Identification of mercury‐resistant Streptomyces isolated from Cyperus rotundus L. rhizosphere and molecular cloning of mercury (II) reductase gene

Streptomyces is one of mercury‐resistant bacteria which can convert Hg2+ into nontoxic Hg0 . This study aimed to identify mercury‐resistant Streptomyces present in the Cyperus rotundus rhizosphere from artisanal small‐scale gold mining (ASGM) area and clone merA gene to the cloning and expression vectors. Molecular identification was conducted using 16s rRNA gene with the maximum likelihood algorithms. Results revealed that the AS1 and AS2 strains were a group of Streptomyces ardesiacus and the BR28 strain was closed to Brevibacillus agri. The AS2 merA gene was cloned to pMD20 cloning vectors, pGEX‐5x‐1 and pET‐28c expression vectors. The transformation was successfully performed in BL21 and DH5α competent cells. The full length of the merA gene was confirmed to be 1,425 bp. This study is the first research on identifying mercury‐resistant Streptomyces and cloning the full‐length merA gene in Indonesia.

Isolation and characterization of mercury-resistant bacteria from wastewater sources in Egypt

An important mechanism for microbial resistance to mercury is its reduction into elemental mercury (facilitated by the merA gene). Thirty-eight microbial isolates from a variety of wastewater sources in Egypt were collected. Approximately 14 of the 38 isolates exhibited not only a high degree of tolerance to mercury (up to 160 ppm) but also a high degree of resistance to other tested heavy metals (Cu, Co, Ni, and Zn). From these 14, the 10 most resistant isolates were selected for further study and were found to include 9 Gram-negative and 1 Gram-positive bacterial strains. Multi-antibiotic-resistance profiles were detected for 6 out of the 10 selected isolates. All the tested Gram-negative isolates (n = 9) harbored a plasmid-encoded merA gene. The mercury removal effectiveness for the 10 selected isolates ranged between 50% and 99.9%, among which Stenotrophomonas maltophilia ADW10 recorded the highest rate (99.9%; at an initial mercury concentration of 20 ppm). To the best of our knowledge, this is the first study to (i) demonstrate the presence of a multimetal-resistant S. maltophilia bacterium with a high mercury tolerance capacity that would make it a suitable candidate for future bioremediation efforts in heavy-metal-polluted areas in Egypt and (ii) report Pseudomonas otitidis as one of the mercury-resistant bacteria.

Mercury Reduction by Bacteria Isolated from Informal Mining Zones

The aim of this research was the selection of bacterial strains resistant to mercury, as well as to demonstrate their capacity to reduce mercury in solution when they are inoculated in pure and mixed cultures. Samples of soil from informal mining gold sites in Peru were collected and fifteen mercury resistant bacteria were isolated. Strains RM6, RM7, RM9, RM11, RM12 and RM13 were selected for their capacity to reduce mercury in solution. The six bacterial strains belong to the genus Pseudomonas. Inoculated in pure cultures, these strains reduce mercury in solution although in different percentages: RM9, RM11 an RM12 reduce 93% to 97% of the mercury, while strains RM6, RM7 y RM13 reduce 80% to 85%. The consortium of all six bacterial strains showed a mercury reduction of 84%. Approximately 91% of mercury in solution was reduced in 1 hour and this reaction was not associated to bacterial growth. Using specific primers, the merA gene was amplified from genomic DNA of the bacterial strains, which would suggest the activity of the mer operon as a mechanism of mercury resistance. Due to their ability to reduce mercury in solution, it is advisable to carry out more research on the selected strains since they could be useful in future bioremediation processes.