An increase in non-toxigenic Corynebacterium diphtheriae infections in Poland — molecular epidemiology and antimicrobial susceptibility of strains isolated from past outbreaks and those currently circulating in Poland

ObjectivesAn increase in non-toxigenic Corynebacterium diphtheriae infections – mainly invasive infections – has been observed in countries with high vaccination coverage. However, reasons for this situation are unknown. In this study we characterized and compared human clinical isolates of non-toxigenic C. diphtheriae strains isolated from infections that have occurred over recent years and C. diphtheriae strains isolated from diphtheria cases from past outbreaks in Poland.MethodsWe determined biotypes, genotypes, the occurrence of plasmids, and antimicrobial susceptibilities of 19 clinical C. diphtheriae strains. Genotypes were determined using pulsed-field gel electrophoresis (PFGE) and enterobacterial repetitive intergenic consensus polymerase chain reaction (ERIC-PCR) techniques.ResultsThe non-toxigenic C. diphtheriae strains isolated over the last few years were found to belong to biotype gravis and were genetically indistinguishable using PFGE and ERIC-PCR techniques. No plasmids were detected in the strains. All tested strains were susceptible to penicillin and erythromycin, as well as to imipenem, vancomycin, daptomycin, gentamicin, tetracycline, clindamycin, trimethoprim/sulfamethoxazole, rifampin, quinupristin/dalfopristin, and linezolid. Of the strains tested, 47% were intermediate for cefotaxime.ConclusionsThe genetic similarity of non-toxigenic C. diphtheriae strains causing infection suggests that the strains represent a single clone. They may possess additional virulence genes in a chromosome, related with higher pathogenicity and invasiveness. The genetic changes have not been followed by resistance to antibiotics.