Abstract: 【Abstract】 Objective To investigate the clinical infection status and trends in drug resistance of a rare pathogen Kluyveromyces marxianus （Candida kefyr）, and to provide experience for clinical diagnosis and treatment. Methods Morphological and molecular biological identification tests and in vitro microdilution drug susceptibility test were conducted on a Kluyveromyces marxianus strain recently isolated from the midstream urine sample of a patient with urinary calculus in the Fungal Laboratory, Changhai Hospital. The ultrastructural damage of the strain caused by different antifungal drugs was observed by scanning electron microscopy. A retrospective analysis was conducted on clinical cases of Kluyveromyces marxianus infection in Changhai Hospital from 2009 to 2021. Results The isolated strain formed smooth, soft, cheese-like yeast colonies on the Sabouraud′s agar medium, and ovoid or slender spores were observed under the microscope. Morphological analysis, mass spectrometry and sequencing analysis identified the strain as Kluyveromyces marxianus. The drug susceptibility test showed that minimum inhibitory concentrations （MICs） of amphotericin B, fluconazole, itraconazole, voriconazole, posaconazole, flucytosine, caspofungin, and micafungin were 0.5, 0.5, 0.03, ≤ 0.03, 0.06, 0.5, ≤ 0.016, and 0.06 μg/ml, respectively. Under the scanning electron microscope, the strain was ovoid to slender before antifungal drug treatment, with a size of （3.0 - 6.5） μm × （5.5 - 11.0） μm; after 24-hour treatment with antifungal drugs at the dose of 1 μg/ml, cell membrane shrinkage was more obvious under the treatment with posaconazole, which exhibited a stronger destructive effect on the strain compared with amphotericin B and voriconazole. From 2009 to 2021, 8 cases of Kluyveromyces marxianus infection were collected, including 6 males and 2 females; the Kluyveromyces marxianus strains were isolated from ascites in 3 cases, bronchoalveolar lavage fluid in 1 case, sputum in 2 cases, and midstream urine samples in 2 cases. Conclusion For suspected Kluyveromyces marxianus infection, it is crucial to determine the pathogenic species as early as possible using various identification methods, and to collect strain as well as evaluate drug susceptibility, which will be beneficial for targeted clinical treatment.

Key words: Saccharomyces, Candida, Microbial sensitivity tests, Microscopy, electron, scanning, Antifungal agents, Amphotericin B, Kluyveromyces marxianus