Bacteria are also a viable source of exoenzymes capable to facilitating the bioremediation of the environment. There are many examples of the use of bacteria for this purpose and their exoenzymes encompass many different classes of bacterial enzymes. Of particular interest in this field are bacterial hydrolases as they have an intrinsic low substrate specificity and can be used for numerous pollutants including solid wastes. Plastic wastes including polyurethanes are particularly hard to degrade, but an exoenzyme has been identified in a Gram-negative bacterium, "Comamonas acidovorans", that was capable of degrading polyurethane waste in the environment. Cell-free use of microbial exoenzymes as agents of bioremediation is also possible although their activity is often not as robust and introducing the enzymes into certain environments such as soil has been challenging. In addition to terrestrial based microorganisms, marine based bacteria and their exoenzymes show potential as candidates in the field of bioremediation. Marine based bacteria have been utilized in the removal of heavy metals, petroleum/diesel degradation and in the removal of polyaromatic hydrocarbons among others.