Final answer:
Streptococcus pneumoniae is the most common cause of bacterial meningitis in patients over 6 months old, particularly after widespread vaccination with the Hib vaccine.
Step-by-step explanation:
The most common bacteria causing meningitis in patients over 6 months old is Streptococcus pneumoniae. After the introduction of the Haemophilus influenzae Type b (Hib vaccine), S. pneumoniae has become the leading cause of meningitis in individuals aged 2 months to adulthood. This encapsulated gram-positive bacterium can be found in the microbiota of the pharynx and can cross the blood-brain barrier in susceptible persons, potentially leading to serious infections.