Formicine ants swallow their highly acidic poison for gut microbial selection and control
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. This challenge is likely especially important for social insects with respect to microbes in food, as they often store food and exchange food among colony members. Here we show that formicine ants actively swallow their antimicrobial, highly acidic poison gland secretion. The ensuing acidic environment in the stomach, the crop, limits establishment of pathogenic and opportunistic microbes ingested with food and improves survival of ants when faced with pathogen contaminated food. At the same time, crop acidity selectively allows acquisition and colonization by Acetobacteraceae, known bacterial gut associates of formicine ants. This suggests that swallowing of the poison gland secretion acts as a microbial filter in formicine ants and indicates a potentially widespread but so far underappreciated dual role of antimicrobials in host-microbe interactions.