<b>Pathobiont and symbiont coordination maintains microbiota homeostasis through Malpighian tubules-gut countercurrent flow in the fly</b><b> </b><b><i>Bactrocera dorsalis</i></b>

Host-gut microbiota interactions are more complex than good and bad. Both gut symbiotic bacteria and pathobionts can provide essential functions to their host in one scenario and yet be detrimental to host health in another. So, these gut-dwelling bacteria must be tightly controlled to avoid harmful effects on the host. However, how pathobionts and other symbiotic bacteria coordinate to establish a host immune defense system remains unclear. Here, using a Tephritidae fruit fly <i>Bactrocera dorsalis</i>, we report that both pathobionts and other gut symbiotic bacteria release tyramine, which is recognized by the host insects. These tyramines induce the formation of conserved Malpighian tubules-gut countercurrent flow upon bacterial infection, which requires tyramine receptors and aquaporins. At the same time, pathobionts but not other gut symbiotic bacteria induce the generation of ROS, which are preserved by the countercurrent flow, promoting bacteria elimination through increasing gut peristalsis. More importantly, our results show that the Malpighian tubules-gut countercurrent flow maintains proper microbiota composition. Our work suggests a model where pathobiont-induced ROS are preserved by both pathobionts and symbiotic bacteria-induced Malpighian tubules-gut countercurrent flow. Furthermore, our work provides a Malpighian tubules-gut interaction that ensures efficient maintenance of the gut microbiota.