Final answer:
The bacterium most commonly associated with sinusitis, otitis, and a leading cause of meningitis is Streptococcus pneumoniae. Vaccines are available for Streptococcus pneumoniae, as well as for other bacteria like Haemophilus influenzae and Neisseria meningitidis, which can also cause bacterial meningitis.
Step-by-step explanation:
The organism most commonly associated with sinusitis (or rhinosinusitis), otitis (including acute otitis media or AOM), and a leading cause of meningitis is Streptococcus pneumoniae, also known as pneumococcus. This encapsulated gram-positive bacterium is commonly found in the nasopharynx of young children and can cross the blood-brain barrier, particularly in susceptible individuals. Streptococcus pneumoniae can cause infections leading to rhinosinusitis and AOM after preceding viral infections compromise the mucosal immune defenses. In addition to pneumococcal meningitis, this organism can cause bacteremia and septicemia. Effective vaccines are available against pneumococcal infections, which help in reducing the incidence and severity of the disease.
Vaccines are also available against other bacteria that can cause bacterial meningitis, such as Haemophilus influenzae and Neisseria meningitidis, the latter being capable of causing epidemic meningitis. Initial bacterial colonization of the nasopharynx and bacterial toxins are factors that can lead to the bacteria gaining access to the central nervous system, eventually causing meningitis.