The causative agents of brain abscess vary according to the age and immunological status of the patient. In adults, aerobic, microaerophilic, and anaerobic streptococci, in particular agents of the Streptococcus milleri group (Streptococcus anginosus and Streptococcus intermedius), are found in 60% to 70% of cases and are common in abscesses arising from sinusitis or dental infections (De Louvois 1980; Kastenbauer et al 2004; Brouwer et al 2014a; Brouwer et al 2014b). Bacteroides species and enteric bacteria, including Escherichia coli, Proteus species, and Pseudomonas species, are present in 20% to 40% of cases (De Louvois 1980; Kastenbauer et al 2004; Brouwer et al 2014a; Brouwer et al 2014b). Staphylococcus aureus is found in 10% to 15% of cases and is the most common isolate from brain abscesses associated with penetrating trauma or neurosurgical procedures (Kastenbauer et al 2004). Methicillin-resistant Staphylococcus aureus is of increasing concern as an etiologic organism (Naesens et al 2009; Sharma et al 2009). Multiple organisms have been documented by culture methods in approximately 18% of cases and are most frequent in brain abscesses associated with sinusitis or otitis (Roche et al 2003; Kastenbauer et al 2004). However, this number may be falsely low. Studies employing 16S rRNA-based amplification, cloning, and high-throughput sequencing suggest that agents in brain abscess are polymicrobial in up to 40 % of cases, with the presence of at least 1 anaerobic bacterium (Mishra et al 2014). Staphylococcus aureus, however, may be present as a single organism in cases associated with trauma or endocarditis (Fritz and Nelson 1997). Other, less frequently isolated organisms may include Listeria monocytogenes and members of the Clostridium, Fusobacterium, and Actinomyces species (Fritz and Nelson 1997; Mylonakis et al 1998). In approximately 25% of cases, no organism is detected (Roche et al 2003; Kastenbauer et al 2004).
Brain abscess in neonates is frequently a complication of meningitis. In this setting, Gram-negative organisms are, thus, the most frequent isolates and include Proteus mirabilis, Escherichia coli, Serratia marcescens and Citrobacter species, especially Citrobacter diversus (Foreman et al 1984; Renier et al 1988; Ersahin et al 1994; Brouwer et al 2014a; Brouwer et al 2014b).
Isolates from brain abscesses in immunocompromised patients frequently differ from those recovered from abscesses in immunologically intact individuals. Enterobacteriaceae and Pseudomonas aeruginosa may be associated with brain abscess in immunosuppressed children and adults, as may Listeria monocytogenes (Brook 1995; Eckberg et al 2001). Fungi including Cryptococcus neoformans, Candida, Mucor, and Aspergillus species may be associated with brain abscess in diabetics and other immunocompromised patients, as well as in patients abusing intravenous drugs (Kastenbauer et al 2004). Although most commonly isolated from immunocompromised patients, Nocardia asteroides may cause brain abscess in patients without known impairment of immune function (Al Tawfiq et al 2013; Alijani et al 2013). Recently, brain abscess due to Nocardia cyriacigeorgica has been reported in patients with multiple myeloma treated with bortezomib and other agents (Pamukcuoglu et al 2014). The most common cause of focal intracranial infection in patients with AIDS is Toxoplasma gondii, followed by Cryptococcus neoformans and Mycobacterium tuberculosis (Price 1997; Menon et al 2011). Although M. avium intracellulare (MAI) is a frequent cause of systemic infection in patients with advanced AIDS, brain abscess due to MAI is infrequent (Karne et al 2012).