Fish disease and intestinal microbiota: A close and indivisible relationship

The gut microbiota is currently one of the most studied ‘organs’ in animals, and fish are no exception. A complex diversity of microbes, including bacteria, archaea, yeast and fungus, constitute the gut microbiota, creating a complex interaction with their host and accomplishing multiple beneficial functions, such as food digestion, nutrient absorption, immune system, endocrine and stress response. The microbiota–pathogen interaction protects the host by mounting colonization resistance, competing for nutrients and space. Changes in the balance of the microbiota community could affect their structure and homeostasis, inducing dysbiosis. In addition, invading pathogens can induce dysbiosis by evading the host's defence barriers, acquiring nutrients from the fish host, using metabolites produced by the microbiota and producing toxins. In this regard, understanding the interactions within the fish gut microbiota is essential to prevent pathogen establishment in the host. Hence, this review describes the close and indivisible relationships and interactions between gut microbiota and pathogens that could lead to dysbiosis and disease.