The development of an Actinomadura madurae grain model in Galleria mellonella larvae

Mycetoma is a neglected tropical disease characterized by large tumorous lesions. It can be caused by fungi (eumycetoma) or bacteria (actinomycetoma). The hallmark of mycetoma is the formation of grains by the causative agent. Grains can only be formed in vivo; therefore, in vivo models are crucial to studying mycetoma. In vivo, grain models have been developed in the invertebrate Galleria mellonella for eumycetoma, but not for actinomycetoma. Here, we aimed to develop the first actinomycetoma grain model in G. mellonella larvae. Actinomadura madurae strains DSM43236 and DSM44005 were used to infect G. mellonella larvae. Larval survival was monitored over 10 days. Grain formation was studied histologically and compared to grains in human tissues. The efficacy of trimethoprim-sulfamethoxazole and amikacin, the current standard treatment for actinomycetoma, was determined. A. madurae infection decreased the survival of G. mellonella larvae in a concentration-dependent manner. Grains were formed within 24 h post-infection. After 72 h these grains became melanised. No significantly enhanced survival was noted with trimethoprim-sulfamethoxazole, amikacin, or a combination thereof. In this model, the melanised A. madurae grains did differ from human grains, most likely due to the immune system of G. mellonella. The lack of therapeutic efficacy could be caused by this melanin or the fact that A. madurae grains, in general, are less susceptible to these drugs. More research will be needed to address this question.