Microplastics (MPs) constitute a global emerging pollution whose ecological effects are not fully understood. Recent studies performed in aquatic ecosystems show that the plastic particles can be colonized by microorganisms (the plastisphere) being the surrounding environment the key factor for the MP-attached microbial community. Nonetheless, the early colonization of MPs in wastewater treatment plant (WWTP) effluents has not been evaluated yet, despite that mature biofilms depend on early colonizers. Also, MPs might be an important hotspot of bacterial pathogens and antibiotic resistance genes (ARGs). Here, we explored the diversity and community composition of bacteria attached on seven types of MPs (including biodegradable and non-biodegradable polymers) incubated in situ into two different WWTP effluents for 48 hours using Illumina MiSeq sequencing of the 16S rRNA. Also, we checked the relative abundance of the ARGs (sulI and tetM) using qPCR in comparison with the surrounding water to check a possible increase of ARGs in the MP-attached community. The diversity in the MP-associated microbial community was significatively higher in comparison with water free-living bacteria. RDA analysis confirmed that the sampling site explained the major changes in community diversity. Hydrophobicity of the polymer also affected the diversity between samples but to a lesser extent. A MPs-core microbiome was identified which included some genera associated with the degradation of plastics such as Acidovorax, Aquabacterium, Pseudomonas and Variovorax. sulI and tetM genes were detected in the two WWTPs effluents but only sulI was clearly concentrated in the MPs. In conclusion, our results highlight the relevance of the early attachment phase in the development of bacterial biofilms on different types of MP polymers and provide novel information about the development of biofilms on MPs and the role that different types of polymers might have facilitating the attachment of specific bacteria, some of which might carry ARGs.