Differences in iron acquisition from human haemoglobin among strains of Actinobacillus actinomycetemcomitans

To get a better insight into the physiology of the high-toxic JP2 clone of Actinobacillus actinomycetemcomitans serotype b, which is strongly associated with juvenile periodontitis in adolescents of African descent, the modes of iron acquisition in this clone were examined and compared to those of other strains of the species. None of the strains examined could utilize human transferrin as a source of iron. This was in accordance with the presence of a non-functional tbpA gene, which normally encodes the A subunit of the transferrin-binding-protein complex. Southern blot analysis indicated that functional duplications of tbpA were not present in the genome. Thus, A. actinomycetemcomitans seems to be in a process of evolution, in which iron acquisition from host transferrin is not essential as in many other members of the Pasteurellaceae. All strains could utilize haem as a source of iron. All 11 A. actinomycetemcomitans strains examined harboured a single genomic sequence with homology to the hgpA gene encoding haemoglobin-binding protein A in Haemophilus influenzae. However, in all three strains belonging to the JP2 clone and in one serotype e strain hgpA was a pseudogene. Seven other strains possessed a functional hgpA gene which, according to insertion mutagenesis experiments, was responsible for the ability of these strains to utilize haemoglobin as a source of iron. Thus, the presence of an hgpA pseudogene and the inability to use human haemoglobin as an iron source discriminate the high-toxic JP2 clone from low-toxic serotype b strains and most other strains of A. actinomycetemcomitans.