Commensal and Opportunistic Bacteria Present in the Microbiota in Atlantic Cod (Gadus morhua) Larvae Differentially Alter the Hosts' Innate Immune Responses.

Ragnhild Inderberg Vestrum,Olav Vadstein,Birgit Luef,Per Winge,Yngvar Olsen,Torunn Forberg,Ingrid Bakke

doi:10.3390/microorganisms10010024

Ragnhild Inderberg Vestrum, Olav Vadstein + Show 5 more

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https://doi.org/10.3390/microorganisms10010024

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Journal: Microorganisms	Publication Date: Dec 24, 2021
Citations: 5	License type: CC BY 4.0

Affiliation: Norwegian University of Science and Technology

Abstract

The roles of host-associated bacteria have gained attention lately, and we now recognise that the microbiota is essential in processes such as digestion, development of the immune system and gut function. In this study, Atlantic cod larvae were reared under germ-free, gnotobiotic and conventional conditions. Water and fish microbiota were characterised by 16S rRNA gene analyses. The cod larvae’s transcriptional responses to the different microbial conditions were analysed by a custom Agilent 44 k oligo microarray. Gut development was assessed by transmission electron microscopy (TEM). Water and fish microbiota differed significantly in the conventional treatment and were dominated by different fast-growing bacteria. Our study indicates that components of the innate immune system of cod larvae are downregulated by the presence of non-pathogenic bacteria, and thus may be turned on by default in the early larval stages. We see indications of decreased nutrient uptake in the absence of bacteria. The bacteria also influence the gut morphology, reflected in shorter microvilli with higher density in the conventional larvae than in the germ-free larvae. The fact that the microbiota alters innate immune responses and gut morphology demonstrates its important role in marine larval development.

Highlights

The roles of the microbiota associated with vertebrate hosts, including fish, have received much attention over the last decade
Two different bacterial strains were added in equal amounts to the rearing bottles: Microbacterium ND 2–7 and Vibrio RD 5–30, both previously isolated from cod and identified as probiotic candidates
Kaplan–Meier survival curves showed a clear separation of the conventional group vs. the germ-free and gnotobiotic group cumulative mortality, and this difference was highly significant (Log rank post hoc p-values 0.000032 and 0.000005 for the pairwise comparisons) (Supplementary Figure S1) (84.9%, 84.8% and 76.0% survival, respectively)

Summary

Introduction

The roles of the microbiota associated with vertebrate hosts, including fish, have received much attention over the last decade. Several studies have shown that the microbiota stimulates the immune system and functions as a barrier against potential pathogens [1,2,3,4], aids in epithelial development and maturation [4,5] and affects the digestion of nutrients [6,7]. There is a bias in the type of animals studied, and still relatively few studies are published on the function of microbiota in fish. Germ-free animals have been popular tools used in studies of host–microbe interactions [13,14], and the use of gnotobiotic zebrafish is well-known [15,16]. Rawls et al [16]

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