First case of an invasive Bacteroides dorei infection detected in a patient with a mycotic aortic aneurysm-raising a rebellion of major indigenous bacteria in humans: a case report and review

Abstract

Background: Bacteroides dorei is an anaerobic gram-negative bacterium first described in 2006. Because of the high similarity in mass spectra between B. dorei and Bacteroides vulgatus, discriminating between these species is arduous in clinical practice. In recent decades, 16S rRNA gene sequencing has been a complementary method for distinguishing taxonomically close bacteria, including B. dorei and B. vulgatus, at the genus and species levels. Consequently, B. dorei has been shown to contribute to some diseases, including type 1 autoimmune diabetes mellitus and atherosclerotic diseases. However, there are no reports on invasive infectious diseases caused by B. dorei. This report describes the first case of direct invasion and colonisation of human tissue by B. dorei, thus providing a warning regarding the previously proposed application of B. dorei as a live biotherapeutic for atherosclerotic diseases.

Case presentation: A 78-year-old Japanese man complained of intermittent chest/back pain and was diagnosed with a mycotic thoracic aortic aneurysm by enhanced computed tomography on admission. Despite strict blood pressure control and empirical antibiotic therapy, the patient's condition worsened. To prevent aneurysmal rupture and eliminate infectious foci, the patient underwent surgical treatment. The resected specimen was subjected to tissue culture and 16S rRNA gene sequencing analysis to identify pathogenic bacteria. A few days after the surgery, culture and sequencing results revealed that the pathogen was B. dorei/B. vulgatus and B. dorei, respectively. The patient was successfully treated with appropriate antibacterial therapy and after improvement, was transferred to another hospital for rehabilitation on postoperative day 34. There was no recurrence of infection or aneurysm after the patient transfer.

Conclusions: This report describes the first case of invasive infectious disease caused by B. dorei, casting a shadow over its utilisation as a probiotic for atherosclerotic diseases.

Keywords: 16S rRNA gene sequencing; Bacteroides dorei; Case report; Indigenous intestinal bacteria; Mass spectrometry; Microbiota; Mycotic aortic aneurysm.

Fig. 1

Contrast-enhanced computed tomography scan of the patient on admission. a Axial view shows a distal aortic arch aneurysm with a perianeurysmal fluid density collection (yellow arrow) and periaortic inflammatory changes (blue arrows). Coronal (b) and sagittal (c) views show the dislocation of intimal calcification (green arrows) and the appearance of aortic dissection

Fig. 2

Pathological examination of the resected aorta. Microscopic examination of the resected specimen was conducted with Elastica-Masson staining (a) and haematoxylin and eosin staining (b, c). a Significant immunocyte infiltration is observed in the sub-adventitial layer, depicting purulent inflammation. b, c Abundant neutrophils infiltrated into the intramural area of the infected arterial wall, forming an abscess

Fig. 3

16S rRNA gene sequence analysis for the identification of pathogenic bacteria. The Bacteroides dorei (DSM 17855) and Bacteroides vulgatus (ATCC 8482^T) 16S rRNA gene sequences were aligned based on the consensus, with a concordance rate as high as 97%. V1-V9 represent the variable regions in the gene sequence, and the red bar indicates the sequencing range for identifying pathogens in this study. B. dorei was identified as the pathogen in this patient, with 100% sequence identity with the available B. dorei sequence