PCBs are toxic to bacterial communities above 1000 mg/kg. However, if the concentration is too low (lower than 50 mg/kg), the degradation slows down significantly, for there is not enough material to stimulate the expression of required genes and support the growth of competent microorganisms. PCBs includes various different compounds with slightly different structures. Those slight differences make big differences in metabolic rate. Generally speaking, the more chlorines in a PCB molecule, the harder for it to be degraded. In particular,  microorganisms cannot degrade di- and tetra-ortho substituted well. It is possible that those structures prevent enzymes from accessing reaction sites.