Most animals and plants harbor microbial symbionts that have beneficial and fundamentally important impacts on host biology. One of the best-characterized examples of mutualistic interaction is termites and gut symbionts that are essential for cellulose digestion. Recently, there is growing interest in the “extended” role of gut microbes beyond their nutritional functions. Here we show that the gut symbionts of a damp-wood termite Zootermopsis nevadensis protect nests from the spread of an environmental bacterium Serraita marcescens with pathogenic potential. Defaunated (gut symbionts removed) termites dispersed S. marcescens which survived through termite gut passage, while control termites did not. Removal of gut symbionts caused significant decrease of intestinal acetate, which is important carbon and energy source for termites produced by gut symbionts. Culture experiments showed that bacteriostatic activity of acetate was lost in such low concentration as in defaunated termite. These result showed that acetate produced by gut microbes contribute to nest hygiene beyond its nutritional roles. This study provide novel insight into understanding multifunctionality of symbiotic microorganisms.