To make a fuel or food or other stuff out of it you need to define the microbes set(mixture). Almost every study claiming plastic to X follows hybrid pattern, they break the polymer into smaller molecules by depolymerization either thermally/chemically for polyolefins like PE/PP/PS, since no enzyme is known to break a C-C backbone; enzymatically for ester-linked plastics like PET/PU, via PETase-type enzymes). Then it goes to the fermentation process, resulting small molecules into lipids (most common), PHA, or specific chemicals.

Sometimes a consortium(Mixture of microbes) is used, with different microbes handling different fractions of the breakdown mixture. Any "protein from plastic" claim is the same logic, the depolymerized substrate is just feeding an yeast, and the protein is the yeast biomass itself, harvested and used crude or extracted.

True microbial degradation of intact plastic polymers, no pretreatment, organism doing all the work, is still a challenging task for biologist.