Discrimination between transient and persistent bacteria harbored in the digestive tract of larval Galleria mellonella

Abstract

A significant emphasis of insect symbiont research has focused on the digestive tract and elucidating whether bacteria colonize and proliferate or transiently pass through is integral to our understanding of microbial community structure and host–microbe interactions. The larval stages of the greater wax moth, Galleria mellonella (Linnaeus) (Lepidoptera: Pyralidae), have become increasingly used in studies that are heavily influenced by host–microbiome dynamics (e.g., plastic biodegradation and innate immunity). However, it is presently unclear whether continual bacterial recruitment is required to sustain the bacterial assemblages, and the extent by which gut bacterial flora is a reflection of their food substrate. Therefore, the objective of this study was to discern between transient and more persistent gut microbes harbored by G. mellonella larvae, and to evaluate their relative contributions to microbial diversity and abundance. We used 16S rRNA sequencing to characterize and compare the bacteriomes of G. mellonella to their natural honeycomb diet throughout larval development, as well as to caterpillars subjected to a 4-day starvation period. Then, we used qPCR to measure relative bacterial abundances at each instar. Our results indicate larval gut bacterial composition and abundance are predominantly diet-driven, with a myriad of bacterial genera seemingly transiently present. However, several genera (e.g., Ralstonia, Pelomonas, and Cutibacterium) appear to be more permanent fixtures, presumably colonizing and proliferating in the digestive tract. Moreover, some bacterial genera co-occur, forming non-random associations that may be indicative of functional synergies. Overall, this study advances our knowledge of lepidopteran gut microbial dynamics and provides valuable information for an emerging invertebrate model.