Accumulation and efflux of polychlorinated biphenyls in Escherichia coli

Abstract

Polychlorinated biphenyls (PCBs) are environmental pollutants that have been associated with numerous adverse health effects in human and animals. Hydroxylated PCBs (HPCBs) are the product of the oxidative metabolism of PCBs. The presence of hydroxyl groups in HPCBs makes these compounds more hydrophilic than the parent PCBs. One of the best approaches to break down and remove these contaminants is bioremediation; an environmentally friendly process that uses microorganisms to degrade hazardous chemicals into non-toxic ones. In this study, we investigated the cellular accumulation and toxicity of selected PCBs and HPCBs in Gram-negative bacteria, using Escherichia coli as a model organism. We found that none of the five PCBs tested were toxic to E. coli, presumably due to their limited bioavailability. Nevertheless, different HPCBs tested showed different levels of toxicity. Furthermore, we demonstrated that the primary multidrug efflux system in E. coli, AcrAB-TolC, facilitated the efflux of HPCBs out of the cell. Since AcrAB-TolC is constitutively expressed in E. coli and is conserved in all sequenced Gram-negative bacterial genomes, our results suggest that the efflux activities of multidrug resistant pumps may affect the accumulation and degradation of PCBs in Gram-negative bacteria.

Response Mechanisms of Bacterial Degraders to Environmental Contaminants on the Level of Cell Walls and Cytoplasmic Membrane

“…Similar results were published with toluene by Segura et al [91]. The ability of E. coli to eliminate PCBs and hydroxyl-PCBs was studied by Geng et al [90]. The correlation between the multidrug resistance and the efflux of toxic pollutant by Pseudomonas aeruginosa was studied in detail by Muller [92].…”

“…It requires a cytoplasmic membrane export system, which acts as an energy-dependent extrusion pump, a membrane fusion protein, and an outer membrane factor [39, 89]. It was found that primary multidrug efflux system AcrAB-TolC facilitated the efflux of hydroxyl-PCBs out of the cells [90]. These multidrug resistant pumps may affect the accumulation and degradation of PCBs by bacteria.…”

Toxicity of hydroxylated polychlorinated biphenyls (HO-PCBs) using the bioluminescent assay Microtox®

“…Parnell et al (2006) exposed the PCB degrader, Burkholderia xenovorans LB400, to the commercial mixture, Aroclor 1242 (500 mg L −1 ), and observed that induction of the biphenyl pathway resulted in overexpression of several detoxification genes, suggesting that the toxicity associated with PCBs was partly due to the formation of PCB metabolites. More recently, Geng et al (2012) reported the inhibitory effect of four ortho -HO PCBs (with 2, 3, 4, and 5 chlorine atoms) on Escherichia coli , although none of the parent PCBs tested exhibited a recordable effect. The authors reported that HO-PCBs were actively pumped out of the cell by the AcrAB-TolC drug efflux system, which is known to be involved in cell resistance to hydrophobic pollutants.…”

“…The authors reported that HO-PCBs were actively pumped out of the cell by the AcrAB-TolC drug efflux system, which is known to be involved in cell resistance to hydrophobic pollutants. Besides PCBs, hydroxylation of other aromatic compounds has been reported to increase their toxicity, which is assumed to be related to the higher solubility of HO-derivatives as compared with the parent compounds (Camara et al 2004; Geng et al 2012). …”

“…A few publications have reported the toxic effects of HO-PCBs on bacteria (Camara et al 2004; Parnell et al 2006; Geng et al 2012), however these studies focused more on PCB biodegradation and considered only a limited number of HO-derivatives. The objective of the present study is to determine and compare the toxic effect of a suite of mono-HO PCB congeners using the microbial bioluminescent assay Microtox ® .…”