Lead and cadmium-resistant bacterial species isolated from heavy metal-contaminated soils show plant growth-promoting traits

Application of metal-resistant rhizobacteria is a promising approach for detoxification and bioremediation of contaminated soils. In order to isolate, identify, and characterize lead and cadmium-resistant bacteria, nearly 30 soil samples were collected from heavy metal-contaminated sites, and five resistant bacterial strains were isolated and identified based on their cultural, physiological, biochemical, and molecular characteristics as Enterobacter cloacae, Enterobacter kobei, Bacillus cereus, Rhizobium pusense, and Agrobacterium tumefaciens. The nucleotide information of these strains is available in GenBank under the accession numbers of MH327251, MH327252, MH327253, MH327254, and MK123361, respectively. The minimum inhibitory concentrations (MICs) against lead and cadmium differed for each isolate and the isolates showed higher MIC against lead (3500 μg ml−1) than cadmium (100 μg ml−1). Assessment of the heavy metal degradation capacity of the species showed 10–60% and 5–40% reduction in concentrations of lead and cadmium, respectively. The highest ability for P-solubilization was measured for the R. pusense, A. tumefaciens, and B. cereus species, while the R. pusense and B. cereus species had the capability to solubilize potassium. The studied species also had the ability to produce indole acetic acid (IAA) and/or hydrogen cyanide production (HCN). Inoculation of ornamental cabbage cultivated in a heavy metal-contaminated soil with the isolated species significantly increased biomass and Pb and Cd uptake of the plant. With respect to plant growth promoting and heavy metal-resistant traits of the studied species, it is concluded that these species can have great significance in bioremediation and management of environmental pollution.