Nutrient factories: metabolic function of beneficial microorganisms associated with insects

Many symbiotic microorganisms in animals, including insects, have parallels to microbial nutrient factories of biotechnology: just as the metabolism of individual microorganisms and microbial communities is modified by biotechnologists to produce specific nutrients, so the many insect-associated microorganisms synthesize specific nutrients that support the sustained growth and reproduction of their animal host. Three broad metabolic functions are mediated by insect-associated microorganisms: (i) fermentation of dietary constituents, releasing products that contribute to host carbon and energy metabolism; (ii) overproduction of nutrients, notably essential amino acids, required by the host and (iii) recycling of host waste metabolites. In many systems, the nutrients that are released from living microbial cells have been identified, with evidence for metabolite cross-feeding and shared metabolic pathways both among different microbial taxa and between microorganisms and the host. However, the flux of nutrients from microbial cells to host has rarely been quantified; our understanding of the processes that regulate nutrient transfer is fragmentary; and the scale and mechanism of metabolic adaptations of microorganisms to host nutritional demand are largely unknown. Recent advances in metabolic, microscopical and modelling techniques offer excellent opportunities to resolve these outstanding issues, with insights that can contribute to the effective design of nutrient factories for biotechnological applications.