Since the 1970s several studies have indicated that certain bacterial species are associated with the destructive process in periodontal tissues. The association of Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis with periodontal diseases is most evident, but also Prevotella inter- media and Prevotella nigrescens are suggested as suspected periodontopathogens (1). P. intermedia sensu lato (currently including P. intermedia and P. nigrescens) are pigmented, moderately saccharo- lytic, indole-positive, gram-negative rods which are frequent members of the oral anaerobic micro- ¯ora in humans (2, 3). In addition, we have recently described a new species to be included in the The Prevotella intermedia group organisms in young children and their mothers as related to maternal periodontal status Ko Èno Ènen E, Wolf J, Ma Ètto È J, Frandsen EVG, Poulsen K, Jousimies-Somer H, Asikainen S: The Prevotella intermedia group organisms in young children and their mothers as related to maternal periodontal status. J Periodont Res 2000; 35: 329± 334. # Munksgaard, 2000. Currently, the Prevotella intermedia group includes three biochemically and phylogenetically related species: Prevotella intermedia, Prevotella nigrescens, and the newly described Prevotella pallens. The two ®rst-named species are mentioned with varying emphasis in connection with periodontal diseases, while such a connection of P. pallens is not known. Mothers serve as a plausible source of bacteria to their children, and conceivably, a mother with periodontitis as a recurrent reservoir of periodontally infecting organisms. In the present study, 23 mothers and their young children were examined for the presence of the P. intermedia group organisms in relation to maternal periodontal status (I: periodontal health, II: initial periodontitis, and III: advanced periodontitis). Species dierentiation was based on established biochemical methods, electrophoretic mobility patterns, SDS-PAGE, and DNA hybridization. P. intermedia was not recovered from children but nearly exclusively from mothers in group III, thus con®rming its association with periodontitis. P. nigrescens and P. pallens were frequently found in mothers and children. To determine bacterial transmission between a mother and her child, 72 isolates from 13 mother±child pairs were analyzed by arbitrarily primed PCR (AP-PCR). Similar AP-PCR types of P. nigrescens and/or P. pallens were recovered from 3/4 pairs in group I, 2/5 pairs in group II, and none in group III. Our results indicate that dierent species within the P. intermedia group have a dierent colonization pattern in childhood and that the periodontal status re¯ects qualitatively their presence in maternal saliva. Intra-familial transmission of P. nigrescens and P. pallens can occur in early childhood, however similar AP-PCR types were most obvious within periodontally healthy mother± child pairs. Eija Ko È no È nen 1,2,4 , Juhani Wolf 2 , Jaana Ma È tto È 2 , Ellen V. G. Frandsen 3 , Knud Poulsen 4 , Hannele Jousimies-Somer 1 , Sirkka Asikainen 2 1 Anaerobe Reference Laboratory, National Public Health Institute, and 2 Institute of Dentistry, University of Helsinki, Helsinki, Finland, 3 Department of Oral Biology, Royal Dental College, and 4 Department of Medical Microbiology and Immunology, University of Aarhus, Aarhus, Denmark Eija Ko Èno È nen, DDS, PhD, Anaerobe Reference Laboratory, National Public Health Institute, Mannerheimintie 166, FIN-00300 Helsinki, Finland Fax: z358 9 4744 8238 e-mail: eija.kononen@ktl.fi Key words: Prevotella intermedia; Prevotella nigrescens; Prevotella pallens; periodontal status; mother±child pairs; bacterial transmission; AP-PCR Accepted for publication May 19, 2000 J Periodont Res 2000; 35: 329±334 Printed in UK. All rights reserved