Abstract
BackgroundDiphtheria remains a major public health concern with multiple recent outbreaks around the world. Moreover, invasive non-toxigenic strains have emerged globally causing severe infections. A diphtheria epidemic in the former Soviet Union in the 1990s resulted in ~5000 deaths. In this study, we analysed the genome sequences of a collection of 93 C. diphtheriae strains collected during and after this outbreak (1996 – 2014) in a former Soviet State, Belarus to understand the evolutionary dynamics and virulence capacities of these strains.ResultsC. diphtheriae strains from Belarus belong to ten sequence types (STs). Two major clones, non-toxigenic ST5 and toxigenic ST8, encompassed 76% of the isolates that are associated with sore throat and diphtheria in patients, respectively. Core genomic diversity is limited within outbreak-associated ST8 with relatively higher mutation rates (8.9 × 10−7 substitutions per strain per year) than ST5 (5.6 × 10−7 substitutions per strain per year) where most of the diversity was introduced by recombination. A variation in the virulence gene repertoire including the presence of tox gene is likely responsible for pathogenic differences between different strains. However, strains with similar virulence potential can cause disease in some individuals and remain asymptomatic in others. Eight synonymous single nucleotide polymorphisms were observed between the tox genes of the vaccine strain PW8 and other toxigenic strains of ST8, ST25, ST28, ST41 and non-toxigenic tox gene-bearing (NTTB) ST40 strains. A single nucleotide deletion at position 52 in the tox gene resulted in the frameshift in ST40 isolates, converting them into NTTB strains.ConclusionsNon-toxigenic C. diphtheriae ST5 and toxigenic ST8 strains have been endemic in Belarus both during and after the epidemic in 1990s. A high vaccine coverage has effectively controlled diphtheria in Belarus; however, non-toxigenic strains continue to circulate in the population. Recombination is an important evolutionary force in shaping the genomic diversity in C. diphtheriae. However, the relative role of recombination and mutations in diversification varies between different clones.
Highlights
Diphtheria remains a major public health concern with multiple recent outbreaks around the world
Major endemic clones of toxigenic and non-toxigenic C. diphtheriae in Belarus A total of 4382 C. diphtheriae isolates were collected in Belarus from 1996 to 2014 (Additional file 2: Table S2)
93 C. diphtheriae isolates were selected including one isolate from 1979. These were isolated from all six provinces of Belarus (Brest, Gomel, Grodno, Minsk, Mogilev and Vitebsk) from asymptomatic carriers (n = 22) and patients that presented with diphtheria (n = 26) or sore throat (n = 45)
Summary
Diphtheria remains a major public health concern with multiple recent outbreaks around the world. A diphtheria epidemic in the former Soviet Union in the 1990s resulted in ~5000 deaths. Diphtheria is a toxin-mediated disease caused by toxigenic strains of Corynebacterium diphtheriae which is characterised by the presence of an inflammatory pseudomembrane in the upper respiratory tract, resulting in breathing difficulties with fatal outcomes [1]. Diphtheria toxin, which is the most prominent virulence factor of C. diphtheriae, inhibits protein synthesis by catalysing NAD+-dependent ADP-ribosylation of elongation factor 2, inducing apoptosis, resulting in the cell death [3]. The cell death caused by the toxin likely makes the host iron sources available to the pathogen [1]. Diphtheria remains endemic to many countries [7] and multiple diphtheria outbreaks have been reported across the globe [8,9,10,11]
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