Aggregatibacter actinomycetemcomitans

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Classification

Kingdom: Bacteria; Phylum: Proteobacteria; Class: Gammaproteobacteria; Order: Pasteurellales; Family: Pasteurellaceae; Genus: Aggregatibacter; Species: Actinomycetemcomitans.

Species

NCBI: HK1651 Taxonomy

Description and Significance

Aggregatibacter actinomycetmcomitans is composed of fimbriae, vesicles, and extracellular amorphous materials. These vesicles (blebs) are lipopolysaccharide structures which contain endotoxin. Endotoxin acts as bone resorption and contains actinobacillin, a gram-negative bacterium. Vescicles also serve adhesive properties, allowing the delivery of toxic materials. Fimbriae are small filamentous hair-like appendages that bundle together. They form two types of colonies: fimbriated strains which are star-positive (with a star shape interior) and non-fimbriated strains which are start-negative.

Periodontitis is a serious gum infection, damaging the soft tissue around teeth. Without treatment, periodontitis can lead to tooth loss as well as gum recession. A. actinomycetemcomitans causes rapid progression of localized aggressive periodontitis (LAP). LAP specifically affects incisors and molars, leading to the breakdown of supporting tooth structures if left untreated. For this learson, A. actinomycetemcomitans is considered a systemic pathogen.

Genome Structure

Aggregatibacter actinomycetemcomitans, a bacterium of clinical significance, exhibits a relatively small genome size compared to other bacteria, ranging from 2.0 to 2.7 Mb. Its genome structure is characterized by a single circular chromosome containing 2,206 genes. Among these genes are 2,129 coding sequences, which contribute to various cellular functions, along with 19 rRNAs and 54 tRNAs responsible for protein synthesis. Additionally, the genome harbors 4 noncoding RNAs, which may play regulatory roles within the bacterium. Whole genome sequencing (WGS) studies have revealed that A. actinomycetemcomitans can be divided into three phylogenetic lineages (I, II, and III) based on differences in genomic content and competence for transformation. Furthermore, serotyping, traditionally used for classification, has been shown to be insufficient for accurately characterizing the species, as certain serotypes are not confined to specific lineages.

In addition to its genomic diversity, A. actinomycetemcomitans exhibits variation in competence for transformation, with specific lineages showing varying degrees of natural competence. This competence for transformation is linked to horizontal gene transfer and may impact the genome size of strains within the species. Moreover, WGS studies have challenged traditional taxonomic classifications of A. actinomycetemcomitans based solely on serotyping. Certain strains, such as those in the clade e' outgroup, have been found to fall outside the species boundary based on genomic relatedness. Therefore, while serotyping and WGS provide valuable insights into the genetic diversity and phylogenetic relationships of A. actinomycetemcomitans, the latter offers a more comprehensive understanding of its genome structure, lineage diversity, and taxonomic implications.

Cell Structure, Metabolism and Life Cycle

Aggregatibacter actinomycetemcomitans, a clinically significant bacterium associated with periodontal disease, exhibits distinctive cell structure and metabolism. As a fastidious, facultatively anaerobic, Gram-negative rod, A. actinomycetemcomitans presents unique characteristics. Microscopically, its cells may appear as cocci in broth and clinical samples, indicating potential pleomorphism. Despite its non-motile nature and lack of flagella, this bacterium demonstrates adaptability to environmental conditions, thriving well in 5% CO2 but growing poorly in ambient air. Colonies on chocolate agar are notably small, rough-textured, and tenacious, adhering strongly to agar surfaces. Additionally, A. actinomycetemcomitans is distinguished by its capability to produce important toxins, including leukotoxin, cytolethal distending toxin, and various enzymes, contributing to its pathogenicity in periodontal infections.