Microplastics are emergent contaminants threatening aquatic organisms including aquacultured fish. This study investigated the effects of high-density polyethylene (HDPE, 100 to 125 μm) on yellow perch (Perca flavescens) based on integrative evaluation including growth performance, nutritional status, nutrient metabolism, fish health, and gut microbial community. Five test diets (0, 1, 2, 4, or 8 g HDPE/100 g diet) containing 41% protein and 10.5% lipid were fed to juvenile perch (average body weight, 25.9 ± 0.2 g; n = 15) at a feeding rate of 1.5% to 2.0% body weight daily. The feeding trial was conducted in a flow-through water system for 9 wk with 3 tanks per treatment and 15 yellow perch per tank. No mortality or HDPE accumulation in the fish was found in any treatments. Weight gain and condition factor of fish were not significantly impacted by HDPE (P > 0.05). Compared to the control group, fish fed the 8% HDPE diet had significantly decreased levels of protein and ash (P < 0.05). In response to the increasing levels of HDPE exposure, the hepatosomatic index value, hepatocyte size, and liver glycogen level were increased, but lipid content was reduced in the liver tissues. Compared to the control treatment, fish fed the 8% HDPE diet had significant accumulations of total bile acids and different metabolism pathways such as bile acid biosynthesis, pyruvate metabolism, and carnitine synthesis. Significant enterocyte necrosis was documented in the foregut of fish fed the 2% or 8% HDPE diet; and significant cell sloughing was observed in the midgut and hindgut of fish fed the 8% HDPE diet. Fish fed the 2% HDPE diet harbored different microbiota communities compared to the control fish. This study demonstrates that HDPE ranging from 100 to 125 μm in feed can be evacuated by yellow perch with no impact on growth. However, dietary exposure to HDPE decreased whole fish nutrition quality, altered nutrient metabolism and the intestinal histopathology as well as microbiota community of yellow perch. The results indicate that extended exposure may pose a risk to fish health and jeopardize the nutrition quality of aquacultured end product. This hypothesis remains to be investigated further.

Adhesion is an important virulence factor of Vibrio alginolyticus. This factor may be affected by environment conditions; however, its molecular mechanism remains unclear. In our previous research, adhesion tender models were obtained by culturing V. alginolyticus under stresses such as Cu2+, Pb2+, Hg2+, and low pH. With high-throughput sequencing and bioinformatics analysis, we found that all of these stress treatments significantly affected the flagellar assembly pathway, which may play an important role in V. alginolyticus adhesion. Therefore, we hypothesized that the environmental stresses of the flagellar assembly pathway may be one way in which environment conditions affect adhesion. To verify our hypothesis, a bioinformatics analysis, QPCR, RNAi, in vitro adhesion assay and motility assay were performed. Our results indicated that 1) the flagellar assembly pathway was sensitive to environmental stresses, 2) the flagellar assembly pathway played an important role in V. alginolyticus adhesion, and 3) motility is not the only way in which the flagellar assembly pathway affects adhesion.

The Siamese crocodile (Crocodylus siamensis) is a freshwater, endangered crocodile with high economic value in the farming industry. Gut microflora plays an essential role in host physiological activity, and it contributes significantly to both the health and diseased states of animals. However, thus far, no study has focused on the correlation between diseases and intestinal bacterial communities in crocodilians. Here, we first compared the composition and function of gut microbial communities in captive juvenile C. siamensis suffering from anorexia and healthy crocodile controls using deep amplicon sequencing. The gut microbial diversity of anorexic crocodiles was much lower than the healthy individuals. Obvious changes in gut microbial composition were observed between sick and healthy crocodiles, except for Cetobacterium somerae of phylum Fusobacteria. In particular, the abundance of Bacteroides luti, Clostridium disporicum, Plesiomonas shigelloides, and Odoribacter sp. in the gut flora of healthy crocodiles was distinctly higher than the diseased group. Conversely, the species Edwardsiella tarda was overrepresented in the gut of anorexic crocodiles compared to the healthy group. Furthermore, in anorexic crocodiles, the predicted microbial functions that were related to amino acid metabolism, biosynthesis of other secondary metabolites, nucleotide metabolism, replication and repair, and translation were significantly reduced, while signal transduction was significantly enriched. These findings of the present study provide a reference to enrich the field of gut microorganism studies in crocodilians and suggest that alterations in the composition and function of gut bacteria in C. siamensis juveniles may be associated with anorexia in crocodiles.

Adhesion capability to fish mucus is considered to be a key virulence factor of Vibrio alginolyticus, while the molecular mechanism is still unclear. Non-coding RNAs (ncRNAs) play a crucial role in the intricate regulation of bacterial gene expression, affecting the pathogenicity of bacteria. With high through-put sequencing and bioinformatics analysis, we identified three ncRNAs predicted to regulate the bacterial chemotaxis pathway in our previous study, which might play a key role in the adhesion process of V. alginolyticus. In the present study, we validated the effects of these three ncRNAs on their predicted target genes and the role of these three ncRNAs in the adhesion process of V. alginolyticus with RNAi, QPCR, capillary assay, and in vitro adhesion assay. According to our results, the expression of these ncRNAs and their predicted target genes were confirmed by QPCR, which reinforced the reliability of the sequencing data and the target prediction. We found that over-expression of these ncRNAs were capable of reducing the chemotactic and adhesion ability of V. alginolyticus, while the expression levels of their target genes were significantly reduced. Our results indicated that: 1) these three ncRNAs were really able to regulate the bacterial chemotaxis pathway; 2) these three ncRNAs played a key role in the adhesion process of V. alginolyticus.