Cytolethal distending toxin of Actinobacillus actinomycetemcomitans Occurrence and association with periodontal disease Kai Soo Tan 1 , Keang-Peng Song 1 , Grace Ong 2 1 Department of Microbiology, Faculty of Medicine, National University of Singapore, Singapore; 2 Department of Preventive Dentistry, Faculty of Dentistry, National University of Singapore, Singapore. Actinobacillus actinomycetemcomitans, a Gram-negative capnophile, has been associated with various forms of peri- odontal disease, including localized juvenile periodontitis, early onset, refractory and adult periodontitis (1–6). Although the pathogenic mechanism by which this bacterial species acts to cause periodontal dis- ease is not known, this organism is able to produce a variety of virulence factors capable of facilitating the colonization, invasion and destruction of the periodontal tissues (7). This bacterial species has also been associ- ated with other human diseases, including endocarditis, meningitis and osteomyelitis (8, 9). The cytolethal distending toxin (Cdt) of A. actinomycetemcomitans is a newly described cytotoxin with immunosuppressive properties. Cdt is encoded by three genes, designated cdtA, cdtB and cdtC, which are organized in an apparent operon (10, 11). Two small open reading frames (ORFs) of unknown function reside upstream of the cdtA gene in A. actin- omycetemcomitans strains. Cdt’s have been reported in pathogenic strains of Escherichia coli, Shigella dysenteriae and Campylobacter jejuni (12–14). RT-PCR analyses performed on Tan K-S, Song K-P, Ong G. Cytolethal distending toxin of Actinobacillus actino- mycetemcomitans. Occurrence and association with periodontal disease. J Periodont Res 2002; 37: 268–272. Ó Blackwell Munksgaard, 2002 The cytolethal distending toxin (Cdt) of Actinobacillus actinomycetemcomitans,a periodontal pathogen, is a newly described cytotoxin with immunosuppressive properties, capable of causing cell cycle arrest of lymphocytes. The objectives of this study were to investigate the occurrence of A. actinomycetemcomitans with the cdt genotype in the subgingival plaque of periodontitis patients and to determine the association of this bacterial genotype with periodontal disease. A total of 146 subgingival plaque samples from periodontitis patients were assayed by the PCR method using oligonucleotide primers targeting the cdt operon of A. actinomyce- temcomitans. Primers targeting the leukotoxin gene A (ltxA) of A. actinomyce- temcomitans was used to determine the occurrence of the bacteria in the plaque samples at baseline. At baseline, A. actinomycetemcomitans was detected in 106 out of 146 (73%) diseased sites studied. Among the 106 diseased sites found to harbor A. actinomycetemcomitans, 13 sites were positive for the bacteria with the cdt genotype (12%). Out of the 13 positive sites, 10 sites were obtained from patients diagnosed with aggressive periodontitis (77%). Thus, A. actinomycetem- comitans with the cdt genetic subtype has low occurrence in the subgingival plaque of periodontitis patients. However, a strong association was observed between the presence of the bacteria and aggressive forms of periodontitis. Thus, the cytotoxic and immunosuppressive properties of A. actinomycetemcomitans Cdt may func- tion to cripple the host immunity and contribute to the pathogenesis of aggressive periodontitis. Keang-Peng Song, Department of Microbiology, Faculty of Medicine, National University of Singapore, 5 Science Drive 2, Singapore 117597, Republic of Singapore Tel: +65 874 3278 Fax: +65 776 6872 e-mail: micskp@nus.edu.sg Key words: Actinobacillus actinomycetemcomi- tans; cytolethal distending toxin; periodontal disease Accepted for publication July 10, 2001 J Periodont Res 2002; 37; 268–272 Printed in the UK. All rights reserved Copyright Ó Blackwell Munksgaard Ltd JOURNAL OF PERIODONTAL RESEARCH ISSN 0022-3484