Background: We dealt with the occurrence of an outbreak of Candida parapsilosis in a neonatal intensive care unit (NICU) in September 2020. There have been several reports of C. parapsilosis outbreaks in NICUs. In this study we describe our investigation into both the transmission route and the biofilm of C. parapsilosis. Methods: C. parapsilosis strains were detected in three inpatients and in two environmental cultures in our NICU. One environmental culture was isolated from the incubator used by a fungemia patient, and another was isolated from the humidifier of an incubator that had been used by a nonfungemia patient. To prove their identities, we tested them by micro satellite analysis. We used two methods, dry weight measurements and observation by electron microscopy, to confirm biofilm. Results: Microsatellite analysis showed the five C. parapsilosis cultures were of the same strain. Dry weight measurements and electron microscopy showed C. parapsilosis formed biofilms that amounted to clumps of fungal cells. Conclusions: We concluded that the outbreak happened due to horizontal transfer through the humidifier of the incubator and that the C. parapsilosis had produced biofilm, which promoted an invasive and infectious outbreak. Additionally, biofilm is closely associated with pathogenicity.

Routine identification of Corynebacterium species can be challenging and may complicate investigation of suspected healthcare-associated transmission. We investigated a suspected cluster of blood culture isolates routinely identified as Corynebacterium jeikeium and retrospectively evaluated similarly identified isolates recovered at our hospital. After three patients in a hematology ward had blood cultures yielding C. jeikeium within approximately one month in 2024, pulsed-field gel electrophoresis (PFGE), environmental sampling, and follow-up surveillance were performed. We subsequently reviewed all blood culture isolates routinely identified as C. jeikeium from 2019 to 2025. Ten isolates from 10 unique patients underwent long-read whole-genome sequencing (WGS). Species identity was assessed by average nucleotide identity (ANI) against type-strain genomes, and genomic relatedness by core-genome phylogenetic and pairwise genome analyses. PFGE showed indistinguishable patterns for two cluster-associated isolates. No environmental isolate was identified as Corynebacterium spp., and no additional temporally associated cases were detected. WGS identified all 10 isolates as C. macclintockiae and did not support a single-clone transmission event; the two isolates with indistinguishable PFGE patterns were closely related but not genomically identical. However, direct or indirect transmission between the corresponding patients could not be excluded. These findings support integrating epidemiological investigation, accurate species identification, and high-resolution genomic analysis when evaluating suspected healthcare-associated transmission involving members of the C. jeikeium complex.