Microcystin-leucine arginine induces skin barrier damage and reduces resistance to pathogenic bacteria in Lithobates catesbeianus tadpoles

Despite the importance of the skin mucosal barrier and commensal microbiota for the health of amphibians, the potential of environmental contaminants to disrupt the skin mucosal barrier and microbiota have rarely been studied in toxicology. In this study, tadpoles (Lithobates catesbeianus) were exposed to 0, 0.5, and 2 μg/L of microcystin-leucine arginine (MC-LR) for 30 days to explore the impacts of environmentally realistic MC-LR concentrations on the physical skin barrier, immune barrier, commensal microbiota, and skin resistance to pathogenic bacterial invasion. MC-LR exposure significantly reduced the collagen fibrils in the dermis of skin tissues and down-regulated tight junction and stratum corneum-related gene transcriptions, suggesting the damage caused by MC-LR to the physical barrier of the skin. Increased skin eosinophils and upregulated transcriptions of inflammation-related genes in the exposed tadpoles underline the development of skin inflammation resulting from MC-LR exposure even at environmentally realistic concentrations. Comparative transcriptome and immunobiochemical analyses found that antimicrobial peptides (Brevinin-1PLc, Brevinin-2GHc, and Ranatuerin-2PLa) and lysozyme were down-regulated in the exposed groups, while complement, pattern recognition receptor, and specific immune processes were up-regulated. However, the content of endotoxin lipopolysaccharide produced by bacteria increased in a dose-dependent pattern. The disc diffusion test showed a reduced ability of skin supernatant to inhibit pathogenic bacteria in the exposed groups. Analysis of microbial 16 S rRNA gene by high-throughput sequencing revealed that MC-LR interfered with the abundance, composition, and diversity of the skin commensal microbiota, which favored the growth of pathogen-containing genera Rhodococcus, Acinetobacter, and Gordonibacter. In summary, the current study provides the first clues about the impact of MC-LR on the integrity and function of skin barrier of amphibians. These new toxicological evidences can facilitate a more comprehensive evaluation of the ecological risk of MC-LR to amphibians.