Abstract
Animals continuously encounter microorganisms that are essential for health or cause disease. They are thus challenged to control harmful microbes while allowing acquisition of beneficial microbes, a challenge that is likely especially important concerning microbes in food and in animals such as social insects that exchange food among colony members. Here we show that formicine ants actively swallow their antimicrobial, highly acidic poison gland secretions after feeding. The ensuing creation of an acidic environment in the stomach, the crop, improves individual survival in the face of pathogen contaminated food and limits disease transmission during mutual food exchange. At the same time, crop acidification selectively allows acquisition and colonization by known bacterial gut associates. The results of our study suggest that swallowing of acidic poison gland secretions acts as a microbial filter in formicine ants and indicate a potentially widespread but so far underappreciated dual role of antimicrobials in host-microbe interactions.
