Survival Strategies of the Plant-Associated Bacterium Enterobacter sp. Strain EG16 under Cadmium Stress

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.

Improvement of cadmium phytoremediation by Centella asiatica L. after soil inoculation with cadmium-resistant Enterobacter sp. FM-1

This study examined the potential of a cadmium-resistant Enterobacter sp. FM-1 to promote plant growth and assist in cadmium accumulation in both mine-type C. asiatica L. and non-mine type C. asiatica L. tissues in highly cadmium-polluted soils. The results indicated that Enterobacter sp. FM-1 significantly promoted growth and alleviated metal toxicity in both types of C. asiatica L. Meanwhile, inoculation with Enterobacter sp. FM-1 in contaminated soil can increased cadmium bioavailability in soil. Furthermore, it will increase plant uptake and the accumulation of cadmium in C. asiatica L. leaves, stems and roots compared to that in an uninoculated plant. However, mine-type C. asiatica L. had better cadmium tolerance than the non mine-type C. asiatica L. Because of its native metal-tolerant ability, which could easily grow and proliferate, and had a better performance under cadmium-contamination conditions. Additionally, inoculation with Enterobacter sp. FM-1 significantly enhanced the bioaccumulation factor (BAF) and the translocation factor (TF) values in both types of C. asiatica L. even under high cadmium concentration soil condition. Hence, based on higher BAF and TF values and strong cadmium accumulation in the leaves and stems, we concluded that inoculation with Enterobacter sp. FM-1 is potentially useful for the phytoremediation of cadmium-contaminated sites by Centella asiatica L.

Alleviation of phytotoxic effects of cadmium on rice seedlings by cadmium resistant PGPR strain Enterobacter aerogenes MCC 3092

Heavy metal resistant PGPR mediated bioremediation, phytostimulation and stress alleviation is an eco-friendly method for sustainable agriculture in the metal contaminated soil. The isolation of such PGPR is highly demanding to reduce heavy metals in contaminated cultivated fields for agricultural benefit. The present study was successful to isolate a potent multi-heavy metal resistant PGPR strain, identified as Enterobacter aerogenes strain K6 based on MALDI-TOF MS, FAME analysis and 16S rDNA sequence homology, from rice rhizosphere contaminated with a variety of heavy metals/metalloid near industrial area. The strain exhibited high degree of resistance to Cd²⁺, Pb²⁺ and As³⁺ upto 4000 μg/mL, 3800 μg/mL and 1500 μg/mL respectively. Intracellular Cd accumulation of this strain was evidenced by AAS-SEM-TEM-EDX-XRF studies. Moreover, it showed several important PGP traits like IAA production, nitrogen fixation, phosphate solubilization, ACC deaminase activity even under high Cd stress (upto 3000 μg/mL). The combined effect of Cd resistance and PGP activities of this strain was manifested to the significant (p < 0.05) growth promotion of rice seedling under Cd stress by reducing oxidative stress (through antioxidants), stress ethylene and Cd uptake in seedlings. Thus K6 strain conferred Cd-tolerance in rice seedlings and could be applied as PGPR in contaminated fields.

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.

Copper and lead removal from aqueous solutions by bacterial consortia acting as biosorbents

A bacterial consortium was selected in the presence of Cu from sediment samples taken from Sepetiba Bay, Brazil, which is a site historically contaminated by metals. Bacteria were exposed to 0, 1, 6, 12.5, 25 and 50μg·mL(-1) Cu, Pb and Cu+Pb for 11days of bioassay. Results showed Alcanivorax dominance (81%) and cell counts of 10(8)cells·mL(-1). However, a reduction in dehydrogenase activity was observed from the fifth day of exposure for all Cu, Pb, and Cu+Pb concentrations tested. Esterase activity tended to increase, indicating higher energy demand to complete the bacterial lifecycle. Pb concentrations in the filtered culture medium (0.2μm) were below the detection limit, indicating biosorption, whereas concentrations of Cu were close to the tested concentrations, indicative of efflux. Results suggest the need for biomarkers, such as esterase and dehydrogenase enzymatic activity, in the assessment of resistance and tolerance of communities previously exposed to stressors.