Aggregatibacter actinomycetemcomitans

by professor

Zainab A. Aldhaher

Aggregatibacter actinomycetemcomitans

This relatively new genus Aggregatibacter (formerly called Actinobacillus) includes species isolated from humans and mammals. (Latin aggregare: to come together, aggregate; bacter: bacterial rod; Aggregatibacter: rod-shaped bacterium that aggregates with others). The only species of this genus routinely isolated from the oral cavity is A. actinomycetemcomitans, so named because it is frequently isolated with Actinomyces spp. from actinomycotic lesions. The reason for this association is unknown. Multiple biotypes and up to six serotypes (a–e) have been described. This species is a major infective agent in particularly aggressive forms of periodontal disease in adolescents (localized aggressive periodontitis) and rapidly destructive periodontal disease in adults.

Habitat and transmission

Primary habitat is unknown but is likely to be subgingival sites.

Characteristics

Gram-negative coccobacilli, microaerophilic or capnophilic (carbon dioxide-dependent).

Small, short (0.4–1 μm), straight or curved rods with rounded ends.. Fresh isolates possess fimbriae (lost on subculture).

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Culture and identification

Grows as white, translucent, smooth, non-haemolytic colonies on blood agar; grows best aerobically with 5–10% carbon dioxide. Selective media are available for identification; the tryptone-soy-serum-bacitracin-vancomycin agar yields white, translucent colonies with a star-shaped or crossed cigar-shaped internal structure on first isolation.

There are two phenotypes: smooth and rough. The latter phenotype is related to fimbriation and to the production of hexoseaminecontaining exopolysaccharide. Cells from rough colonies grow in broth as granular, autoaggregated cells that adhere to the glass and leave a clear broth. Identified by sugar fermentation and assimilation reactions and acid end products of carbohydrate metabolism.

Pathogenicity

A number of virulence factors, including lipopolysaccharide (endotoxin), a leukotoxin, collagenase, cytolethal distending toxin (cdt), epitheliotoxin-bone resorption inducing factor and a protease-cleaving IgG have all been isolated from A. actinomycetemcomitans. The leukotoxin, in particular, is thought to play a significant role in subverting the host immune response in the gingival crevice. Together with other coagents, A. actinomycetemcomitans is involved in localized aggressive periodontitis and destructive periodontal disease in adults. Also isolated from cases of infective endocarditis, and from brain and subcutaneous abscesses.

• Main intraoral sites and infections: periodontal pockets; implicated in aggressive forms of periodontal disease (e.g. localized and generalized aggressive periodontitis). Often isolated from cervicofacial Actinomyces infections as co-pathogens.

Treatment

This species is sensitive to tetracycline.