atypical insertion in the rrs. Ubiquitism and stability of this major clone suggested a clonal succes in a particular niche. Events of genomic reduction were detected in the population and the deleted genomic content was described for one isolate. O. intermedium displayed allopatric characters associated to a tendancy of genome reduction suggesting a specialization process. Considering its relatedness with Brucella, this specialization might be a commitment toward pathogenic life-style that could be driven by technological selective pressure related medical and industrial technologies.