Aggregatibacter actinomycetemcomitans is a bacterium belonging to the Pasteurellaceae family and is part of the group of slow-growing microorganisms called (HACEK). It is not the only species of this genus but it is one of the most important. Formerly this microorganism was classified as an Actinobacillus.
This bacterium, like the species A. aphrophilus, are present in the oral microbiota of humans and primates and have been associated with serious and recurrent infectious processes in the oral cavity, such as aggressive or chronic periodontitis.
However, it has also been involved in extra-oral infections, among which we can mention: endocarditis, bacteremia, wound infections, subphrenic abscesses, brain abscesses, mandibular osteomyelitis, among others..
Most extra-oral infections are due to an invasion of the microorganism from the oral cavity into the interior. This occurs due to the progressive destruction caused by this microorganism in the tissues that make up the insertion and protective periodontium, producing an infection by contiguity..
Fortunately, most of the time this bacterium is susceptible to tetracycline and other antibiotics. However, strains resistant to tetracycline have already been reported, due to the presence of the plasmids tetB.
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Klinger, in 1912, first isolated this microorganism, who called it Bacterium actinomycetum comitans.In 1921 the name was reduced to Bacterium comitans by Lieske.
Eight years later, the name was modified again, but this time by Topley and Wilson who called it Actinobacillus actinomycetemcomitans. In 1985 Potts reclassified it to the genre Haemophilus (H. actinomycetemcomitans).
Later, thanks to a DNA study carried out in 2006 by Neils and Mogens, a new genus called Aggregatibacter., in which this microorganism was included and they called it Aggregatibacter actinomycetemcomitans, being his current name.
Similarly, other bacteria that were previously in the genus Haemophilus, such as: Haemophilus aphrophilus, H. paraphrophilus Y H. segnis, were reclassified and fitted into this new genus, due to their genetic similarity.
If we break down the name of the species actinomycetemcomitans, we can see that it is a combination of words.
The term acts means lightning, referring to the star shape that the colony of this microorganism presents on the agar.
The word mycetes means mushroom. This term was included because actinomycetes were formerly considered fungi..
Finally, the word comitans means 'common', expressing the intimate relationship between Actinobacillus and Actinomycetem, sometimes causing joint infections.
Kingdom: Bacterium
Edge: Proteobacteria
Class: Gammaproteobacteria
Order: Pasteurellales
Family: Pasteurellaceae
Gender: Aggregatibacter
Species: actinomycetemcomitans.
There are 5 well-defined serotypes of this microorganism. These are designated by the letters a, b, c, d and e according to the composition of the O antigen..
There are other serotypes that have not been able to be typed. Serotype (b) is known to be the most virulent and the most frequently isolated of aggressive periodontitis lesions in people from the USA, Finland, and Brazil..
Meanwhile, the second most frequent serotype is (c), which has been found mainly in patients from China, Japan, Thailand and Korea. This serotype has been isolated more frequently in extra-oral lesions.
Virulence factors can be divided into those elements that influence to favor colonization, those that modify the immune response, those that favor tissue destruction and invasion, and those that inhibit tissue repair..
The production of an amorphous extracellular material of a protein nature, together with the adhesion capacity conferred by its fimbriae and the production of adhesins released in its vesicles, play a fundamental role in the formation of biofilms and therefore colonization.
That is why this microorganism is capable of strongly adhering to certain surfaces, such as: glass, plastic and hydroxyapatite, as well as among themselves..
Its main virulence factor is represented by the hyperproduction of a leukotoxin, stored and released by cytoplasmic vesicles. As its name indicates, leukotoxin has a high cytotoxic activity on leukocytes (polymorphonuclear cells and macrophages).
Notably, the vesicles also release endotoxins and bacteriocins. Endotoxins stimulate the production of pro-inflammatory cytokines, while bacteriocins act by inhibiting the growth of other bacteria, creating an imbalance in the oral microbiota in their favor..
Similar to leukotoxin is the cytolethal stretching toxin, or also called cytoskeletal stretching cytotoxin (CDT).
This exotoxin has the ability to block growth, distort morphology, and impede the proper functioning of CD4 lymphocytes. It is also possible that it activates the apoptosis process (programmed cell death) of these cells. In this way the immune response is weakened.
The immune response is also affected due to the inhibition of the opsonization process, since the Fc fractions of the antibodies are attracted by certain proteins located in the cell wall of the microorganism..
This union prevents the complement from doing its job. In addition to this, there is inhibition in the synthesis of IgM and IgG antibodies..
Finally, this bacterium also produces substances that inhibit the chemotactic attraction of leukocytes, especially polymorphonuclear cells, as well as preventing the production of hydrogen peroxide in these same cells..
The capacity for destruction and invasion of tissues that this microorganism possesses is mainly due to the production of epitheliotoxins, collagenases and a protein called GROE1.
The former destroy the intercellular junctions at the level of the hemidesmosomes, the latter destroy the connective tissue of the periodontium and the third has osteolytic activity (destruction of bone).
