Biotransformation of Thiomersal by Naturally Mercury Resistant Isolates and Genetically Engineered Microorganisms

Abstract

Thiomersal (TH), an aryl-alkyl-organomercurial bactericide, has been used for vaccine production and preventing bacterial contamination. Presently, there is no remediation for wastewater contaminated with TH available. Therefore, this study was conducted to assess the feasibility of aerobic treatment of TH contaminated solutions. The potentialities for TH detoxification were determined by naturally mercury resistant isolates and two genetically engineered microorganisms. Generally, all isolates showed resistance up to 2 ppm TH, however with different transformation rates (8−443 ng Hg/ml/min). Ps. putida Spi3 proved in this study to be an excellent strain for TH detoxification. It reduces very high concentrations of TH up to 140 ppm. Analyzes showed that the optimal TH transformation was determined at 30°C and pH 7. Additionally, the results of the growth experiments showed that the cells at lag phase exhibited the highest TH transformation rates, followed by those at the exponential phase and stationary phase. The genetic structure of mercury resistant operons from all environmental strains was studied with special attention to organomercurial lyase (MerB). All sequenced strains carried multiple mer operons. They all harbour a narrow spectrum resistance operon (merNS) beside their resistance to TH that resulted in the presence of a broad spectrum resistant operon (merBS) including merB gene. The strain Ps. putida Spi3, however, carried four mer operons, one conferring merNS and three merBS. Analysis showed that the three merB genes are located on different mer operons. Two merB genes were mapped upstream of regulatory genes (merD). In contrast, the gene arrangement of the third merB gene is peculiar; merB3 is mapped between the merR and merT gene and has its own promoter and ribosomal binding site that may account for Spi3’s extremely high resistance to TH. These four mer operons constitute Spi3’s high resistant to mercuric ions and organomercurials.