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Non-toxigenic Corynebacterium diphtheriae have become increasingly recognised as emerging pathogens across Europe causing severe invasive disease. A subset of non-toxigenic C. diphtheriae are 'non-toxigenic tox gene-bearing' (NTTB) strains; these strains are genotypically toxpositive, but do not express the protein. The circulation of NTTB strains was first observed during the 1990s upsurge of diphtheria in Eastern Europe but has not been reported in other European countries. Circulation of NTTB strains could be considered an increased risk for diphtheria and other related diseases, given their possible role as a tox gene reservoir with the theoretical risk of re-emerging toxin expression. Here we report the characterisation of 108 non-toxigenic C. diphtheriae biovar mitis isolates submitted to the World Health Organization (WHO) Global Reference Centre for Diphtheria at Public Health England, London, between 2003 and 2012, in order to determine the presence of NTTB strains. Using molecular methods, five NTTB isolates were identified; four human isolates (MLST type 212) and one isolate from a companion cat (MLST type 40). The emergence of these strains could indicate continuation of the circulation of potentially toxigenic strains and appropriate laboratory diagnostic methods should be used for detection. Given the complacency that currently exists in Europe awareness with regards to diphtheria diagnostics must be enhanced. .


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