Polyethylene terephthalates (PET), which are highly resistant to degradation under natural environmental conditions due to their inert, hydrophobic and high molecular weights, are collected in landfills or dispersed as waste. Considering the increasing use of PET, the interest in multidisciplinary studies aimed to obtaining high value added products from PET wastes has increased. In this study, the potential of using waste plastics made of Polyethylene terephthalate (PET) treated with bacteria as an aggregate in the production of mortar was investigated. First, the bacterial strains, which secreted esterase or cutinase, were screened for their ability to attach to hydrophobic surfaces and biofilm formation. TH401, which showed potential for the modification of the PET aggregates, was identified as Bacillus safensis. To enhance the compatibility of the PET, the waste PET aggregates were treated with submerged fermentation via B. safensis TH401. The FTIR analyses conducted after the treatment showed that the chemical structure of the PET had been modified. Mortar samples were prepared by using the modified PET aggregates. The compressive and flexural tensile strength values of the samples were measured on days 7, 28 and 56. The compressive strength values of the bacteria modified samples were 4.87%, 7.51% and 13.13% higher than the non-bacterial samples at 22 degrees C, 30 degrees C and 50 degrees C, respectively. The increase in curing temperatures had a positive effect on compressive and flexural tensile strength values. The highest strength values were obtained in the samples cured at 50 degrees C. In the EDX spot of the mixtures, major peaks of C, O, Ca were detected indicating the presence of CaCO3 and bacteria. Stratification on the PET surface as a result of bacterial modification was clearly observed in the SEM images. The test results showed that bacterial modification contributed to the increase in the strength of the waste PET aggregate concrete. It can be said that mortars produced with bacteria modified PET aggregates can be used as an economical and environmentally friendly material alternative in the construction industry. (C) 2020 Elsevier Ltd. All rights reserved.