Understanding how local bacterial communities interact with different plastic types in freshwater ecocystems remains limited, despite the continuous accumulation of synthetic polymers. The objective of this study was to use a raw microbial community from the Tigris River as a natural inoculum to evaluate its biodegradation potential. As the sole carbon sources, High-density polyethylene (HDPE) and polyethene terephthalate (PET) were subjected to 90 days of controlled laboratory conditions. Analysis of colony-forming units (CFU/ml) revealed significant differences in colonization dynamics (p = 0.001 HDPE showed irregular semistable growth, while PET supported continuous bacterial increase. 16S rRNA sequencing showed a consortium dominated by Pseudomonas aeruginosa (GJ-TIG1), Bacillus subtilis (GJ-TIG2), and Bacillus subtilis (GJ-TIG3), suggesting functional cooperation. The GJ-TIG1 strain likely acts as the first colonizer by oxidizing the plastic surface, which enables GJ-TIG2 and GJ-TIG3 to use enzymatic hydrolysis of ester bonds. These biological findings were confirmed by physical and chemical analyses. Field-emission scanning electron microscopy (FESEM) showed distinct damage in PET compared with limited surface changes in HDPE. Fourier transform infrared (FTIR) and gas chromatography-mass spectrometry (GC-MS) analyses supported this difference by detecting breaking of the ester bonds and the formation of by-products such as phthalates and hydrocarbons. These results indicate partial biodegradation by a natural consortium, with its effectiveness deppending on the type of polymer. This study therefore provides basic insights into bioremediation of microplastic contamination in the Tigris River, which often suffer from a scarcity of such studies.