A strategic model of a host-microbe-microbe system reveals the importance of a joint host-microbe immune response to combat stress-induced gut dysbiosis.

Microbiomes provide key ecological functions to their host; however, most host-associated microbiomes are too complicated to allow a model of essential host-microbe-microbe interactions. The intestinal microbiota of salmonids may offer a solution since few dominating species often characterize it. Healthy fish coexist with a mutualistic sp. species, while stress allows the spread of pathogenic strains, such as sp. Even after a skin infection, the does not recover; sp. often remains the dominant species, or - coexistence was occasionally observed. We devised a model involving interactions among the host immune system, sp. plus a toxin-producing pathogen. Our model embraces a complete microbiota community and is in harmony with experimental results that host- mutualism prevents the spread of pathogens. Contrary, stress suppresses the host immune system allowing dominance of pathogens, and does not recover after stress disappears.