Fire in the belly: Stress and antibiotics induce dysbiosis and inflammation in the gut of common carp

Fish are exposed to numerous stressors which negatively affect their immune response and increase infection susceptibility. The risk of bacterial infections results in the excessive and preventive use of antibiotics. Therefore, we aimed to study how antibiotic treatment and restraint stress will affect the stress response, microbiota composition, gut morphology, and inflammatory reaction in common carp. Both restraint stress and antibiotic treatment increased cortisol level. Moreover, antibiotics induced dysbiosis in fish gut, manifested by a decrease in the total abundance of bacteria, and a shift in bacteria diversity, including a reduced number of Aeromonas, Bacteroides, Barnesiellaceae, Cetobacterium and Shewanella and an increased abundance of Flavobacterium. To a lesser extent, stress modified gut microbiota, as it decreased bacteria number and slightly changed the microbiota composition by decreasing Cetobacterium abundance and increasing Vibrio abundance. Microbiota of the antibiotic-treated and stressed fish shifted from the beneficial bacterial genera - Cetobacterium and Bacteroides, to the increased presence of unfavorable bacteria such as Brevinema, Flavobacterium and Desulfovibrionaceae. Stress and antibiotic-induced changes in the gut microbiota were related to the changes in the gut morphology when the higher abundance of goblet and rodlet cells and increased secretion activity of goblet cells were observed. Moreover, up-regulation of the expression of genes encoding pro-inflammatory mediators and cytokines involved in the Th17 immune response was present in the gut of the antibiotic-treated and stressed fish. We conclude that in carp antibiotics and stress alter the abundance and composition of the microbiota and induce Th17-dependent inflammatory reaction in the gut. Moreover, our results strongly suggest the interplay of the stress axis and the brain-gut-microbiota axis.