Cadmium resistance mechanisms of a functional strain Enterobacter sp. DNB-S2, isolated from black soil in Northeast China

Enterobacter sp. DNB-S2 is an efficient DBP-degrading bacterial strain, which was proved to be highly resistant to cadmium (MIC was 12.5 mM) and the involved mechanisms were verified in this study. The results showed the maximu values of extracellular adsorbed and intracellular accumulated Cd2+ contents accounted for 24.67% and 4.78% of the initial Cd2+ content in the medium. The extreme low intracellular Cd accumulation reduced the risk of cadmium interfering with normal cell metabolism. TEM exhibited cell structure of DNB-S2 kept intact even at high Cd concentrations. Some precipitation particles were formed inside cells, which were further identified to be Cd–O–P, Cd (OH)2, CdCO3, and CdS by XPS. Furthermore, Cd resistant genes were predicted on DNB-S2 genome, and the presence of 11 alkaline phosphatase encoding genes provided genetic basis for the formation of cadmium phosphate precipitation, which illustrated intracellular precipitation is one of the important intracellular Cd detoxification mechanisms in strain DNB-S2. Noteworthily, 8 efflux proteins were annotated possibly responsible for cadmium efflux and they were all predicted located on cytoplasmic membrane. In a word, integrity of cell structure, the formation of cadmium precipitation and efficient exclusion primarily constitute Cd tolerance mechanisms of strain DNB-S2. This strain exhibits promising application to the bioremediation of cadmium-DBP contaminated bites.