Polychlorinated biphenyls are harmful chemicals found in the environment. In this research, scientists studied how a type of soil microbe called Aspergillus niger breaks down different types of polychlorinated biphenyls. They found that some of these chemicals were transformed into less harmful substances, while others were not broken down at all. The main metabolites produced were mono- and di-hydroxy biphenyls, which can have negative effects on the endocrine system. This study helps us understand how these toxic chemicals can be broken down in the environment.