Susceptibility Testing of Exophiala dermatitidis to Inform Peri-Operative Prophylaxis at Time of Lung Transplantation

Abstract

Purpose Exophiala dermatitidis is a black fungus that frequently colonises the lungs of people with cystic fibrosis (CF). Mucoid and non-mucoid variants can be detected from the same sputum for some patients, but their role in chronic CF lung disease is not fully understood and attempts at early eradication are not routinely performed. Invasive infections with this species are rare; however in 2014 we reported our first case of fatal, invasive infection with a highly mucoid strain of E. dermatitidis in a 34 year old post lung transplant patient with CF. This prompted us to include peri-operative antifungal prophylaxis for all patients colonised with this species. To inform the choice of optimal antifungal, we performed susceptibility testing of 38 isolates from 34 patients including lung transplant patients. The ability of these isolates to produce biofilm was also investigated. Methods The minimum inhibitory concentration (MIC) of 8 antifungal agents was measured using a commercial broth microdilution assay. Biofilm formation was assessed using a standard in vitro assay using staining with crystal violet. Results Using breakpoints for Candida albicans, all isolates were susceptible to voriconazole and amphotericin B with most isolates also susceptible to itraconazole (89% of isolates) and posaconazole (71% of isolates). In contrast, all isolates were non-susceptible to caspofungin and most isolates (92%) non-susceptible to fluconazole. All strains were inhibited by ketoconazole at ≤0.25 mg/ L whereas MICs of 5-fluorocytosine ranged from 1->64 mg/ L. 31 of 38 isolates were biofilm producers including 4 isolates that were classified as strong biofilm producers. We did not find a correlation between mucoid phenotype and biofilm production. Conclusion E. dermatitidis is commonly found to colonise the lungs of patients with CF, typically produces biofilm, and may rarely produce serious infection post transplantation. We demonstrate in vitro susceptibility to voriconazole and amphotericin B. For patients colonised with this species, we now elect to include voriconazole as part of the peri-operative prophylactic regimen.