Dietary iron supplementation increases the severity of Enterococcus faecalis infections in Manduca sexta larvae

Iron is essential for the growth of all types of organisms, including insects and insect pathogens. Insects fed a diet supplemented with extra iron accumulate more iron than insects fed an unsupplemented diet, and, because infectious microbes acquire iron from their hosts, we hypothesized that a high iron diet would increase the severity of bacterial infections in insects. To test this hypothesis, we evaluated the effects of iron supplementation on extracellular bacterial infections in Manduca sexta larvae. Larvae were fed a typical artificial diet or the same diet supplemented with 10 mM iron. For the first set of experiments, the diet contained antibiotics; for the second set of experiments, the diet was antibiotics-free. Larvae were inoculated with the non-pathogenic Escherichia coli or the entomopathogenic Enterococcus faecalis. The severity of infections was evaluated by measuring larval mortality and the number of bacteria in hemolymph. For larvae inoculated with E. coli, iron supplementation had no effect on infection severity, regardless of the presence or absence of antibiotics in the diet. In contrast, for larvae inoculated with E. faecalis, iron supplementation combined with antibiotics increased infection severity, whereas iron supplementation without antibiotics decreased infection severity. These results indicate that the relationship between dietary iron and susceptibility to infection is complex. This study is part of a larger project focused on understanding the mechanisms of iron homeostasis during infection in insects, and it provides a basis for follow-up studies focused on optimizing the health of farmed insects.