Microbial Degradation of Polychlorinated Biphenyls

Abstract

Polychlorinated biphenyls (PCBs) are toxic xenobiotic pollutants that are today found in all regions of the Earth. Because of their thermal stability and high dielectric constant, PCBs have been used for a variety of industrial applications, including as lubricants, dielectric fluids, and plasticizers. PCBs were manufactured widely over a half century and an estimated 1.5 million tons of PCBs has been produced worldwide. Because of their toxicity and persistence in the environment, PCBs were banned in most countries in the late 1970s. Because of their high volatility and chemical stability PCBs have been widely dispersed by atmospheric route. Environmental cycling of PCBs has resulted in their presence and positive detection in virtually every compartment of the ecosystem, including air, water, soil, sediments, and living organisms. Because of their bioaccumulation in living tissues and their documented toxicity, PCBs are today considered as a major class of environmental pollutants. Traditional remediation strategies of PCB-polluted sites require soil excavation and transport, prior to off-site treatment, which is costly, damaging for the environment, and, in many cases, practically infeasible due to the range of the contamination. Hence, there is a considerable interest in developing cost-effective bioremediation alternatives. Although PCBs were long believed to be unalterable through biological processes, evidence has accumulated that PCBs can be significantly metabolized by several kinds of organisms, including mammals, plants, fungi, and bacteria. Under certain conditions, microbial transformation of PCBs can result in their full mineralization into innocuous products. Two major bacterial metabolic routes are known: the anaerobic and aerobic pathways. Under anaerobic conditions, higher chlorinated congeners undergo preferentially reductive dechlorination resulting in lesser chlorinated congeners. In the presence of oxygen, lesser chlorinated congeners are more susceptible to aerobic oxidation, a process that can potentially lead to the complete mineralization of the PCB molecule. The objective of this article is to present an overview of the current knowledge about the biodegradation of PCBs by bacteria. Recent advances in the development of transgenic bacteria for the treatment of PCBs are also presented.