Multiple Lines of Evidences Reveal Mechanisms Underpinning Mercury Resistance and Volatilization by Stenotrophomonas sp. MA5 Isolated from the Savannah River Site (SRS), USA

Journal Article · 03 April 2019 · Cells

DOI: https://doi.org/10.3390/cells8040309 · OSTI ID:1628317

Agarwal, Meenakshi ^[1]; Rathore, Rajesh Singh ^[1]; Jagoe, Charles ^[2]; Chauhan, Ashvini  ^[1]

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1. Florida A & M University, Tallahassee, FL (United States). School of the Environment, Environmental Biotechnology Laboratory
2. Florida A & M University, Tallahassee, FL (United States). School of the Environment. Environmental Toxicology Laboratory

A largely understudied microbially mediated mercury (Hg) bioremediative pathway includes the volatilization of Hg²⁺ to Hg⁰. Therefore, studies on Hg resistant bacteria (HgR), isolated from historically long-term contaminated environments, can serve as models to understand mechanisms underpinning Hg cycling. Towards this end, a mercury resistant bacterial strain, identified as Stenotrophomonas sp., strain MA5, was isolated from Mill Branch on the Savannah River Site (SRS); an Hg-impacted ecosystem. Minimum inhibitory concentration (MIC) analysis showed Hg resistance of up to 20 µg/mL by MA5 with 95% of cells retaining viability. Microcosm studies showed that the strain depleted more than 90% of spiked Hg²⁺ within the first 24 h of growth and the detection of volatilized mercury indicated that the strain was able to reduce Hg²⁺ to Hg⁰. To understand molecular mechanisms of Hg volatilization, a draft whole genome sequence was obtained, annotated and analyzed, which revealed the presence of a transposon-derived mer operon (merRTPADE) in MA5, known to transport and reduce Hg²⁺ into Hg⁰. Based on the whole genome sequence of strain MA5, qRT-PCR assays were designed on merRTPADE, we found a ~40-fold higher transcription of mer T, P, A, D and E when cells were exposed to 5 µg/mL Hg²⁺. Interestingly, strain MA5 increased cellular size as a function of increasing Hg concentrations, which is likely an evolutionary response mechanism to cope with Hg stress. Moreover, metal contaminated environments are shown to co-select for antibiotic resistance. When MA5 was screened for antibiotic resistance, broad resistance against penicillin, streptomycin, tetracycline, ampicillin, rifampicin, and erythromycin was found; this correlated with the presence of multiple gene determinants for antibiotic resistance within the whole genome sequence of MA5. Overall, this study provides an in-depth understanding of the underpinnings of Stenotrophomonas-mercury interactions that facilitate cellular survival in a contaminated soil habitat.

• Research Organization: Savannah River Nuclear Solutions (SRNS), Aiken, SC (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: AC09-08SR22470
• OSTI ID: 1628317
• Journal Information: Cells, Vol. 8, Issue 4; ISSN 2073-4409
• Publisher: MDPICopyright Statement
• Country of Publication: United States
• Language: English