BACKGROUND: Toxigenic Corynebacterium ulcerans can cause a diphtheria-like 
illness in humans and have been found in domestic animals, which were suspected 
to serve as reservoirs for a zoonotic transmission. Additionally, toxigenic C. 
ulcerans were reported to take over the leading role in causing diphtheria in 
the last years in many industrialized countries.
METHODS: To gain deeper insights into the tox gene locus and to understand the 
transmission pathway in detail, we analyzed nine isolates derived from human 
patients and their domestic animals applying next generation sequencing and 
comparative genomics.
RESULTS: We provide molecular evidence for zoonotic transmission of C. ulcerans 
in four cases and demonstrate the superior resolution of next generation 
sequencing compared to multi-locus sequence typing for epidemiologic research. 
Additionally, we provide evidence that the virulence of C. ulcerans can change 
rapidly by acquisition of novel virulence genes. This mechanism is exemplified 
by an isolate which acquired a prophage not present in the corresponding isolate 
from the domestic animal. This prophage contains a putative novel virulence 
factor, which shares high identity with the RhuM virulence factor from 
Salmonella enterica but which is unknown in Corynebacteria so far. Furthermore, 
we identified a putative pathogenicity island for C. ulcerans bearing a 
diphtheria toxin gene.
CONCLUSION: The novel putative diphtheria toxin pathogenicity island could 
provide a new and alternative pathway for Corynebacteria to acquire a functional 
diphtheria toxin-encoding gene by horizontal gene transfer, distinct from the 
previously well characterized phage infection model. The novel transmission 
pathway might explain the unexpectedly high number of toxigenic C. ulcerans.