Histoplasma capsulatum in planktonic and biofilm forms: in vitro susceptibility to amphotericin B, itraconazole and farnesol.

It is believed that most microbial infections are caused by pathogens organized in biofilms. Recently, it was shown that the dimorphic fungus Histoplasma capsulatum, estimated to be the most common cause of fungal respiratory diseases, is also able to form biofilm. Although the antifungal therapy commonly used is effective, refractory cases and recurrences have been reported. In the search for new compounds with antimicrobial activity, the sesquiterpene farnesol has gained prominence for its antifungal action. This study aimed to evaluate the in vitro susceptibility of H. capsulatum var. capsulatum to the antifungal agents itraconazole and amphotericin B, and farnesol alone and combined, as well as to determine the in vitro antifungal activity of these compounds against biofilms of this pathogen. The results show that farnesol has antifungal activity against H. capsulatum in the yeast and filamentous phases, with MIC values ranging from 0.0078 to 0.00312 µM. A synergistic effect (fractional inhibitory concentration index ≤0.5) between itraconazole and farnesol was found against 100 and 83.3 % of the isolates in yeast and mycelial forms, respectively, while synergism between amphotericin B and farnesol was only observed against 37.5 and 44.4 % of the isolates in yeast and filamentous forms, respectively. Afterwards, the antifungal drugs, itraconazole and amphotericin B, and farnesol alone, and the combination of itraconazole and farnesol, were tested against mature biofilms of H. capsulatum, through XTT (2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium hydroxide) metabolic assay, and the itraconazole and amphotericin B showed lower antibiofilm activity when compared to farnesol alone and farnesol combined with itraconazole. In conclusion, farnesol showed promising results as an antifungal agent against H. capsulatum and also showed adjuvant action, especially when combined with itraconazole, increasing the fungal susceptibility to this drug.