Paving the way to acceptance of Galleria mellonella as a new model insect

Abstract

First paragraph: The larva of the greater wax moth Galleria mellonella is an alternative host used commonly in studies of microbial infection and innate immunity. Indeed, this insect host is often used when quantifying or comparing the virulence of bacterial and fungal pathogens of vertebrates and it has been used successfully to establish the importance of microbial virulence factors and to determine the relative virulence of different isolates of the same species. The recent popularity of G. mellonella as an alternative host system stems from numerous benefits, including the ability to perform experiments at a range of temperatures including human body core temperature; the technical simplicity of establishing infections by various routes such as through feeding, topical application or injection; the convenient size of the insect, which means it is large enough to permit simple injection of inoculums or chemicals but small enough to require little space in the laboratory; the ability to assess the efficacy and toxicity of antimicrobial therapies; and the ease and reliability with which these insects can be sourced in their final instar stage from commercial suppliers. It has also found approval amongst many researchers due to the favourable reproducibility between experiments in the same laboratory. Nevertheless, relatively small variations in susceptibility to infection can occur between batches of larvae from the same supplier and such variation probably arises from factors such as age, size and nutritional status on receipt; conditions encountered during transit to the laboratory; and the presence of any underlying natural infections. These issues are largely uncontrollable when purchasing larvae from a commercial supplier but on reaching the laboratory standardised pre-experimentation storage conditions can improve reproducibility between studies. In recent years the Kavanagh group have raised awareness for the role of a number of variables during storage that require consideration to ensure optimal reproducibility when experimenting with this insect, and factors influencing G. mellonella susceptibility to infections include physical stress, incubation temperature and access to food. In this edition of Virulence, the Kavanagh group report that larvae become increasingly susceptible to infection by pathogens as laboratory storage time increases, highlighting the need to consider this parameter when using the G. mellonella model. Browne et al. elaborate further in the study and relate this observation to a reduction in the total abundance of haemocytes that function in immune defence against pathogens and changes in the relative flux of metabolic pathways. Interestingly, the number of haemocytes after 3 weeks of incubation was approximately half that compared to the population at one week, while qualitative changes in the relative abundance of the various types of haemocytes were also reported. Both these factors probably contribute to reduced immune capacity and thus increased susceptibility to infection.