Insects as models to study wound healing, infection and the wound microbiome

Why did we fund this project?

This award aims to model burn infections using the wax moth Galleria mellonella larvae, replacing the need for mammalian models in some burn wound studies.

Wound infection, by pathogens such as Pseudomonas aeruginosa or Acinetobacter baumannii, is the leading cause of death in patients with severe burns. This is exacerbated by the growing resistance of bacteria to antibiotics. In vitro models of burn infections can be used to characterise the cellular signalling events in response to burns but do not recapitulate the full complexity in vivo. A number of mammalian species are used in burn studies, including rabbits, dogs and pigs, but the most commonly used are mice and rats. Rodents are anaesthetised and burns induced across a predetermined surface area of the skin, typically using scalding water, with burns ranging from superficial to full thickness. These models can cause severe suffering. Dr Ronan McCarthy has developed a burn wound model in Galleria larvae and demonstrated the burns can be topically infected with pathogens, with infection resembling that seen in human patients including biofilm formation, local tissue necrosis and pathogen dissemination. Based on current thinking, Galleria larvae are not considered capable of suffering and therefore provide a partial replacement for the use of other animals.

The student will now use the Galleria larval model to study the colonisation of the burn wound by bacterial pathogens over time, including biofilm formation. They will then investigate the transcriptional response of both the Galleria and the pathogen and the role of the microbiome in wound repair. The student will gain skills in RNA-seq, omics and work with an industrial collaborator to further disseminate the model.

Almost all people at some point in their lives will experience a burn. Many of these burns are minor and can be remedied with a simple plaster to prevent infection. However, for individuals whose burns are large or deep enough to require a hospital visit, they run a higher risk of the burn wound becoming infected. In fact, infection is responsible for 75% of deaths among burn patients. This exceptionally high mortality rate has meant studying burn wound infection is a top priority. However, due to the physiological complexity of a burn wound, wound healing and infection cannot be adequately studied in vitro.

Up until this proposal, studying wound healing and infection in vivo was only possible using mammalian models and procedures that are classed as severe. There are numerous animals commonly used in burn wound studies with the most popular being mice and rats. In 2019, there were over 450 publications published using mouse burn models, with each study using between 45-105 mice. There has also been a worrying trend of increasing numbers of larger mammals being used in burn wound studies including rabbits, dogs and pigs. There has been a doubling in the use of porcine models over the decade, with over 90 publications in 2019 with between 3-10 animals per study. Burn wound models are associated with high levels of severity, high cost, prolonged suffering and restricted reproducible biological outputs. The increase in the use of mammalian models is due to a lack of alternative less ethically challenging models.

Our work has developed for the first time an invertebrate model of burn wound trauma and concomitant infection using the greater wax moth larvae, Galleria mellonella. This model has led to an 80% reduction in the use of mouse models by our group. Invertebrate models of infection such as G. mellonella are well-established and accepted models of pathogenicity for a whole host of human pathogens. Their use has exploded over the last 10 years from 61 publications in 2010, to over 270 in 2019. This surge in growth has been spearheaded by their low cost and the absence of ethical issues associated with their use. There are also numerous similarities between the mammalian immune system and that of G. mellonella making them an attractive model to study the host immune response. We have demonstrated that the G. mellonella burn wound model has remarkable similarities with mammalian burn wound models with key paradigms such as the role of fluid resuscitation in survival, and the link between percentage burn surface area and survival prognosis being conserved. We have also demonstrated that these larval burn wounds can be infected topically by leading burn wound pathogens such as Pseudomonas aeruginosa or Acinetobacter baumannii, replicating the mode of infection seen in the hospital environment. Our work has also shown that the infection progression cycle matches what is seen in human cases, with biofilm formation, local tissue necrosis, dissemination from the burn wound site, and ultimately mortality. This proposal will utilize this model of infection further to gain insights into the temporal kinetics underpinning bacterial colonisation of the burn site. We also aim to explore the host/pathogen transcriptional response to burn trauma and infection. Gaining these insights will open up this model to a wider audience and demonstrate that it can used as an alternative to mammalian models to study wound infection and the host immune response. We will also explore the role of the skin microbiome in promoting wound healing and preventing infection and demonstrate how this model can be used as an alternative to mammalian models to study the wound microbiome.

We expect that the insights that emerge from this proposal will lead to a 20% drop in the numbers of mammals used in wound studies as they will demonstrate the versatility of this model and its capacity to replace mammals in drug discovery, pathogenicity and microbiome studies.