Community-acquired acute bacterial meningitis in adults is most commonly caused by which bacteria?

The most common causes of community-acquired acute bacterial meningitis in adults are Streptococcus pneumoniae and Neisseria meningitidis. These pathogens are significant because they are the leading organisms responsible for bacterial meningitis in this population, particularly in young adults and the elderly.

Streptococcus pneumoniae is a common inhabitant of the respiratory tract and can gain access to the central nervous system, especially following a respiratory infection. Neisseria meningitidis, also known as meningococcus, is known for causing outbreaks and is part of the normal flora of the nasopharynx, making it a notable cause of meningitis in college dormitories or military barracks.

In contrast, while Escherichia coli and Staphylococcus aureus can be pathogens in other contexts, they are not the primary causes of community-acquired acute bacterial meningitis in otherwise healthy adults. Haemophilus influenzae type b was historically significant but became less common due to vaccination, particularly in children. Therefore, the prominence of S pneumoniae and N meningitidis in the context of community-acquired meningitis makes this the most accurate choice.