Reliable differentiation of a non-toxigenic tox gene-bearing Corynebacterium ulcerans variant frequently isolated from game animals using MALDI-TOF MS.

Corynebacterium (C.) ulcerans is a zoonotic member of the C. diphtheriae group and is known to cause abscesses in humans and several animal species. Toxigenic strains, expressing the tox gene encoding diphtheria toxin, are also able to cause diphtheria in humans. In recent years, a non-toxigenic but tox gene-bearing (NTTB) variant of C. ulcerans has been identified that was frequently isolated from clinically healthy as well as from diseased wildlife animals, especially wild boars (Sus scrofa scrofa) in Germany and Austria. The described clinical cases showed similar signs of disease and the isolated corynebacteria displayed common genetic features as well as similar spectroscopic characteristics, therefore being assigned to a so called wild boar cluster (WBC). This study describes the establishment and validation of a method using MALDI-TOF mass spectrometry for a reliable differentiation between various members of the C. diphtheriae group at species level as well as a reliable sub-level identification of C. ulcerans isolates of the WBC variant. For this study 93 C. ulcerans isolates from wildlife animals, 41 C. ulcerans isolates from other animals and humans, and 53 isolates from further representatives of the C. diphtheriae group, as well as 26 non-diphtheriae group Corynebacteria collected via the MALDI user platform from seven MALDI users were used. By assigning 86 C. ulcerans isolates to the WBC the extensive geographical distribution of this previously less noticed variant in two Central European countries could be shown.