Temperature-dependent Galleria mellonella mortality as a result of Yersinia entomophaga Infection

The bacterium Yersinia entomophaga is pathogenic to a range of insect species, with death typically occurring within 2–5 days of ingestion. Per os challenge of larvae of the greater wax moth (Galleria mellonella) confirmed that Y. entomophaga was virulent when fed to larvae held at 25°C, but was avirulent when fed to larvae maintained at 37°C. At 25°C, a dose of approximately 4 × 107 colony forming units (CFU) per larva of a Yen-TC toxin complex deletion derivative, Y. entomophaga ΔTC, resulted in 27% mortality. This low level of activity was restored to near wild-type levels by augmentation of the diet with a sub-lethal dose of purified Yen-TC. Intra-hemocoelic injection of approximately three Y. entomophaga or Y. entomophaga ΔTC cells per larva gave a 4-day median lethal dose, with similar levels of mortality observed at both 25°C and 37°C. Following intra-hemocoelic injection of a Yen-TC YenA1 green fluorescent protein-fusion strain into larvae maintained at 25°C, the bacteria did not fluoresce until the population density reached 2 × 107 CFU ml−1 of hemolymph. The observed cells also took an irregular form. When the larvae were maintained at 37°C, the cells were small and the observed fluorescence was sporadic and weak, being more consistent at a population density of ~3 × 109 CFU ml−1 of hemolymph. These findings provide further understanding of the pathobiology of Y. entomophaga in insects, showing that the bacterium gains direct access to the hemocoelic cavity, from where it rapidly multiplies to cause disease.