Phylogenetic variation of Aggregatibacter actinomycetemcomitans serotype e reveals an aberrant distinct evolutionary stable lineage

The periodontal pathogen Aggregatibacter actinomycetemcomitans that comprises six serotypes (a–f), is often identified by PCR-based techniques targeting the 16S rRNA gene. In this study, 16S rRNA gene sequence analysis revealed an aberrant cluster of 19 strains within serotype e, denoted as serotype e′. The 16S rRNA gene sequence similarities found between serotype e′ strains ranged from 99.7% to 100.0%, whereas 96.8–97.5% sequence similarity was obtained with members of the other serotypes, indicating that the serotype e′ strains might not be true members of A. actinomycetemcomitans. However, DNA–DNA hybridizations between a representative serotype e′ strain and representative strains of serotypes b, d and e of A. actinomycetemcomitans revealed 68–75% DNA–DNA relatedness, demonstrating that the serotype e′ strains do belong to the species A. actinomycetemcomitans. AFLP analysis of 33 A. actinomycetemcomitans strains, representing all serotypes (a–f), but mainly serotype e′ strains, showed that the latter form a distinct cluster, demonstrating that these strains are also closely related on the whole genome level. Moreover, the serotype e′ strains were unable to ferment starch and glycogen in contrast to almost all other A. actinomycetemcomitans strains tested. Overall, the data obtained in this study suggest that the serotype e′ strains form an evolutionary relatively stable distinct subgroup within A. actinomycetemcomitans.