Exploring the antifungal effects and underlying mechanism of Carica papaya seed essential oil against Candida tropicalis in vitro and in vivo.

ETHNOPHARMACOLOGICAL RELEVANCE: Carica papaya seeds can improve indigestion, relieve rheumatic pain and be applied as a paste for treating ringworm and psoriasis in traditional medicine, which indicates that they possess antibacterial, antifungal, and anti-inflammatory properties. As one of the most common non-albicans Candida species, Candida tropicalis has led to a notable rise in morbidity and mortality rates because of increased drug resistance. Papaya seed essential oil (PSEO) contained over 99% benzyl isothiocyanate and exhibited antifungal activity against C. tropicalis, particularly fluconazole resistant C. tropicalis in our previous study. However, the underlying inhibitory mechanism remains elusive and the therapeutic efficacy of PSEO on C. tropicalis in vivo has not yet been studied.

MATERIALS AND METHODS: In this study, the antifungal effects of PSEO in vitro on various aspects including the hyphal formation, cell morphology, membrane integrity, protein concentration, mitochondrial membrane potential (MMP) and reactive oxygen species (ROS) against both sensitive and resistant C. tropicalis were systematically investigated. The transcriptome analysis was carried out by RNA-seq after C. tropicalis exposing to PSEO. The in vivo therapeutic efficacy of PSEO on resistant C. tropicalis was evaluated.

RESULTS: PSEO had powerful antifungal effects against both sensitive and resistant C. tropicalis, with minimum inhibitory concentration values of 32.0-64.0 μg/mL and minimum fungicidal concentration values of 128.0 μg/mL. When treated with PSEO, C. tropicalis exhibited inhibited hyphal growth, damaged cell wall and membrane, reduced protein concentration and MMP, and induced ROS accumulation. The transcriptome analysis revealed that PSEO regulated differential expression genes associated with the cell surface, cell wall, ribosomes and mitochondria, which corroborated the alterations in the phenotypic parameters observed above. Moreover, compared to sensitive C. tropicalis, PSEO exhibited a more pronounced effect on resistant C. tropicalis. In vivo, PSEO showed a superior therapeutic efficacy on mice model of systemic resistant C. tropical infection compared with that of fluconazole, significantly improving survival rates and body weight, reducing fungal burden in multiple organs, and alleviating pathological damage in the heart, kidneys, and liver.

CONCLUSIONS: These findings revealed the in vitro and in vivo antifungal effects and underlying mechanism of PSEO against C. tropicalis, especially resistant C. tropicalis. It validates the use of Carica papaya seed in ethnopharmacology and provides novel insights for the development of PSEO as a natural antifungal agent in the fight against emerging C. tropicalis drug resistance.