Changes in MLST profiles and biotypes of Corynebacterium diphtheriae isolates from the diphtheria outbreak period to the period of invasive infections caused by nontoxigenic strains in Poland (1950-2016)

Abstract

Background: Corynebacterium diphtheriae is a re-emerging pathogen in Europe causing invasive infections in vaccinated persons and classical diphtheria in unvaccinated persons. In the presented study we analysed genetic changes in C. diphtheriae isolates collected in Poland from the period before the introduction of the mass anti-diphtheria vaccination to the present time when over 98% of the population is vaccinated.

Methods: A total of 62 C. diphtheriae isolates collected in the 1950s-1960s, 1990s and 2000-2016 in Poland were investigated. Examined properties of the isolates included toxigenic status, presence of tox gene, biotype, MLST type (ST) and type of infection.

Results: A total of 12 sequence types (STs) were identified among the analysed C. diphtheriae isolates. The highest variability of STs was observed among isolates from diphtheria and asymptomatic carriers collected in the XX century. Over 95% of isolates collected from invasive and wound infections in 2004-2016 belonged to ST8. Isolates from the XX century represented all four biotypes: mitis, gravis, intermedius and belfanti, but the belfanti biotype appeared only after the epidemic in the 1990s. All except three isolates from the XXI century represented the biotype gravis.

Conclusions: During a diphtheria epidemic period, non-epidemic clones of C. diphtheriae might also disseminate and persist in a particular area after the epidemic. An increase of the anti-diphtheria antibody level in the population causes not only the elimination of toxigenic strains from the population but may also influence the reduction of diversity of C. diphtheriae isolates. MLST types do not reflect the virulence of isolates. Each ST can be represented by various virulent variants representing various pathogenic capacities, for example toxigenic non-invasive, nontoxigenic invasive and nontoxigenic non-invasive.

Keywords: Corynebacterium diphtheriae; Diphtheria; Invasive infections; MLST.

Fig. 1

Distribution of C. diphtheriae biotypes in different time periods with consideration of toxigenicity of the isolates (n – number of isolates included in the study)

Fig. 2

Minimum spanning tree of the MLSTs of the 62 C. diphtheriae isolates. Each circle corresponds to an ST. The area of each circle corresponds to the number of isolates. Each ST is colour coded according to its corresponding time period of isolation: red – 1950s-1960s, yellow – 1990s-2001, green – 2004-2016. The relationships between the isolates are indicated by the connections between the isolates and the lengths of the branches linking them. The numbers between the circles indicate the number of allelic differences

Fig. 3

Distribution of STs by type of infections (n – number of isolates included in the study). The charts do not include isolate 72/E (site of isolation is not known)