To make matters worse, the presence of lipopolysaccharide (LPS) in its cell wall (endotoxin) cannot be ignored..
LPS acts as a stimulant for the production of interleukin 1 (IL-1B), tumor necrosis factor alpha (TNF-α), among other inflammatory mediators, in addition to promoting bone resorption.
On the other hand, it should be noted that there are indications that this bacterium can live and multiply intracellularly, especially within epithelial cells..
Cell invasion occurs in specific sites, such as connective tissue, alveolar bone, intracellular spaces, among others.
In addition to all the aforementioned, this bacterium also produces other cytotoxins that delay the renewal of damaged tissue, by destroying fibroblasts, generating true chaos..
It is a Gram negative coccobacillus that does not have flagella, therefore it is immobile. It does not form spores but it does have a capsule and fimbriae. Each bacterium is approximately 0.3-0.5 µm wide and 0.6-1.4 µm long..
In the Gram, a certain pleomorphism can be observed, that is, some individuals are more elongated (coccobacilli) and others shorter (coccoid), with the coccobacillary forms predominating over the coctaceous ones when the Gram comes from a culture medium.
While the cocaceous forms predominate when it comes from a direct sample, they are distributed singly, in pairs or forming clumps or clusters.
Aggregatibacter actinomycetemcomitans develops in transparent, rough colonies of rounded shape and subtly serrated edges.
Pathognomically in its center, a star-shaped figure of greater density is observed. This characteristic is noticeable especially in cultures that have had a prolonged incubation..
This figure is believed to be formed by the presence of fimbriae. Another important characteristic is that the colonies adhere strongly to the agar, being difficult to remove..
This microorganism inhabits the oral cavity as a local microbiota. As long as it is in equilibrium with the rest of the microorganisms it does not cause a problem, but an increase in its population can initiate the destruction of the periodontium..
It is a facultative anaerobic microorganism, characterized by being capnophilic, that is, in the presence of oxygen it requires an environment with 5-10% COtwo to be cultivated. It also grows under conditions of strict anaerobiosis.
Its growth in culture media is slow, therefore, well-defined colonies will be observed on solid media after 48 to 72 hours of incubation. It grows at a temperature of 37 ° C.
This microorganism being Gram negative does not grow on MacConkey agar. This bacteria requires certain nutrients that are provided by the culture medium trypticase soy, horse serum, bacitracin and vancomycin (TSBV).
Adding yeast or cysteine extracts to the medium can further enhance your recovery.
Müeller Hinton agar without any additives is used to perform the antibiotic susceptibility test..
This microorganism is an opportunistic pathogen. When the conditions are given, it behaves like a very virulent bacteria, which can cause serious damage and can even lead to death.
It is known that it initiates the process of destruction of the tissues that make up the periodontium.
It is because of that, A. actinomycetemcomitans it is considered a periodontopathogen along with other bacteria, such as: Phorphyromonas gingivalis, Fusobacterium nucleatum, Prevotella intermedia, Treponema denticola, Prevotella nigrescens, Tannerella forsythensis, Campylobacter rectus and Peptoestreptococcus micros, among others.
Some studies have revealed the presence of several pathogens together in periodontal disease. The presence of A. actinomycetemcomitans next to Phorphyromonas gingivalis, it's a bad prognosis.
The destruction of the tissue progresses rapidly, and can cause important contiguous lesions, such as: brain abscesses, liver abscesses, glomerulonephritis, pleuropulmonary infections, cervical lymphadenitis, among other conditions..
It can reach the blood and cause endocarditis, bacteremia, septic arthritis, endophthalmitis, epidural abscess and infections in the intra-abdominal cavity (subphrenic abscesses)..
Endocarditis cases are associated with the presence of a malformation or previous condition in the patient, such as the presence of valvular heart disease or prosthetic valves. On the other hand, this bacterium increases the risk of heart attacks, since it thickens the atheromatous plaque in the coronary arteries.
In patients with periodontitis, swish with chlorhexidine 0.12 - 0.2%, 2 times a day for 10-14 days, can be used as a local treatment (oral cavity)..
In the treatment of periodontitis, it is important to perform a supra-gingival and sub-gingival scaling (above and below the gum respectively) and also a root polishing to smooth the surface, since on a smooth surface it is more difficult for tartar to accumulate.
However, this is not enough and systemic treatment with antibiotics such as ciprofloxacin, metronidazole, amoxicillin or tetracycline is needed..
The use of antimicrobial combinations is recommended for more efficient bacterial eradication. The combinations of amoxicillin and metronidazole or ciprofloxacin with metronidazole have been very useful, but not so that of metronidazole with doxycycline, according to some studies carried out..
This strain generally expresses resistance against penicillin, ampicillin, amikacin, and macrolides..
To prevent an infection by this microorganism it is recommended to take care of and maintain good oral health. For this, it is necessary to visit the dentist periodically and remove dental plaque and tartar with frequent cleanings..
Smoking is a factor that favors periodontal disease, which is why it should be avoided.
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