'Bacterium actinomycetem comitans' was first described by Klinger (1912) as coccobacillary bacteria isolated with Actinomyces from actinomycotic lesions in humans. It was reclassified as Actinobacillus actinomycetemcomitans by Topley & Wilson (1929) and as Haemophilus actinomycetemcomitans by Potts et al. (1985). The species has attracted attention because of its association with localized aggressive periodontitis.
Recent studies have shown a phylogenetic similarity of A. actinomycetemcomitans and Haemophilus aphrophilus, H. paraphrophilus, and H. segnis, suggesting the new genusAggregatibacter for them.
It is one of the bacteria that might be implicated in destructive periodontal disease. Although it has been found more frequently in localized aggressive periodontitis, prevalence in any population is rather high. It has also been isolated from actinomycotic lesions (mixed infection with certain Actinomyces species, in particular A. israelii). It possesses certain virulence factors that enable it to invade tissues, such as the pore-forming toxin leukotoxin A. It has also been isolated from women with bacterial vaginosis and as an etiologic agent in endocarditis. The pore-forming toxin LtxA of A. actinomycetemcomitans may be a trigger of the autoimmune disease rheumatoid arthritis due to its ability to stimulate protein citrullination, a post-translational protein modification targeted by autoantibodies in this disease.
a strain, for example ATCC 29523, frequently in oral cavity, variable leukotoxin expression
b strain Y4, most frequently in localized aggressive periodontitis, high leukotoxin expression; of the b subset, clone Jp2 is particularly leukotoxic
c strain ATCC 33384, low leukotoxin expression
serotypes d, e, f, g
within each serotype, leukotoxin expression can be highly variable between strains
In bacteria, small RNAs are involved in gene regulation. Jorth et al. identified 9 sRNA by Northern blotting from computer-predicted candidates in strain VT1169 and 202 sRNA by RNA seq in strain 624. A systematic screen by RNA-seq and RT-PCR in HK1651 strain (a clinical isolate from an aggressive periodontitis patient), quantified 70 sRNAs and further identified 17 differentially expressed sRNAs during growth phases. Target prediction indicated possibility of sRNA interaction with several virulence genes.  This study confirmed the presence of one of previously identified Fur regulated sRNAs JA04 identified in strain HK1651.
^ abNørskov-Lauritsen N, Kilian M (September 2006). "Reclassification of Actinobacillus actinomycetemcomitans, Haemophilus aphrophilus, Haemophilus paraphrophilus and Haemophilus segnis as Aggregatibacter actinomycetemcomitans gen. nov., comb. nov., Aggregatibacter aphrophilus comb. nov. and Aggregatibacter segnis comb. nov., and emended description of Aggregatibacter aphrophilus to include V factor-dependent and V factor-independent isolates". International Journal of Systematic and Evolutionary Microbiology. 56 (Pt 9): 2135–46. doi:10.1099/ijs.0.64207-0. PMID16957111.
^Haubek, Dorte (28 September 2010). "The highly leukotoxic JP2 clone of Aggregatibacter actinomycetemcomitans: evolutionary aspects, epidemiology and etiological role in aggressive periodontitis". APMIS. 118 (130): 1–53. doi:10.1111/j.1600-0463.2010.02665.x. PMID21214629.