## 1 Introduction  
Brain abscess (BA), defined as a focal infection within the brain parenchyma, which starts as a localized area of cerebritis and subsequently is converted into a collection of pus within a well-vascularized capsule [1], represents a universal health problem with a high long-term morbidity, and a mortality rate of up to 9.5% reported in some studies [2; 3; 4]. The reported incidence of BAs was estimated at 0.3 to 1.3 cases per 100,000 people per year [5], with a male predominance (ratio resulted from 2:1 to 3:1) and a median age of 30 to 40 years in most pediatric and adult series, although the age distribution varies depending on the predisposing condition leading to the formation of BA [5]. Approximately 5-7% of all BAs are caused by dental infection and manipulation. Dental procedures in the setting of periodontal disease can provoke focal oral infections. Multiple abscesses account for 5-50% of all BAs, which are often seen, but not only, in immunocompromised patients and are the result of hematogenous spread. Some microorganisms are more common causative than others; a well-known condition, although not fully clarified [6; 7].  
The human oral cavity contains a very broad range of microorganisms. These microorganisms constitute the human oral microbiota, which represents one of the most complex microbial communities in the human body [8]. Some authors reported about 500 different species and 700 kinds of microorganisms [8], and that all known microorganisms associated with humans are at some time found in the oral cavity as either transient (the majority) or resident (only a few) species [9]. It seems that the bacteria that cause odontogenic infections are generally saprophytes. Some microorganisms are usually more responsible for these illnesses compared to others (in well-known conditions, still to be clarified) [10].  
In recent years, the role of oral microbiota in the setting of oral diseases such as caries, periodontal disease, oral cancer and systemic infections has attracted the attention of physicians and researchers [11; 12; 13; 14; 15]. There is also evidence that oral microbiota is closely related to some systemic diseases, including rheumatoid arthritis, adverse pregnancy outcomes, cardiovascular disease and BA [16; 17; 18; 19; 20; 21]. In addition, a recent study reported the role of saliva and saliva early mediators of oral microbiota, like serum and salivary Galectin-3 levels, in predicting periodontitis [21; 22; 23]. Patients suffering from severe untreated periodontal disease frequently experience bacteria after tooth brushing, flossing and chewing [23]. Frequent bacteremia and systemic spill of proinflammatory cytokines [24] from periodontal pockets result in the release of leukocyte elastase and acute phase proteins. The link between oral infections and adverse systemic conditions such as multiple BAs has attracted much attention in the research community in the last few years and with it, also the role of oral microbiota and the mechanism underlying its spreading from mouth to brain, through transient bacteremia resulting in bacterial colonization in extra-oral sites. It seems that the composition of primary odontogenic infections reflects the normal oral microbiota and the method of dissemination.  
To the best of our knowledge, this is the first review of multiple BAs of odontogenic origin. The aim of this review is to assess the more common pathogens involved in multiple BAs, the possible surgical and medical treatments, the antibiotic therapies taken into account and the role of microbiota in the onset of oral infections and blood dissemination, leading to the development of multiple BAs.