Brain abscesses (BA) are defined as focal infections of the central nervous system consisting of a localized zone of necrosis, encapsulated by a membrane formed by the immune response. The incidence of BA has decreased over the last few years, ranging from 0.3 to 1.3 per 100,000 people per year, depending on the consulted series [1,3,4]. This disease can be produced by a wide variety of microorganisms, from bacteria, mycobacteria, or fungi to parasites, both in the adult and pediatric populations [1,2]. Anaerobic bacteria might be overlooked due to their fastidious nature, and their accurate identification might be a challenge for most laboratories. The most common identified anaerobic microorganisms in BA are, in descending frequency, according to the systematic review of Cobo F [5]: Bacteroides spp., Gram-positive anaerobic cocci, Fusobacterium spp., Prevotella spp., and Cutibacterium acnes. However, cultures are often polymicrobial [1,2].

BA formation can occur by contiguous, hematogenous, or metastatic routes. In fact, infection of the paranasal, frontal, or sphenoid sinuses is commonly found as the origin of BA in contiguous cerebral structures [6]. Furthermore, other risk factors include immunosuppression or disruption of the blood-brain barrier, among others [1,2,5]. Specifically, about 10% of total cases of BA seem to be caused by odontogenic foci, with approximately 140 cases reported in the literature up to now [7].

In this manuscript, we describe the case of a 65-year-old man with a BA coinfected with Eubacterium nodatum and Porphyromonas gingivalis, oral commensals usually associated with periodontal lesions [8,9]. P. gingivalis is a gram-negative, anaerobic, rod-shaped bacteria that requires the presence of heme or hemin and vitamin K to grow. This microorganism has been reported as a cause of BA mainly in patients with periodontitis [[10], [11], [12], [13]], which highlights the importance of previous dental focus as an important risk factor. However, in this case, the patient did not present an oral infection prior to the BA, and no lesions were observed during the inspection.

E. nodatum is also a known periodontal pathogen [9]. It is an obligately anaerobic, gram-positive, branching rod that closely resembles Actinomyces israelii in its cellular and colonial characteristics. Hill GB showed that both microorganisms could adhere to and colonize foreign bodies [14]. This obligate anaerobe has been rarely reported to cause human infections, and only three reports of infections were found during the literature review [[14], [15], [16]]. In two of these reports, E. nodatum was described as causing monomicrobial and polymicrobial brain abscesses, making this the third case report in the scientific literature.