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Curr Microbiol (2008) 56:236239
DOI 10.1007/s00284-007-9064-x
The cadA Gene in Cadmium-Resistant Bacteria from Cadmium-Polluted Soil in the Zhangshi Area of Northeast China
Yan Zhang Huiwen Zhang Xinyu Li
Zhencheng Su Chenggang Zhang
Received: 29 July 2007 / Accepted: 24 September 2007 / Published online: 5 January 2008 Springer Science+Business Media, LLC 2008
Abstract Cadmium-resistant bacteria were isolated from the farmland soil in Zhangshi Irrigation Area in Shenyang of Northeast China, an area has been polluted by heavy metals, especially cadmium, for more than 40 years. The cadA gene was detected in 4 Bacillus strains and for the rst time in one Flavobacterium strain. The high sequence identity (93%99%) of cadA gene, shared indels in different bacterial species and genera, and the phylogenetic incongruence between 16S rDNA gene tree and cadA gene tree suggested that lateral gene transfer (LGT) occurred among Bacillus and Flavobacterium spp. The LGT of cadA gene might play a vital role in promoting the spread of cadmium-resistant phenotypes throughout soil microbial communities.
Introduction
Soil heavy metals pollution gives a strong stress on soil microbes, resulting in a response of the increase of preexisted resistant bacteria and the appearance of new
resistant microbes via lateral transfer of resistant genes [3, 12]. Cadmium-resistance of soil bacteria is mediated by several genetic systems [9, 13], and the well-studied one is the cadmium efux system coded by cadmium-resistant gene cadA operon [10]. The homologues of cadA gene were reported to be widespread in Gram-positive bacteria such as Staphylococcus aureus, Bacillus rmus [4], Listeria monocytogenes [6] and Lactococcus lactis [7], and in Gram-negative bacterium Stenotrophomonas maltophilia [1].
The farmland soil in Zhangshi Irrigation Area in Shenyang of Northeast China has been polluted by heavy metals, especially by cadmium for more than 40 years [15]. In our previous study, the heavy metals in this soil seemed to have less toxic effects on soil microbial populations, enzymatic activities, and many other bioindicators, suggesting that the microbes in this soil could be already adapted to the severe environmental stress. The objectives of this study were to isolate and identify the cadmium-resistant bacteria in the farmland soil in Zhangshi Irrigation Area, and approach the distribution patterns of cadA gene in the isolated bacterial strains.
Materials and Methods
Soil Sampling and Isolation...