Emerging Corynebacterium diphtheriae ST-212 cluster with truncated tox gene linked by possible sexual transmission.

PURPOSE: Despite low incidence, Corynebacterium (C.) diphtheriae remains a pathogen of concern, particularly due to emerging clusters of toxigenic and non-toxigenic strains causing respiratory, wound or invasive infections, and even recent fatal cases. This study aimed to investigate genetic relationships, characteristics, and epidemiological links among recent C. diphtheriae isolates of sequence type (ST)-212 from Germany, Austria, and Switzerland, focusing on their tox gene status and transmission routes.

METHODS

Four isolates collected 2022-2024 underwent whole genome sequencing (WGS). Analyses included multi locus sequence typing (MLST) and core genome MLST (cgMLST). Toxigenicity was assessed by qPCR, Elek, and lateral flow immunoassay (LFIA). Comparative genomic analyses used virulence factor databases and insertion sequence identification. Epidemiological data were gathered through patient interviews.

RESULTS

The four isolates belonged to the rare ST-212 and showed close genetic relatedness (cgMLST allelic distances 0-8, compared to a default cluster threshold of 14). Each harboured an identical truncated tox gene with a 5' deletion of 336 nucleotides and an upstream insertion sequence (IS) element (IS1132) disrupting the gene. The isolates were tox PCR positive but negative for diphtheria toxin (DT) production (non-toxigenic tox gene bearing; NTTB). Epidemiological investigation revealed shared exposure of two cases at a Berlin nightclub and suggested transmission through close or sexual contacts or indirect transmission via a contaminated environment within the nightclub. Comparison with historical and public ST-212 genomes confirmed the unique tox truncation and IS element insertion.

CONCLUSION

WGS led to the discovery of previously unknown epidemiological links and unexpected genetic markers in C. diphtheriae. The findings could indicate fort sexual transmission and form the basis for the hypothesis that tox gene truncation and its consequence of NTTB strains may attenuate clinical severity. Molecular-epidemiological integration is vital for surveillance and outbreak control.