Antifungal susceptibilities of emerging and novel filamentous fungal pathogens

Abstract

The incidence of fungal diseases is increasing with the inevitable aging population and the expansion of immunocompromised patients. Through the advancement of molecular technologies, it has been revealed that many rare/cryptic and novel fungal species were associated with human diseases. However, knowledge of their antifungal susceptibility profiles and clinical spectrum is very limited, especially in Hong Kong. In the present study, it was hypothesized that the antifungal susceptibility profiles and clinical spectrum of these emerging and novel filamentous fungi are different compared to those of the commonly encountered species. The present project aimed to study the diversity of emerging and novel filamentous fungi and characterize their in vitro antifungal susceptibilities. A total of 362 fungal isolates were included in this study. This study is divided into three parts. In part 1, 24 phenotypically non-dermatophyte, non-Aspergillus isolates were recovered from 24 independent cases of onychomycosis. In part 2, seven Talaromyces marneffei isolates were recovered from seven independent cases of systemic talaromycosis. In part 3, 331 non-duplicated Aspergillus isolates were recovered from cases of pulmonary and extrapulmonary aspergillosis. These fungi were studied by molecular phylogenetics (based on the ITS and other secondary DNA barcodes), or matrix-assisted laser desorption/ionization–time-of-flight mass spectrometry (MALDI–TOF MS), and antifungal susceptibility testing (AFST). Among the 24 phenotypically non-dermatophyte, non-Aspergillus mold isolates, AFST showed that 33.3%, 41.7% and 95.8% of the 24 isolates exhibited minimum inhibitory concentrations (MICs) of >1 mg/L to terbinafine, itraconazole and fluconazole, respectively, which are the first-line treatment of onychomycosis. All seven T. marneffei isolates possessed low MICs to amphotericin B, itraconazole, voriconazole, posaconazole, and the newest approved isavuconazole, but high MICs to anidulafungin. As for the 331 Aspergillus isolates, AFST showed that the rare/cryptic species possessed very different susceptibility profiles when compared to the classical pathogenic Aspergillus species. Notably, most isolates of the cryptic species in series Nigri of section Nigri and series Versicolores of section Nidulantes were less susceptible to the triazole antifungals. Fortunately, majority of the cryptic species in section Nigri were not resistant and wild type to amphotericin B. They also exhibited low minimum effective concentrations (MECs) to all three echinocandins. On the other hand, amphotericin B is not useful for species in section Flavi, Terrei, Nidulantes and Circumdati. Overall, the geometric mean, MIC50 and MIC90 for posaconazole were the lowest amongst the four triazoles tested across all Aspergillus sections. Regarding to clinical spectrum, the rare/cryptic Aspergillus species had quite similar spectrum of infections when compared to the common/classical Aspergillus species. In conclusion, a great diversity of less well-known fungi has been recognized as emerging pathogens in humans. Both accurate identifications to species level as well as AFST should be performed in clinical microbiology laboratories to guide proper patient management.