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Current understanding on the roles of gut microbiota in fish disease and immunity.

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PubMed ID: 29976843

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Xiong JB, Nie L, Chen J

Zool Res. Mar 2019. doi: 10.24272/j.issn.2095-8137.2018.069

COMMENT: According to FAO report, aquaculture is the fastest growing animal food production sectors. Intensive farming practices have increased the frequency and severity of fish diseases.  The use of antibiotics is under strict control and regulatory measures because of drug resistance and residue related issues. As the gut microbiota plays an indispensable role in fish health, its modulation could be a feasible strategy to mitigate these emerging diseases. This mini review is focused on current knowledge on the interplay among gut microbiota, fish inmunity and disease. 

OBJECTIVE: 

In this commentary, we summarize current knowledge on the associations between fish immunity, gut microbiota, and invading intestinal pathogens. We also highlight recent progress in uncovering the ecological processes of fish diseases. 

MAIN RESULTS:

A preponderance of evidence has demonstrated that more diverse gut communities exert greater protective effects on the host (...). In this regard, gut microbial diversity in fish should be maximized to reduce pathogenic invasions in aquaculture systems. 

The introduction of pathogens into hosts is antagonized by environmental pressure, fish filtering, and colonization resistance of gut commensals. In healthy fish, the gut microbiota directly antagonizes the colonization or overgrowth of pathogens (...). These effects include competition for resources, niche exclusion, and suppression of virulence factors. In addition, pathogens are suppressed by immune clearance.

In diseased fish, balances in the protective commensal microbial community and host immunity are disturbed by external factors. For example, antibiotic usage can decrease species diversity and alter gut microbial community structure in fish (...). Pathogenic infections have been shown to significantly disrupt interspecies interactions in the fish gut microbiota (...). These alterations may open up ecological niches for pathogenic invasions. Furthermore, environmental stresses may impose additional pressure on fish, leading to compromised immunity. Lastly, the expression of virulence genes in pathogens can also be induced by poor water quality (...). These detrimental effects cumulatively attenuate resistance to colonization by pathogens and allow overgrowth of harmful colonies that may lead to disease.

CONCLUSIONS:

Given the functional importance of the gut microbiota in improving host fitness, introduction or augmentation of beneficial microbes may be a promising approach for protecting fish from emerging diseases (...).  However, various studies have identified long lists of implicated microbes that may contribute to the gut microbiota dysbiosis-disease relationship, and these associations may reflect biomarkers of disease. Therefore, future work is required to explore the causal links between fish disease and specific taxa, which may enable us to optimize gut microbiota composition to mitigate fish disease.

To understand the mechanisms underlying fish disease, one should focus on the infection process from an ecological prospective (...) instead of isolating potential pathogens from diseased fish. Next generation sequencing has allowed the identification of universal gut microbial biomarkers (common features of affected individuals) in various fish diseases from different regions. Therefore, we recommend that relevant information should be deposited into a public database, which could enable convenient cross-disease comparisons. This would facilitate rapid diagnosis as well as promote prediction of the course and prognosis of disease. 

Contributor

Raquel Ruiz-Arroyo