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.

Bacterial and archaeal communities in saline soils from a Los Negritos geothermal area in Mexico

In recent years,there has been a growing need to understand how salinity affects microbial communities in agricultural soils.Archaeal and bacterial community diversities and structures were investigated by high-throughput sequencing analysis of their 16S rRNA in two arable soils with low electrical conductivity(EC)(2.3 and 2.6 dS m^(-1))and a saline soil(EC=17.6 dS m^(-1)).The dominant bacterial phyla in the soils were Proteobacteria(relative abundance(RA)=46.2%),followed by Acidobacteria(RA=13.1%)and Actinobacteria(RA=10.0%),whereas Serratia(RA=6.0%)and Bacillus(RA=4.0%)were the dominant bacterial genera.Candidatus Nitrososphaera(53.5%)was the dominant archaeal phylotype in the arable soils,whereas Nitrosopumilus(RA=0.4%)dominated in the saline soil.The archaeal and bacterial community structures were different between the soils and significantly correlated with soil sand,arsenic,barium,and antimony contents,but not with soil salinity.