Contribution of gut symbionts to the host physiology of a wood-boring beetle

Psacothea hilaris hilaris is a xylophagous beetle belonging to the Cerambycidae family. Its larvae grow inside mulberry tree and fig trunks, reaching the length of up to three centimetres and damaging the plants to death. In Japan, this is a serious pest for sericulture, while its recent establishment in northern Italy could threaten the fig cultivation in the Mediterranean basin. In this study, we characterized the gut bacterial communities associated to wild and laboratory-reared P. h. hilaris larvae by molecular and culture-dependant methods. PCR-Denaturant Gradient Gel Electrophoresis (DGGE) was applied to assess the significant effect of the diet and the gut tract in modifying the taxonomical composition of the gut bacterial community. Field-caught larvae and larvae reared on diet in presence of antibiotics and preservative showed richer communities than larvae reared on artificial diet, not exposed to antibiotics and/or preservatives; in this last case the larvae were dominated by Enterococcus and Leuconostoc sequences. This probably was due to the overgrowth of these microbes on the diets where antibiotics and/or preservatives were removed. PCR-DGGE analysis showed a significant difference in the bacterial community composition between the midgut and the hindgut, probably correlated to the different physiological conditions. By using a collection of bacterial isolates from the guts of wild larvae we evaluated the possible contribution of the isolates to the host physiology (in terms of contribution to carbon or nitrogen uptake) through in vitro tests. Results suggested that symbiont isolates can contribute to the cellulose digestion or exploit the by-products of the degradation of cell wall compounds and help their host to absorb nitrogen converting waste molecules (uric acid and urea) or proteins to ammonia and smaller peptides, or even fixing atmospheric nitrogen. From our observations, the cultivable gut bacterial community of P. h. hilaris appears to include many different commensals, which are suitable to exploit the food sources in the gut and to give a contribution to the host metabolism.