Arsenic and chromium resistance mechanisms in the Micrococcus luteus group

High arsenic(As)and chromium(Cr)concentrations are currently receiving attention because of their negative effects on the environment and human health.Microorganisms inhabiting contaminated environments have developed resistance mechanisms against the toxicity of these pollutants.Indeed,members of the bacterial genus Micrococcus have been isolated from different toxic metal-contaminated environments;however,knowledge concerning its resistance mechanisms to As and Cr toxicity remains limited.Micrococcus luteus strains(An24,Mh,NE2TL6,and NE2TTS4)were isolated from the endosphere and soil of two heavy metal-contaminated sites in Mexico to identify differences in the resistance mechanisms by the M.luteus group.The strains were resistant to As(As^(3+)and As^(5+)),chromate,dichromate,cobalt,copper,nickel,and zinc.Genome analysis indicated that the heavy metal-resistant strains(An24,Mh,NE2TL6,and NE2TTS4)could be assigned to the M.luteus group and had more heavy metal-resistant genes(transporters,chaperones,and enzymes)compared to reference strains of the M.luteus group,M.luteus NCTC 2665^(T)and Micrococcus endophyticus JCM 16951^(T).The resistant bacteria were able to biotransform As^(3+)and As^(5+)through a carbon source-dependent mechanism.The biotransformation of As5+was potentially carried out in the cytoplasm through a thioredoxin-dependent pathway,which may be coupled with biosorption.A qualitative analysis of organic acids(OAs)identified a change in the OA profile of the metal-resistant strains that was As-or Cr-dependent.Our genomic and phenotypic findings suggest that the four M.luteus group strains evaluated in the current study have developed resistance mechanisms that may enable their survival in contaminated sites.

Plant Growth-Promoting Traits in Rhizobacteria of Heavy Metal-Resistant Plants and Their Effects on Brassica nigra Seed Germination

Phytoremediation is an emerging technology that uses plants and their associated microbes to clean up pollutants from the soil, water, and air. In order to select the plant growth-promoting rhizobacteria(PGPR) for phytoremediation of heavy metal contamination, 60 bacterial strains were isolated from the rhizosphere of two endemic plants, Prosopis laevigata and Spharealcea angustifolia, in a heavy metal-contaminated zone in Mexico. These rhizobacterial strains were characterized for the growth at different pH and salinity, extracellular enzyme production, solubilization of phosphate, heavy metal resistance, and plant growth-promoting(PGP) traits, including production of siderophores and indol-3-acetic acid(IAA). Overall, the obtained rhizobacteria presented multiple PGP traits. These rhizobacteria were also resistant to high levels of heavy metals(including As as a metalloid)(up to 480 mmol L(-1)As(V), 24 mmol L(-1)Pb(Ⅱ), 21 mmol L(-1)Cu(Ⅱ), and 4.5 mmol L(-1)Zn(Ⅱ)). Seven rhizobacterial strains with the best PGP traits were identified as members of Alcaligenes, Bacillus, Curtobacterium, and Microbacterium, and were selected for further bioassay.The inoculation of Brassica nigra seeds with Microbacterium sp. CE3R2, Microbacterium sp. NE1R5, Curtobacterium sp. NM1R1,and Microbacterium sp. NM3E9 facilitated the root development; they significantly improved the B. nigra seed germination and root growth in the presence of heavy metals such as 2.2 mmol L(-1)Zn(Ⅱ). The rhizobacterial strains isolated in the present study had the potential to be used as efficient bioinoculants in phytorremediation of soils contaminated with multiple heavy metals.