Osmolality as a Novel Mechanism Explaining Diet Effects on the Outcome of Infection with a Blood Parasite.

Recent research has suggested that the outcome of host-parasite interactions is dependent on the diet of the host, but most previous studies have focused on "top-down" mechanisms, i.e., how the host's diet improves the host immune response to drive down the parasite population and improve host fitness. In contrast, the direct impacts of host nutrition on parasite fitness and the mechanisms underpinning these effects are relatively unexplored. Here, using a model host-pathogen system (Spodoptera littoralis caterpillars and Xenorhabdus nematophila, an extracellular bacterial blood parasite), we explore the effects of host dietary macronutrient balance on pathogen growth rates both in vivo and in vitro, allowing us to compare pathogen growth rates both in the presence and absence of the host immune response. In vivo, high dietary protein resulted in lower rates of bacterial establishment, slower bacterial growth, higher host survival, and slower speed of host death; in contrast, the energy content and amount of carbohydrate in the diet explained little variation in any measure of pathogen or host fitness. In vitro, we show that these effects are largely driven by the impact of host dietary protein on host hemolymph (blood) osmolality (i.e., its concentration of solutes), with bacterial growth being slower in protein-rich, high-osmolality hemolymphs, highlighting a novel "bottom-up" mechanism by which host diet can impact both pathogen and host fitness.

Diet modulates the relationship between immune gene expression and functional immune responses.

Nutrition is vital to health and the availability of resources has long been acknowledged as a key factor in the ability to fight off parasites, as investing in the immune system is costly. Resources have typically been considered as something of a "black box", with the quantity of available food being used as a proxy for resource limitation. However, food is a complex mixture of macro- and micronutrients, the precise balance of which determines an animal's fitness. Here we use a state-space modelling approach, the Geometric Framework for Nutrition (GFN), to assess for the first time, how the balance and amount of nutrients affects an animal's ability to mount an immune response to a pathogenic infection. Spodoptera littoralis caterpillars were assigned to one of 20 diets that varied in the ratio of macronutrients (protein and carbohydrate) and their calorie content to cover a large region of nutrient space. Caterpillars were then handled or injected with either live or dead Xenorhabdus nematophila bacterial cells. The expression of nine genes (5 immune, 4 non-immune) was measured 20 h post immune challenge. For two of the immune genes (PPO and Lysozyme) we also measured the relevant functional immune response in the hemolymph. Gene expression and functional immune responses were then mapped against nutritional intake. The expression of all immune genes was up-regulated by injection with dead bacteria, but only those in the IMD pathway (Moricin and Relish) were substantially up-regulated by both dead and live bacterial challenge. Functional immune responses increased with the protein content of the diet but the expression of immune genes was much less predictable. Our results indicate that diet does play an important role in the ability of an animal to mount an adequate immune response, with the availability of protein being the most important predictor of the functional (physiological) immune response. Importantly, however, immune gene expression responds quite differently to functional immunity and we would caution against using gene expression as a proxy for immune investment, as it is unlikely to be reliable indicator of the immune response, except under specific dietary conditions.

Development of a Real-Time qPCR Assay for Quantification of Covert Baculovirus Infections in a Major African Crop Pest.

Many pathogens and parasites are present in host individuals and populations without any obvious signs of disease. This is particularly true for baculoviruses infecting lepidopteran hosts, where studies have shown that covert persistent viral infections are almost ubiquitous in many species. To date, the infection intensity of covert viruses has rarely been quantified. In this study, we investigated the dynamics of a covert baculovirus infection within the lepidopteran crop pest Spodoptera exempta. A real-time quantitative polymerase chain reaction (qPCR) procedure using a 5' nuclease hydrolysis (TaqMan) probe was developed for specific detection and quantification of Spodoptera exempta nucleopolyhedrovirus (SpexNPV). The qPCR assay indicated that covert baculovirus dynamics varied considerably over the course of the host life-cycle, with infection load peaking in early larval instars and being lowest in adults and final-instar larvae. Adult dissections indicated that, contrary to expectation, viral load aggregation was highest in the head, wings and legs, and lowest in the thorax and abdomen. The data presented here have broad implications relating to our understanding of transmission patterns of baculoviruses and the role of covert infections in host-pathogen dynamics.