Effect of virgin low density polyethylene microplastic ingestion on intestinal histopathology and microbiota of gilthead sea bream

Polyethylene microparticles are common contaminants in the marine environment, which are easily ingested by fish and can provoke physical damage or toxic reactions, resulting in inflammation, stress-related alterations and behavioural disorders. This work focuses on the extend of the alterations by microplastic ingestion on the intestinal health of Sparus aurata, in an integrative effort to relate the effect on gut microbiota and histopathological damage. Virgin, low density, polyethylene microplastic (LDPE-MP) particles were included into a control diet (D1, without MP), at a 0.12% in two different size ranges in diet D2 (200–500 μm) and D3 (501–1000 μm). Gilthead sea bream were fed D1, D2 or D3 diets for 30 days, and then sacrificed, biometrical data recorded, anterior intestine samples taken for histological analyses and anterior intestine mucus collected for Illumina sequencing of the V3-V4 region of the 16S rRNA, to evaluate the adherent bacterial communities. Intestines of fish fed D2 and D3 contained plastic aggregates and showed macroscopic signs of distension, liquid accumulation and inflammation. Their histopathological analysis revealed severe mechanical lesions (haemorrhages and epithelial desquamation and necrosis), inflammatory reaction (eosinophilia, lymphocytosis, and melanomacrophage presence) and severe epithelial vacuolization, caused by erosion and accumulation of MPs with sharp and rough-edges. Illumina sequencing resulted in over 7 million high-quality reads that were assigned to 589 OTUs. No significant differences were found in richness, diversity or composition at the phylum level among all three experimental groups. However, small but significant differences were found in differential expression analyses, with the D2 diet changing 6.2% and D3 changing 2.9% of the total bacterial composition in anterior intestine. The altered microbial composition seems to be directed as an attempt to counteract the mechanical damage and inflammation inflicted by MPs ingestion, in agreement with metagenome prediction and pathway analysis, which revealed that the slight effect of diets containing MPs (D2 and D3) on gut microbiota has the potential to change the metabolic capacity of these populations, including a decrease in pathways related to bacterial infection and inflammation. Thus, virgin LDPE-MPs ingestion in gilthead sea bream did not significantly affect growth and gut microbiota composition over a 30-day feeding period, but the slight alterations observed could derive in growth problems and dysbiosis upon prolonged microplastic ingestion, which are mainly driven by the severe mechanical damage inflicted to the digestive tract.