A Falciformispora senegalensis grain model in Galleria mellonella larvae.

Eumycetoma is a subcutaneous implantation mycosis often found in the foot. One of the hallmarks of eumycetoma is the formation of grains. These grains are either black or white and the consistency and morphology differs per causative agent. The two most common causative agents of black-grain eumycetoma are Madurella mycetomatis and Falciformispora senegalensis. Since grains cannot be formed in vitro, in vivo models are needed to study grain formation. Here, we used the invertebrate Galleria mellonalla to establish an in vivo grain model for F. senegalensis. Three different F. senegalensis strains were selected and four different inocula were used to infect G. mellonella larvae, ranging from 0.04 mg/larvae to 10 mg/larvae. Larval survival was monitored for 10 days. Grain formation was studied macroscopically and histologically. Efficacy of antifungal therapy was determined for itraconazole, amphotericin B and terbinafine. A concentration of 10 mg F. senegalensis per larvae, was lethal for the majority of the larvae within 10 days. At this inoculum, grains were formed within 24 hours after infection. The grains produced in the larvae resembled those formed in human patients. Amphotericin B given at 1 mg/kg 4 h, 28 h and 52 h after infection prolonged larval survival. No enhanced survival was noted for itraconazole or terbinafine. In conclusion, we developed a F. senegalensis grain model in Galleria mellonella larvae in which grains were formed which were similar to those formed in patients. This model can be used to monitor grain formation over time and to study antifungal efficacy.