A Case Report of Non-toxigenic Corynebacterium belfantii With Suspected Diphtheria Infection.

We report a case of a 69-year-old woman with aspiration pneumonia in whom Corynebacterium diphtheriae (C. diphtheriae) was initially identified by mass spectrometry but subsequently confirmed as Corynebacterium belfantii (C. belfantii) through whole-genome analysis. Since sputum culture isolation and subsequent mass spectrometry identification yielded C. diphtheriae strain 2511-21 and Pseudomonas aeruginosa (P. aeruginosa), we notified public health authorities. We implemented isolation measures in accordance with the Infectious Disease Control Law. Diphtheria Toxin production assays, polymerase chain reaction (PCR), and serum antibody tests were all negative, and isolation measures were discontinued following appropriate infection control protocols. Average nucleotide identity (ANI) analysis of the whole-genome sequence revealed that isolate strain 2511-21 showed 99.37% homology with the C. belfantii type strain (CIP111412), confirming its identification as C. belfantii. Conventional phenotypic identification methods and mass spectrometry cannot reliably differentiate between C. diphtheriae and C. belfantii, making molecular biological confirmation essential for epidemiologically important bacterial species under infectious disease legislation. Although both C. belfantii and P. aeruginosa were identified as potentially causative pathogens based on Gram stain and culture findings, the misidentification of C. belfantii as C. diphtheriae by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) necessitated significant public health measures, highlighting the clinical and regulatory importance of accurate species identification. This case represents the first reported isolation of C. belfantii in Japan.