## 5 Conclusions  
There are growing concerns about the potential toxic effects of MPs on aquatic organisms. These toxic pollutants may have far-reaching disadvantages on the aquaculture industry and on human health. This study offered new insight about the effect of virgin polypropylene MPs (3-4 mm) on commercial fish health. Firstly, the results presented here do not support the hypothesis that the addition of MPs has an adverse effect on fish growth. After a four-week trial, the body length and weight of the treatment largemouth bass were indistinguishable from the control group. Secondly, MPs exposure had no significant effect on the community composition or diversity of intestinal microbial, although it could partly influence intestinal morphology, and the recombination process of the intestinal microbial community. Virgin MPs did not cause imminent harm to largemouth bass. However, the results only offer temporary relief, since pollutants in the natural environment are more complex. We used MPs that were brand new. There were no weathering effects or biofilm on their smooth surface. This is unrealistic for natural pond MPs pollution. Furthermore, the choice of size and polymer of MPs affects the results. More attention should be addressed on combined toxicity of MPs and other contaminants in further research.  
The following are available online at [[https://www.mdpi.com/article/10.3390/app112411921/s1](https://www.mdpi.com/article/10.3390/app112411921/s1)]([https://www.mdpi.com/article/10.3390/app112411921/s1](https://www.mdpi.com/article/10.3390/app112411921/s1)), Table S1: Weight and length information of largemouth bass before and after the experiment; Table S2: Alpha diversity of intestinal microbiota; Figure S1: Identification of materials; Figure S2: The process of making fodder contained MPs; Figure S3: Villi length of largemouth bass among each group after the trial; Figure S4: Rarefaction curves; Supporting Text S1: Specific steps of PCR reactions; Supporting Text S2: Processing of raw fastq files; Supporting Text S3: Intestinal microbial analyses.  
Conceptualization, C.Z. and G.X.; methodology, S.W. and Z.P.; software, Q.W.; validation, J.Z., Q.W. and S.W.; formal analysis, C.Z.; investigation, D.S. and Y.C.; resources, J.Z.; data curation, Q.W.; writing--original draft preparation, C.Z.; writing--review and editing, G.X.; visualization, J.Z.; supervision, G.X.; project administration, J.Z.; funding acquisition, G.X. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Key Realm R&D Program of Guangdong Province (No. 2020B0202080005), and China Agriculture Research System of MOF and MARA (CARS-45-50).  
**Institutional Review Board Statement:** The study was conducted according to the Animal Care and Use Committee of South China Agricultural University (identification code: 20200127; date of approval: 20 May 2020).  
**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study. Written informed consent has been obtained from the patient(s) to publish this paper.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
**Conflicts of Interest:** The authors report no conflict of interest.