Combiningin vivoandin vitroapproaches to better understand host-pathogen interactions

Abstract

ABSTRACTNutrition often shapes the outcome of host-parasite interactions, however understanding the mechanisms by which this occurs is confounded by the intimate nature of the association and by the fact that the host and parasite may compete for the same limiting nutrients. One way of disentangling this interaction is to combinein vivoandin vitroapproaches. Here, we explore the role of host nutrition in determining the outcome of infections using a model insect-bacterium system: the cotton leafwormSpodoptera littoralisand the blood-borne bacteriumXenorhabdus nematophila.Spodoptera littoralislarvae were reared on one of a series of 20 chemically-defined diets ranging in their protein: carbohydrate (P:C) ratio and caloric density. They were then challenged with either a fixed dose ofX. nematophilacells (live or dead) or were sham-injected. Survivorship of larvae challenged with live bacterial cells was strongly dependent on the protein levels of the diet, with mortality being highest on low-protein diets. This trend was reflected in the bacterial growth ratein vivo, which peaked in larvae fed low-protein diets.To determine whetherin vivobacterial growth rates were driven by blood nutrients, rather than an enhanced host immune response, we generated 20 synthetic haemolymphs (‘nutribloods’) that mimicked the nutritional content of host blood. Bacterial growth rate in the nutribloods was also negatively impacted by their protein content suggesting that nutrient availability and not host immunity was driving the interaction. By comparing standardized bacterial growth ratesin vivoandin vitro, we conclude that the outcome of this host-parasite interaction is largely driven by ‘bottom-up’ effects of nutrients on bacterial growth, rather than by ‘top-down’ effects of nutrients on host-mediated immune responses. The outcome of host-parasite interactions is typically assumed to be strongly determined by the host immune response. The direct effects of nutrition have been underexplored and may have broad consequences for host-parasite interactions across taxa.