Effect of iron on the infection in G. mellonella larvae

Mycetoma is a neglected, chronic granulomatous infection of the subcutaneous tissue, most often caused by the fungal pathogen Madurella mycetomatis. Characteristic of the infection is the formation of grains. However, knowledge of the function and formation of the grain is limited. To map the processes leading to M. mycetomatis grain formation, we used a Galleria mellonella larvae infection model and time-course transcriptomic profiling. In the infected G. mellonella 88.0% of the RNA sequence reads mapped to G. mellonella, only 0.01% mapped to M. mycetomatis. Differential Gene Expression analysis revealed that 3.498 G. mellonella and 137 M. mycetomatis genes were differentially expressed during infection. Most of the enriched GO terms of both host and pathogen are linked to energy pathways, nucleobase metabolic process, and cation and iron transport. Genes related to iron transport were highly expressed by both G. mellonella (transferrin and ferritin) and M. mycetomatis (SidA, SidD and SidI). A protein-protein interaction network analysis of D. melanogaster homologues genes in M. mycetomatis revealed the expression of the entire siderophore biosynthesis pathway throughout infection. Many host and pathogen genes were differentially expressed during infection. The identification of the importance of iron acquisition during grain formation can be exploited as a potential novel diagnostic and therapeutic strategy for mycetoma. Overall design: Galleria mellonella larvae were injected with FeCl2 and FeCl3 to a final concentration of 25 µM to create an iron overload, and 332 µM ethylenediamine-N,N'-diacetic acid (EDDA) based on previous findings of Dunphy et al. in order to create an iron deficit [PMID:12770052]. Healthy larvae were first used to determine the toxicity of the different conditions to the larvae. Next, larvae were injected with either FeCl2, FeCl3 or EDDA to final concentrations as stated above. After 30 minutes, the larvae were infected with M. mycetomatis strain MM55 and survival was monitored for 10 days. Additionally, after 4, 24, and 72 hours larvae were sacrificed for RNA isolation and histological examination.