Transient Microbes in Insects: Fleeting but Functional

Many insects' gut microbiota derive partly or wholly from environmental sources. These microbes may be transient, passing through in a matter of hours, days, a developmental stage, or a host generation. There is increasing recognition of the presence of transient microbes in the insect gut, but it is often assumed that these microbes are commensal and serve no function for their hosts. Here, we explore different definitions of microbial transience and review results from diverse insect systems showing that transience does not always preclude, and in some cases enables, important contributions of environmentally acquired microbes to host fitness. Moving past the assumption that microbes must always be tightly associated with a host to serve beneficial functions will help us develop a more accurate and nuanced understanding of the functions of the gut microbiota in insects and other animals.