Development and role of the indigenous gut microbiota of Spodoptera littoralis

Microbial symbionts are known to live in close associations with animals, plants, and insects. Insects harbor commensal microbes that provide the host with essential nutrients, aid in food digestion, and protection against pathogens. The cotton leafworm, Spodoptera littoralis (Lepidoptera: Noctuidae) is one of the most successful polyphagous insect pests that causes economic losses in agricultural industry. The gut microbial communities of S. littoralis are well characterized, and the insect is a popular model to study insect-microbe and microbe-microbe interactions. It is known that the gut of S. littoralis is predominated by Enterococcus mundtii and Clostridium sp. towards the late stage of larvae. However, the composition of gut microbiome of S. littoralis in other stages (eggs, pupae, and adults) was not defined in previous studies. The 16S rRNA amplicon sequencing based on DNA and cDNA levels has shown that E. mundtii was present in the eggs, pupae, and adults (Article II). Thus far, no study has been done to investigate the colonization of E. mundtii in the gut tissues across life stages of the host insect. To gain better understanding of its survival strategies, GFP-tagged E. mundtii was constructed to track its colonization in the intestine throughout different stages of development (larvae, pupae, adults, and eggs). Fluorescent bacteria survived and proliferated in the intestinal tract of the insect for all life stages, eventually entering second generation offspring following ingestion (Article I). This shows that symbiotic bacterium was vertically transmitted from the mother to progeny. However, the adaptation mechanisms of symbiotic Enterococcus in the gut environment are unknown. Hence, we sequenced the genome of E. mundtii to better decode genes that are important for gut colonization (Article III). To better explore real-time metabolic activities, transcriptome analysis of bacteria isolated from the foregut and hindgut of the insect by RNA-sequencing was performed (Unpublished results part II). ...