Thirty six chromium resistant bacterial strains were isolated from tanneries effluent (CrT-I, CrT-2, CrT-3, CrT-4, CrT-5, CrT-6, CrT-7, CrT-8, CrT-9, CrT-l0, CrT11, CrT-12, CrT-13, CrT-14, CrT-15, CrT-16, CrT-17) electroplating effluent (CrM-1, CrM-2, CrM-3, CrM-4, CrM-5, CrM-6, CrM-7, CrM-8, CrM-9, CrM-l1, CrM-l1, CrM-12, CrM-13, CrM-14) and from chromium polluted soil (S-3, S-4, S-6, S-7 and S-8) samples. All the s trains could resist very high concentrations of hexavalent chromium both in rich (20 to 60 mg m1-1 K2CrO4) as well as minimal (2 to 10 m1-1 K2CrO4) medium. Although strains showed some difference in resistance to different metallic salts and antibiotics, but all of them exhibited multiple resistance to metals (MnSO4, CU SO4, HgC12, Zn SO4, Ni SO4, CrCl3, CoC12 and Pb (NO3)2) and antibiotics (ampicillin, kanamycin, streptomycin, chloramphenicol and tetracycline). On the basis of different morphological and biochemical characterization strains CrM-1, S-6 were affiliated with family Bacillaceae; CrM-5, CrM-6, S-7, S-8 with Micrococaccae; CrT-13, Brevibacteriaceae; CrM2, CrM-3, CrM-11, CrM-12, CrM-14 to Enterobacteriaceae and -12, CrT-13, CrT-I4, CrT-IS, CrT-16, CrT-17, CrM-4, CrM-5, CrM-6, CrM-7, CrM-CrT-1, CrT-2, CrT-3, CrT-4, CrT-5, CrT-6, CrT-7, CrT-8, CrT-9, CrT-l0, CrT-11, CrT8, CrM-9, CrM-l0 and CrM-13 shared most of their characteristics with family Pseudomonadaceae. All the strains have ability to take .up chromate both by active and passive pathways. In majority of the strains optimum chromium (VI) reduction was observed at pH 7 and 37°C. They also reduced handsome amount of chromium (VI) even at very high initial chromium (VI) concentration (1000 µg m1-1 K2CrO4). These observations revealed that increase in size of inoculums (four times) did not lead to proportionate reduction of chromate and in majority of strains the amount of Cr(VI) reduced with 9.6x 107 cells m1-1 was slightly more as compared to 2.4x 107 cells m1-1. The rate of chromium (VI) reduction increased when culture were agitated 150 rpm in an incubating shaker as compared to stationary cultures. Salts of different heavy metals (Ni, 200 µg m1-1; Mn, 200 µg m1-1; Zn, 200 µg m1-1; Cu, 200 µg m1-1; Co, 50 µg m1-1 and Ag, 50 µg m1-1) did not appear to have any significant effects on t he chromium reduction potential of these strains. All the strains have capacity to reduce toxic chromium (VI) into chromium (III) present in industrial effluent samples directly. Not only bacterial cell but supernatant obtained from these cultures was also able to reduce chromium (VI) to chromium (III). Under chromium stress, different growth (seed germination, seedlings length leaf area, fresh weight, dry weight per gram fresh weight) and biochemical parameters (protein content, auxin content, acid phosphatase and chromium content) of all three crops (Triticum aestivum, Helimzthus aannuus and Vigna radiata) were severely affected. Majority of bacterial inoculations resulted in an increment in all the growth as well as biochemical parameters in these three economically important crops as compared to their respective non-inoculated control. In pots experiments, in all the three crops (Triticum aestivum, Helianthu annuus and Vigna radiata) chromium resistant bacterial strains (especially CrT-1, CrT-4, CrT-6, CrM-l and S-6) significantly increased the growth and yield parameters not only in the chromium free but also in the presence of both trivalent (300 µg g-1 CrCl3 as well as hexavalent chromium (300 µg g-1 K2CrO4) stresses. Therefore, the presence of chromium resistant bacteria appears to promote root growth and enhance hexavalent chromium removal from solution by protecting roots from hexavalent chromium toxicity through a process of Cr(VI) reduction in to the Cr(III) near the root surface. Application of hexavalent chromium caused disintegration of cells/tissues in root, shoot and leaves of Triticum a estivum, Helianthus annuus and Vigna radiata. But in bacterized seedlings the toxic effects of hexavalent chromium were less obvious because bacterial strains give some alleviation to the plants by decreasing the availability of toxic Cr (VI) to plants roots. Hence in addition to others factors contributing by the bacterial strains and chromium salts, it appears that auxin production in plants was stimulated the by these bacterial strains which caused some modification in the internal morphology of the different tissues of T. aestivum, H. annuus and V. radiata seedlings both under control and under chromium stress conditions. Due to chromium stress especially Cr(Vl) much reduction in the length parameters was observed leading to stunted growth which caused some increment in width of cells/tissues.
Bacterial strains especially (CrT-1, CrT-l3, CrM-l, CrM-3 and S-6) were very efficient in the reduction of hexavalent chromium into trivalent chromium and this activity extenuated in the presence of Eichonzia crassipes plants. Among these three chromium resistant bacteria CrT-1, CrT -13 and S -6 were selected for 16S rRNA gene sequence analysis. Strains CrT -1, CrT -13 and S-6 were isolated from the effluent of tanneries and from chromium contaminated soil. All the three strains were very efficient in the uptake and reduction of toxic hexavalent chromium. Beside this strains were also able to improve the growth of different crops (Triticum aestivum, Helianthus annuus and Vigna radiata) efficiently as compared to others strains from the respective source. In the present study the resistance level of all three strains Ochrobactrum intermedium CrT-1, sp. CrT-13 and Bacillus cereus S-6 is much high as compared to the others strains from the same source. Plant growth promoting effect of these strains was also better as compared to others strains. Nevertheless CrM-1 and CrM-3 also proved to be efficient chromate reducers and also inhibit growth promoting activity. Their molecular identification is targeted in future. Hence these strains could easily utilized in the bioremediation of chromium contaminated waste waters/soil as well as plant growth promoting tool for various economically important cash crops.