INTRODUCTION

Trichosporon species are found widely in nature, comprising of yeast-like fungi belonging to the phylum Basidiomycota (1). A recent taxonomic revision identified 20 species within the genus, using IGS1 rDNA sequence analysis (2). Among these species, T. asahii, T. asteroides, T. inkin, T. ovoides, and T. faecale were reported as infectious in humans. The most common cause of invasive trichosporonosis and urinary tract infections (UTIs) is T. asahii (3,4).

Trichosporon species, which are members of saprophytic flora of the skin or found in the respiratory, gastrointestinal, and genitourinary tracts of humans, are causing superficial as well as invasive infections with increasing frequency (1,4). Nosocomial UTIs are the most common infections, particularly in intensive care units (ICUs). Most of these infections are reported to be related to the presence of a urinary catheter and tend to develop following urinary catheterization (5). Candida species are the most common types of yeasts isolated in UTIs, followed by T. asahii (6).

Virulence factors play an important role in the development of infection. Trichosporon species produce extracellular enzymes such as lipase, protease, and esterase and form biofilms (1,7). Most UTI-causing T. asahii isolates were shown to form biofilm on polystyrene plates (8,9). Moreover, studies have reported that a significant relationship exists between biofilm formation and antifungal resistance (8,10).

Amphotericin B and triazole antifungal agents are usually used for the treatment of trichosporonosis (1). Previous studies have reported that amphotericin B has inadequate fungicidal activity and limited in vivo activity with evidence of in vitro resistance (11). Triazole drugs, particularly voriconazole, are effective for treatment (12,13). Moreover, echinocandins, another drug group, are naturally ineffective against Trichosporon species (14-16). Thus, the present study aimed to determine the susceptibility of 100 T. asahii strains, isolated from urine samples, to various antifungal agents.

METHODS

Non-Invasive Research Ethics Committee approval was obtained from Haydarpasa Numune Education and Research Hospital (02.09.2019, HNHEAH-KAEK 2019/103-955). Among the 1,442 urine samples sent to the Central Laboratory of the Department of Public Hospital Services-2 in İstanbul between 2015 and 2016 that yielded yeast on culture, Candida species were detected in 1,332 (92.3%) samples, T. asahii in 106 samples (7.3%) and other yeasts in four samples (0.2%).

Identification was performed using the matrix-assisted laser desorption ionization-time of flight mass spectrometry-ITEK MS IVD V.2 (Bio-Mérieux, Marcy l’Etoile, France) automated system-as well as conventional methods (macroscopic and microscopic morphologies, appearance on corn meal agar with Tween 80, and urease positivity). Isolates were stored at -80 °C until the time of analysis and revived with two passages in Sabouraud dextrose agar.

Broth microdilution is standardized only for Candida and Cryptococcus species on CLSI M27-A3, which is intended for antifungal susceptibility testing; however, similar to previous studies, (3,4,9) our study investigated the in vitro susceptibility profiles of T. asahii for antifungal agents according to CLSI M27-A3 (17). Only the minimum inhibitory concentration (MIC) values obtained were specified because clinical thresholds of antifungals for the genus Trichosporon are still unestablished. Antifungal agents used in the study included amphotericin B (Sigma Chemical Co., St. Louis, MO, USA), fluconazole (Sigma Chemical Co.), voriconazole (Sigma Chemical Co.), itraconazole (Sigma Chemical Co.), posaconazole (Sigma Chemical Co.), and isavuconazole (Toronto Research). Microdilution plates were prepared with a final antifungal concentration of 32-0.06 µg/L for fluconazole; 16-0.03 µg/L for amphotericin B and itraconazole; 8-0.015 µg/L for voriconazole, posaconazole, and micafungin; and 4-0.008 µg/L for isavuconazole. The experiment was repeated twice for each strain. C. krusei ATCC 6258 and C. parapsilosis ATCC 22019 were used as quality control strains. The yeast suspensions resulted in concentrations of 2.5 × 10³ cells/mL, and MIC was defined as the lowest antifungal concentration capable of promoting a 50% inhibition for azoles and 90% for amphotericin B at the end of 24 and 48 hours.

RESULTS

Among the 100 T. asahii strains isolated from the urine samples, 68 (68%) were isolated from male and 32 (32%) from female patients, and 66 (66%) of the total were from patients aged ≥70 years. The mean age of the patients was 69.94 (±20.30164) years. Among the 82 patients admitted in the ICU, 56 (68%) were male and 26 were female, and 68% were aged 70 and above. Table 1 presents the MIC ranges, MIC₅₀, MIC₉₀, and geometric mean values of isolates against amphotericin B, micafungin, and the five azole antifungal agents. The growth evaluation at 24 h revealed that most of the strains (89%) had a MIC value of ≥1 µg/mL for amphotericin B, while the rate was 96% at 48 h. Azole antifungal agents, including voriconazole, itraconazole, posaconazole, and isavuconazole, showed similar and low MIC values at both time points, while the MIC values for fluconazole were higher than those for other azole agents. The MIC values of all strains for micafungin were >8 mg/L Overall, T. asahii colonies became more prominent, and MIC values were more accurately determined at 48 h.

