Signatures of dysbiosis in fish microbiomes in the context of aquaculture

Fish microbiome plays an important role in maintaining host homeostasis, with many bacterial functions directly linked to host fitness. Fish microbiome research is advancing fast, especially in the context of aquaculture where several stressors are known to disrupt stability of host-associated bacteria, prompting dysbiosis. Therefore, understanding the signatures of dysbiosis in different fish mucosae and their association with such stressors is fundamental to set up efficient health-monitoring strategies, as well as sound and objective working hypothesis for future research. Herein, we reviewed studies that employed culture-independent approaches to assess the effects of disease, chemotherapeutics and water quality changes on several diversity metrics of gut, skin and gill microbiomes. We conclude that increases in abundance of potential pathogens and changes to bacterial community structure are reliable indicators of dysbiosis in fish. The gut microbiome emerged as being highly susceptible to salinity changes and chemotherapeutics, whereas external microbiota seems to be more susceptible to dysbiosis caused by disease and temperature changes. Our analysis showed that treatments with tetracyclines and florfenicol are more likely to elicit severe dysbiosis compared to quinolones and disinfectants that cause lesser disturbance to fish microbiome. Bacterial diseases also frequently elicit severe dysbiosis (enteritis in particular), whereas parasitic diseases are far less deleterious. Regarding impacts on water quality, only changes to salinity and temperature are reasonably studied. Recent developments in metagenomics, that include a fast turn-around time of results, can be used to detect changes to fish homeostasis during critical periods of fish production, assisting aquaculture management.