The plastic-degrading capacity of some insects has been investigated over the past decade, with the aim of identifying gut microorganisms potentially involved in plastic degradation. However, plastic-only diets impose severe nutritional constraints, potentially driving microbial selection independently of plastic exposure. Here, we examined how nutritional stress influences gut bacterial community and the identification of plastic-associated bacteria in two plastivorous insects, Galleria mellonella and Tenebrio molitor, using polyurethane (PU) as a representative polymer. Bacterial communities were characterized by 16S rRNA gene sequencing under contrasted dietary conditions, including starvation, and complemented by a culture-dependent isolation approach using PU as the sole carbon source. In both species, gut bacterial communities under plastic-only feeding closely resembled those observed under starvation, whereas they differed from nutritionally balanced conditions. Differential abundance analyses reflected this pattern, as taxa enriched under plastic feeding were also enriched under starvation. This convergence was strong and structured in T. molitor, but weaker and more variable in G. mellonella. In addition, bacterial strains were isolated from the gut of T. molitor under both PU-amended and carbon-free conditions. Overall, our results demonstrate that nutritional stress is a driver of gut bacterial community restructuring under plastic-based diets and can bias the identification of candidate plastic-associated bacteria.