Nutribloods: novel synthetic lepidopteran haemolymphs for understanding insect-microbe interactions in vitro

Understanding the role of nutrients in microbial population dynamics relies on a sound appreciation of their nutritional environment and how this may vary in different habitats. For microbial pathogens and commensals, this can be especially challenging because the microbe may share nutritional resources with its host. Here we design a series of 20 synthetic haemolymphs (nutribloods) that mimic haemolymph nutrient profiles of caterpillars fed on one of 20 chemically-defined diets, that vary in their protein:carbohydrate (P:C) ratio and caloric density. Using these, we are able to simulate the range of nutritional conditions that insect blood pathogens might face, providing a model system for understanding the role of nutrition in microbial growth. We tested this using the entomopathogen, Xenorhabdus nematophila, a gram-positive extracellular bacterium of insect hosts. This revealed that whilst bacterial fitness peaked in nutriblood nutrient space that was high in carbohydrates and low in proteins, levels of amino acids in the nutribloods also appear to be an important driving force for bacterial growth. Using synthetic haemolymphs that had average levels of all nutrients other than carbohydrate, protein or amino acids, we also established that bacterial growth is generally enhanced by carbohydrate and amino acids but reduced by proteins. Here, we have established a tractable model system for examining the role that nutrition plays in the growth of an entomopathogenic bacterium. In future work, this model host-pathogen system can be used to test a range of nutritionally-driven processes, including competition during co-infection and interactions with the host microbiome, as well as comparative studies of other entomopathogens.