Abstract
Cadmium is a natural heavy-metal element. It is highly toxic and a vital industrial pollutant to the environment. In order to survive in heavy-metal polluted environments, some bacteria can withstand high concentrations of heavy metals in environment due to their specific mechanisms, including the transport of heavy metal ions out of the cell. Three kinds of membrane-bound-proteins are known to participate in this transport. The objective of this study was to determine the cadmium-resistant properties and possible mechanism among strains of Cupriavidus metallidurans (= Ralstonia eutropha), C. Taiwanensis (=R. Taiwanensis) and Ralstonia solanacearum isolated from soil and plants in Taiwan. Strains tested include six strains, rcd 6, 8, 19 (C. metallidurans) and rcd 12, 14, 21 (C. Taiwanensis), from cadmium-polluted soil, seventeen strains, nod 1 to nod 5 and nod s1 to s12 (C. Taiwanensis) from root nodules of two Mimosa species (M. pudica and M. diplotricha), and ten strains of R. solanacearum isolated from different diseased plants. Sequence analysis of 16S-23S rDNA ITS and nifH gene regions were used to confirm the taxonomic classification of these bacteria. However, the 624-bp PCR product of nifH gene was only amplified from strains nod 1 to nod 3 and nod s1 to s12, but not from other tested strains of Cupriavidus and Ralstonia spp. These tested strains could tolerate cadmium within the range from 0.67 to 6.70 mM. The gene, czcC, thought to encode the outer membrane factor (OMF) of czc efflux system, was found in fifteen tested strains (rcd 6, 8, 12, 14, 19, 21; nod s1 to s6, s9, s11; PSS161) by PCR. Especially, strains rcd 12, 14, and 21 which could tolerate higher cadmium concentration (6.70 mM) harbored the entire czc operon. In addition, the PCR products revealed that cadmium-tolerant strains contained at least a portion of the czc operon and efflux mechanism was confirmed by cadmium uptake test. The results here indicated that cadmium resistant capability in Cupriavidus spp. and R. solanacearum was related to the presence of czcC or czc operon.
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