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Aggregatibacter actinomycetemcomitans and Aggregatibacter aphrophilus are closely related gram-negative bacteria mainly found in the oral cavity. Periodontitis in adolescents is associated with the presence of A. actinomycetemcomitans, but not with the presence of A. aphrophilus. However, both can cause extra-oral infections such as endocarditis and brain abscesses. Several unknown virulence and environmental factors could influence their prevalence in different populations. A shift in the microbiological and demographical characteristics could also influence the prevalence of periodontitis in adolescents residing in Sweden.

The aims of this PhD thesis were to study mechanisms that might influence serum resistance in A. actinomycetemcomitans and A. aphrophilus. In relation to this we wanted to study the presence of these bacterial species in two different adolescent populations (Västerbotten, Sweden and Kenya), factors influencing their prevalence, and the prevalence of periodontitis in adolescents in Sweden.

In paper I, serum resistance was detected in all tested strains of A. actinomycetemcomitans and A. aphrophilus. The OmpA paralogues, OmpA1 and OmpA2 were important for serum resistance in both species, i.e., inactivation of their genes, respectively resulted in significantly reduced serum resistance. In A. actinomycetemcomitans, ompA1 mutant derivatives were fortuitously found with regained serum resistance as a result of increased expression of OmpA2. OmpA1 and OmpA2 from A. actinomycetemcomitans, and OmpA1 from A. aphrophilus were able to bind C4bp, an inhibitor of parts of the activating paths of the complement system, suggesting this to be one mechanism to mediate bacterial serum resistance.

In paper II, we investigated the effects of outer membrane vesicles (OMV) on the serum resistance of A. actinomycetemcomitans. The survival rate of an almost totally serum sensitive A. actinomycetemcomitans ompA1 and ompA2 double mutant increased 100-fold when OMVs from a wildtype strain were supplemented. This effect was similar when OMVs from the ompA1 ompA2 double mutant were used, suggesting an OmpA-independent effect. On the other hand, OMVs from an LPS-deficient A. actinomycetemcomitans strain did not contribute to serum survival, consistent with an LPS-dependent serum resistance carried out by the vesicles. This appeared to be due to a substantial consumption of serum complement components by LPS in the OMVs.

In paper III, the salivary prevalence of A. actinomycetemcomitans and A. aphrophilus was high (71.8% and 99% respectively) in the adolescent population from Maasai Mara, Kenya. A. aphrophilus was found in higher amounts compared to A. actinomycetemcomitans. Cultivation from subgingival samples of a subset of the population yielded a lower prevalence of A. actinomycetemcomitans (22.1%). The highly leukotoxic genotype, JP2, was discovered in one salivary sample, indicating the first JP2-genotype identified in East Africa.

In paper IV, screening of dental radiographs from an adolescent population born in 2001 and residing in Västerbotten County, Sweden (n=1656) was performed. This resulted in 24 individuals (1.45%) positive for marginal bone loss (>2 mm). The dropout-rate was rather high and, eventually, thirteen of these individuals (cases) were periodontally examined, as well as 26 controls who did not display any marginal bone loss. Twelve of the cases were diagnosed with periodontitis, indicating an increase in the prevalence of periodontitis in adolescents. Moreover, this emphasized the potential of radiographs to identify small changes on marginal bone levels to diagnose periodontitis in adolescents. The cases had significantly higher values of clinical attachment loss, pocket probing depth and bleeding on probing. Significantly higher loads and prevalence of A. actinomycetemcomitans, Porphyromonas gingivalis and Filifactor alocis were generally detected in the saliva and subgingival plaque samples from the cases. In contrast, A. aphrophilus was more prevalent in the controls.

In conclusion, outer membrane proteins OmpA1 and OmpA2, and outer membrane vesicles display protective effects contributing to serum resistance in A. actinomycetemcomitans and A. aphrophilus. The prevalence of these bacteria varies in different populations, and they are also affected by environmental factors. Finally, the prevalence of periodontitis in adolescents in Västerbotten is suggested to have increased and is associated with distinct microbiological characteristics and geographic origin.