Final answer:
Solitary brain abscesses commonly arise from sinusitis or otitis media with direct pathogen extension, with Streptococcus pneumoniae and Haemophilus influenzae being the most common pathogens involved.
Step-by-step explanation:
Most cases of solitary brain abscess are usually due to the direct extension of infections from nearby structures such as the sinuses or the middle ear. When the frontal lobe is affected due to sinusitis, the common pathogens include Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. In cases of a temporal lobe lesion resulting from otitis media with mastoiditis, the typical causative agents are also S. pneumoniae, H. influenzae, and, especially in neonates, other bacteria like Escherichia coli and Enterococcus spp. Furthermore, meningitis, which can complicate sinusitis and otitis media, is commonly caused by Neisseria meningitidis, S. pneumoniae, and H. influenzae. These organisms can spread from person to person and may cross the mucous membranes of the upper respiratory tract to enter the bloodstream and reach the brain, causing an abscess.