DISCUSSION

Infections caused by Trichosporon species often arise from endogenous flora, and the risk of such infections increases especially in patients with immunosuppression or in patients admitted in the ICU due to facilitating factors such as microbial translocation through the gastrointestinal mucosa and presence of vascular or urinary catheters (1). In a previous study, the prevalence of UTIs caused by Trichosporon in the ICU in a two-year period was 6% and the mortality rate was 20%. The prevalence was higher among men (65%) and individuals aged >70 years (55%) (6).

Among Trichosporon species, T. asahii is the most common cause of UTIs (4,9). T. asahii is an emergent pathogen in older patients with urinary catheter (12). Our study also identified T. asahii as the most commonly isolated species, with prevalence being higher among patients in the ICU (82%) and in male patients (68%). Furthermore, 66% of such patients were ≥70 years. Although UTIs are typically more common in women because of their anatomical structure (short urethra, vagina-anus proximity, etc.), (6) those caused by the genus Trichosporon were more common among men in our patient population, consistent with some other studies (6,12).

Triazole antifungal agents and amphotericin B are usually used for the treatment of Trichosporon infections (1). Previous studies have reported that amphotericin B has inadequate fungicidal activity against some Trichosporon strains and has limited in vivo activity along with evidence of in vitro resistance (11). Susceptibility test results vary from study to study. There are reports of low MIC (0.06-1) values, (8) as well as high MIC values (14). Although the fungus appears to be susceptible to amphotericin B in vitro, in vivo resistance may develop through a biofilm layer formed by the Trichosporon species; as a result, the desired effect is not observed (13). In our study, the MIC value for amphotericin B was ≥1 µg/mL in 89% and 96% of the strains at 24 h and 48 h, respectively.

Studies have reported that triazole antifungal agents, particularly voriconazole, are superior to amphotericin B in terms of efficacy in trichosporonosis treatment, and this group of agents are more commonly preferred for treatment (13,16). That said, there are reports of fatal pediatric cases (18) and treatment failures due to T. asahii infection, despite treatment with amphotericin B and voriconazole (19,20). In addition to the antifungal susceptibility of the agent, the patient’s immunity system and neutrophil count play an important role in treatment success (21).

The 2014 clinical guidelines for the diagnosis and management of rare invasive yeast infections drawn up by the European Society for Clinical Microbiology and Infectious Diseases recommends the use of triazoles, particularly voriconazole, for the treatment of invasive infections caused by T. asahii. (15) Studies that compared the in vitro efficacy of triazole antifungal agents against T. asahii strains have reported fluconazole as the triazole antifungal agent with the lowest activity, whereas voriconazole demonstrated the highest activity. Other triazole antifungal agents, such as itraconazole, posaconazole, and isavuconazole, showed comparable activity (3,4,14,16,22,23) The findings of the present study were consistent with the results of such studies. To the best of our knowledge, only a few studies have investigated the susceptibility of the genus Trichosporon to isavuconazole, which is the newest member of triazole antifungals. The MIC₅₀-MIC₉₀ values of clinical T. asahii isolates for isavuconazole, were found to be as follows: Hazirolan et al. (22) (n=90), 0.125-0.25 µg/mL; Thompson et al. (24) (n=40), 0.125 µg/mL; and the present study, 0.12-0.25 µg/mL, which is consistent with the previous data. Table 2 presents the in vitro susceptibility test results of T. asahii strains to several antifungal agents after 48 h of incubation, as reported in various studies.

Echinocandins, which are another group of antifungal agents, have demonstrated limited and inadequate in vitro activity against Trichosporon species (15,25). The MIC values for all strains against micafungin were >8 mg/L in the present study. Patients developing breakthrough invasive trichosporonosis while undergoing echinocandin therapy were also reported. Therefore, the risk of breakthrough infections of Trichosporon should not be ignored in patients with high risk status undergoing empirical or prophylactic therapy with echinocandins (16,25,26).

The generalization of the study’s results is limited by the lack of differentiation between infection and colonization in patients; as a result, isolates deemed as potential causes are being considered as “related to the clinical picture.” Standardization is required to differentiate between colonization and infection by Trichosporon species, particularly among patients in the ICU.

CONCLUSION

In conclusion, previous studies have identified various susceptibilities to antifungal agents and have shown that in vitro activity does not always correlate with efficacy in vivo. Our study established that voriconazole, an azole antifungal agent, was the most effective antifungal against T. asahii isolates in vitro. Considering the high MIC values, breakthrough infections of Trichosporon should be considered in patients with high risk status receiving empirical or prophylactic therapy of echinocandin or fluconazole.

ETHICS

Ethics Committee Approval: The study were approved by the Haydarpasa Numune Education and Research Hospital of Local Ethics Committee (Protocol number: HNHEAH-KAEK 2019/103-955).

Informed Consent: Consent form was filled out by all participants.

Authorship Contributions

Surgical and Medical Practices: D.T., Concept: D.T., Design: D.T., S.A., Data Collection or Processing: D.T., Analysis or Interpretation: D.T., A.B., Literature Search: F.Ö., Ş.D.D., Writing: D.T., A.B.,

Conflict of Interest: No conflict of interest was declared by the authors.

Financial Disclosure: The authors declared that this study received no financial support.