Prevalence of cdtABC genes encoding cytolethal distending toxin among Haemophilus ducreyi and Actinobacillus actinomycetemcomitans strains

The aim of this study was to investigate the presence of the three cdtABC g enes responsible for production of cytolethal, distending toxin (CDT) in Ha emophilus ducreyi and Actinobacillus actinomycetemcoinitans strains. Of 100 H. ducreyi strains from the culture collection of the University of Gotebo rg (CCUG), 27 strains with low or intermediate cytotoxic titre (<1 in 10(4) ) and 23 of the remaining isolates with a high cytotoxic titre (greater tha n or equal to1 in 10(4)) were selected. Twenty-nine strains of H. ducreyi w ere isolated recently from patients with chancroid and 50 A. actinomycetemc omitans strains from patients with periodontitis. The cytotoxic activity on HEp-2 cells and the presence of cdtABC genes were studied by cytotoxicity assay of bacterial sonicates and PCR with primers specific for individual c dtA, B. and C genes of H. ducreyi in bacterial DNA preparations, respective ly. All strains that manifested a cytotoxic titre in sonicate greater than or equal to1 in 100 possessed all the three cdt genes. Eighteen of the 50 s trains selected from the culture collection were negative and 32 positive f or cdt genes. As all strains with a high cytotoxic titre gave positive PCR results, it, can be assumed that the remaining 50 strains, which have high cytotoxic titre, would have been positive as well. Thus, it can be estimate d that 82% of the culture collection strains had cdtABC genes. Similarly, 2 4 (83%) of 29 recent H. ducreyi isolates expressed the CDT activity and dis played all cdtABC genes. Forty-three (86%) of 50, strains of the closely re lated A. actinomycetemcomitans, expressing a cytotoxic activity greater tha n or equal to1 in 100, also possessed all three genes. Furthermore, the nuc leotide sequence of the cdtABC genes was highly conserved among H. ducreyi strains from different geographic areas. These results indicate that the ma jority of pathogenic H. ducreyi and A. actinomycetemcomitans strains expres s a CDT activity encoded by all three cdtABC.