Abstract
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.
Highlights
Infections caused by toxigenic strains of Corynebacterium diphtheriae have become uncommon in Europe as a result of widespread immunisation, implemented during the first half of the 20th Century
Pure cultures for the 108 C. diphtheriae biovar mitis strains were mainly referred from throat swabs (n=87; 80.6%), wound swabs (n=10; 9.3%) and from blood culture (n=1), nasal swab (n=2) swab/other (n=1), pus (n=1), skin (n=2), ulcer (n=1), for three specimens, no information was provided; 62 specimens were from men (57.4%) and one from an animal
Primers were selected for sequencing of subunit A and subunit B of the tox gene.Primers indicated in dotted lines were designed for sequencing and determination of the promoter and upstream region of the four human non-toxigenic tox gene-bearing Corynebacterium diphtheria isolates
Summary
Infections caused by toxigenic strains of Corynebacterium diphtheriae have become uncommon in Europe as a result of widespread immunisation, implemented during the first half of the 20th Century. Diphtheria is still present in all six World Health Orgainization (WHO) regions and new epidemics are regularly reported [2]. These and the Eastern European epidemic in the 1990s with more than 157,000 cases and 5,000 deaths between 1990 and 1998, clearly demonstrate the unbroken threat of the disease in the post-vaccine era [3]. Integration of toxcarrying bacteriophages into the bacterial genome can convert non-toxigenic strains into toxigenic and virulent strains. This transformation has been described for example in patients but is generally believed to occur rarely in nature [4,5]
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