Sheehan [⁵] presented a patient with aortitis due to Bacteroides spp. that developed MAA, which ruptured following translumbar aortography. Reddy et al. [⁶] reported a patient with a 7 cm saccular MAA due to B. fragilis that emerged in the suprarenal abdominal area.

Suddleson et al. [⁷] presented a case of mycotic suprarenal MAA caused by B. fragilis. The patient was treated only with continuous antibiotic therapy and had a complete recovery. Taylor et al. [⁸] described a patient with infected abdominal aortoiliac mycotic aneurysm caused by Bacteroides spp. The patient was treated with appropriate antibiotics and a single operative procedure consisting of preliminary extraanatomic bypass followed by complete aneurysm excision and posterior drainage of the retroperitoneum. Jewkes and Black [⁹] reported a case with MAA caused by a Gram-negative anaerobic organism. The MAA was in the abdominal area and was a consequence of an abscess of the appendix.

Reddy et al. [¹⁰] performed a 30-year retrospective review and identified 13 patients treated for infected aneurysms of the abdominal aorta or iliac arteries. Four (31%) of the patients died within 30 days of operation, three of whom of aneurysmal rupture. Salmonella spp., B. fragilis, S. aureus and Pseudomonas aeruginosa accounted for all deaths, ruptures, and suprarenal aneurysm infections which occurred in 10 (77%) of the patients.

Hsu et al. [¹¹] performed a retrospective review of all 19 patients with MAA treated at a single center over 5 years. The most common pathogens were Salmonella spp. (74%) and Streptococcus spp. (11%). There was a single case of B. fragilis who did not survive.

Matsuyama et al. [¹²] successfully-treated a patient with acute MAA due to B. fragilis by placing a dacron graft replacement between the distal arch and the proximal descending aorta with teflon felt reinforcement, and covering the graft with omental flap.

Tsuji et al. [¹³] successfully treated a patient with a mycotic aneurysm caused by B. fragilis of the common iliac artery complicated with vertebral spondylitis. Under midline laparotomy, complete debridement of the infected tissues, in-situ replacement of the common iliac artery with cryopreserved aortic allograft, and iliac bone autotransplantation and omentopexy to fill the debrided cavity were performed.

Beland et al. [¹⁴] described a patient suffering from leprosy that developed B. fragilis sepsis and MAA.

Takano et al. [¹⁵] reported a case of mycotic aneurysm of the infrarenal abdominal aorta infected by C. septicum and reviewed 18 addition cases. Of the total of 19 cases of aneurysm caused by C. septicum that have been reported, [^16-33] at the time of the review the aneurysm was located in the abdominal aorta in 6, the thoracic in 4, the thoracoabdominal in 2, the iliac artery in 2, the popliteal artery in 1, and the thoracic aorta and abdominal aorta (double aneurysm) in 1 patient. In addition, two cases of aortic dissection caused by C. septicum infection have been reported [³², ³³]. In these 19 cases, there were 14 with colon neoplasm, 11 of which were colon cancer. Ten of the 12 patients who underwent vascular surgery survived, whereas all 7 who did not undergo surgery died, mostly from aneurysmal rupture. Thus, surgical treatment seems to be needed to achieve optimal results.

A typical finding of clostridial mycotic aneurysms in the CT scan is gas formation surrounding the aorta or peripheral arteries [¹⁵]. Clostridia can proliferate in tissues when oxidation-reduction falls or the pH is reduced, which may occur with arterial injury, necrotic tissue, or anoxic tissue with lactic acid accumulation. For this reason, clostridial infection is frequently associated with gastrointestinal or hematologic malignancy. Kornbluth et al. [³⁴] reported an associated malignancy in 81% of patients with C. septicum infection, and other studies have reported similar findings [¹⁷, ³⁵]. It is believed that ulcerative lesions of the gastrointestinal tract, especially colon carcinoma, can allow clostridial organisms to enter the bloodstream and seed an atherosclerotic focus in the aorta, resulting in the development of MAA [³⁶]. The diagnosis of clostridial MAA requires therefore a thorough search for an occult malignancy.

Three additional cases of MAA due to C. septicum not included in the above review were also recently reported [^37-39].

Clostridium paraputrificum was also reported to be associated in a single case of AMA in a patient with a myeloproliferative disorder and a necrotic carcinoma of the colon [⁴⁰].

In addition to the patient with P. acnes MAA, described by Brook and Frazier [⁴], another patient was reported by Galema et al. [⁴¹].

Data recently reported by Marques da Silva et al. [⁴²] sheds light about the potential role of Propionibacterium species as well as other anaerobic bacteria in the etiology of aortic aneurysms. The authors evaluated 53 samples from aneurysm walls that were collected from 49 patients during reconstructive surgery. The tissue specimens were sectioned and cultured under anaerobic conditions. Anaerobic cultivation yielded bacteria in 14 of the 53 samples (26.4%). All bacteria were gram-positive cocci or rods from nine genera and 12 species. Five cultures (35%) were mixed, containing two bacterial species. Mixed aerobic and anaerobic species were found in four samples (28.5%). Anaerobic bacteria were recovered from 10 of 14 positive cultures (71%). Among the anaerobes found were P. acnes (5 instances), and Propionibacterium granulosum, Actinomyces viscosus, Actinomyces naeslundii, and Eggerthella lenta (1 each). In further investigation Marques da Silva et al. [⁴³] examined 10 aortic aneurysms for the presence of bacterial DNA using polymerase chain reaction (PCR) targeting the 16S ribosomal RNA (rRNA) gene, followed by cloning and sequencing. Sequences of P. acnes were identified in five samples and Prevotella melaninogenica were detected in one sample.

The authors concluded that the presence of bacteria in aortic aneurysms did not necessarily imply a causal relationship in aneurysm initiation and development, and some of these bacteria might have been secondary colonizers of a previously damaged vessel. However, their presence demonstrates the ability of anaerobic bacteria to reach the aortic aneurysms and participate in some instances in the mycotic process.