Aggregatibacter actinomycetemcomitans (Aa, previously Actinobacillus actinomycetemcomitans) is a Gram-negative facultative non-motile rod oral commensal often found in association with localized aggressive periodontitis, a severe infection of the gingiva, although it is also associated with non-oral infections. Its role in periodontitis was first discovered by Danish-born periodontist Jørgen Slots, currently a professor of dentistry and microbiology at University of Southern California School of Dentistry.
'Bacterium actinomycetem comitans' was described by Klinger (1912) as coccobacillary bacteria isolated together with Actinomyces from actinomycotic lesions of man. 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 (Slots & Ting, 1999; Haubek et al., 2001). 
Recent studies have shown a phylogenetic similarity of Actinobacillus actinomycetemcomitans and Haemophilus aphrophilus, Haemophilus paraphrophilus, and Haemophilus segnis suggesting the new genus Aggregatibacter for them.
It is one of the bacteria which 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 Actinomyces israelii). It possesses certain virulence factors that enable it to invade tissues, such as leukotoxin.
Virulence factors 
- Immunosuppression factors that inhibit blastogenesis, antibody production and activate T-suppressor cells
- Inhibition of PMNs functions
- Resistant to complement mediated killing.
- Surface antigens
- Heat shock proteins
- Antimicrobial resistance
A. actinomycetemcomitans serotypes 
- a strain, for example ATCC 29523, frequently in oral cavity, variable leukotoxin expression.
- b strain Y4, most frequently in localized aggressive periodontitis, leukotoxin expression.
- c strain ATCC 33384, low leukotoxin.
- serotypes d, e
- Nørskov-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". Int. J. Syst. Evol. Microbiol. 56 (Pt 9): 2135–46. doi:10.1099/ijs.0.64207-0. PMID 16957111.
- Slots J (January 1976). "The predominant cultivable organisms in juvenile periodontitis". Scand J Dent Res 84 (1): 1–10. PMID 1061986.