ABSTRACT
The environmental consciousness about polychlorinated biphenyls (PCBs) began in the late 1960s as a result of concern over possible health effects. PCBs were manufactured by direct chlorination of biphenyl to produce complex mixtures containing up to 10 chlorines. By this process, theoretically, 209 different compounds (referred to as congeners) can be produced. The congeners used in commercial PCB mixtures are mostly tri-to hexa-chlorinated ones. The total amount that has been produced in the world is estamated to be 1.2 million tons. PCBs were marketed under the trade name 'Aroclor' in the U.S., 'Clophen' in Europe, and 'Kaneclor' in Japan. In the case of Aroclors, e.g., Aroclor 1254, the first two digits are 12 for 12 carbon atoms of biphenyl, and the last two digit number indicate the percent chlorine by weight. PCBs were widely used for a variety of industrial purposes due to their chemical and physical stability. Because of their lipophilic property, PCBs tend to accumulate in biota. Ahmed and Focht (1973) first reported two Achromobacter strains that degrade several PCB congeners. Since then a number of PCB-degrading bacteria have been isolated and characterized, and biochemical and genetic approaches have been extensive. The reductive dehalogenation of PCBs is important in anaerobic environments. Microbial reductive dehalogenation of PCBs was also extensively studied in soil and aquatic sediments under anaerobic conditions. Several review articles concerning microbial degradation of PCBs have been published to date (Abramowicz 1990, Furukawa 1982, 1994, Unterman 1996). The present review is focused on more recent works on biochemical and molecular genetic approaches to microbial degradation of PCBs.
