In this study we performed a phylogenetic analysis of a culturable bacterial community isolated from heavymetal-contaminated soil from southwest Slovakia using 16S rRNA (16S rDNA) and heavy-metal resistance genes. The soil sample contained high concentrations of nickel (2,109 mg/kg), cobalt (355 mg/kg) and zinc (177 mg/kg), smaller concentrations of iron (35.75 mg/kg) and copper (32.2 mg/kg), and a trace amount of cadmium (<0.25 mg/kg). A total of 100 isolates were grown on rich (Nutrient agar No. 2) or minimal (soil-extract agar medium) medium. The isolates were identified by phylogenetic analysis using partial sequences of their 16S rRNA (16S rDNA) genes. Representatives of two broad taxonomic groups, Firmicutes and Proteobacteria, were found on rich medium, whereas four taxonomic groups, Actinobacteria, Bacteroidetes, Firmicutes and Proteobacteria, were represented on minimal medium. Forty-two isolates grown on rich medium were assigned to 20 bacterial species, while 58 bacteria grown on minimal medium belonged to 49 species. Twenty-three isolates carried czcA-and/or nccA-like heavy-metal-resistance determinants. The heavy-metalresistance genes of nine isolates were identified by phylogenetic analysis of their protein sequences.
Cells of Escherichia coli increase greatly the synthesis of a small primarily cytoplasmic protein as soon as the cell growth rate falls below the maximal growth rate supported by cadmium exposure, after which the mature product is exported to the periplasm. This protein was previously identified as the product of the E. coli yodA open reading frame. We now report the isolation of an E. coli mutant defective in YodA synthesis because of insertional inactivation of the corresponding gene. In experiments to test the ability of both the wild-type and yodA mutant E. coli cells to bind cadmium, we have used gamma-labeled [(109)Cd]. Whereas the wild-type E. coli strain was able to bind metal, the yodA mutant strain failed to do so. In addition, analysis of such a mutant demonstrated that it grows at a rate distinguishable from that of the isogenic parent in the presence of cadmium ions. However, challenging cells with hydrogen peroxide and additional metals such as zinc, copper, cobalt, and nickel did not significantly affect the growth rate of the mutant. This growth phenotype was found to be the result of the loss of its ability to bind cadmium. These results suggest that the role of YodA protein might be to decrease the concentration level of cadmium ions in E. coli cells during cadmium stress by its ability to bind heavy metal.
This research work was oriented to outlining the diversity of Gram-negative culturable portion of the bacterial community in three fruit plants rhizosphere. Rhizosphere samples were taken from European chestnut (Castanea sativa Mill), true service tree (Sorbus domestica L.) and cornelian cherry (Cornus mas L.) plants. Experiments were conducted for three years during the vegetation period, and the bacterial community structure was assessed with cultivation-dependent approach. Many Gram-negative isolates (n = 251) from the rhizosphere survived sub culturing and were identified by biochemical tests. A total of 57 species belonging to 29 genera were identified and assigned to four broad taxonomic groups (Bacteroidetes, Alpha-, Beta-and Gamma-proteobacteria). Several specific bacterial cluster communities were identified inside all the three rhizospheres. Most of the species belonged to the genera Moraxella, Pseudomonas, Pantoea, Enterobacter and Acinetobacter. In addition, while, using the plate count analysis, large discrepancies in numbers among physiological groups of bacteria cultured from three rhizosphere samples have not been revealed, more expressive distinctions among bacterial populations were obtained concerning the relative abundance of different genera, different taxonomic groups as well as different diversity indices. Furthermore, the number of cultured bacteria and their taxonomic distribution in the rhizosphere of all three plants changed not only explicitly during vegetation period but continually during the three years of investigation. It seems that rhizosphere bacterial populations of each plant are under the influence of the specific rootreleased materials.
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