Final answer:
The primary organisms causing meningitis in infants are Streptococcus agalactiae, Neisseria meningitidis, and Haemophilus influenzae type b, all of which are encapsulated bacteria that can be spread through respiratory secretions and can cause severe infections.
Step-by-step explanation:
The top three organisms causing meningitis in infants are Streptococcus agalactiae, Neisseria meningitidis, and Haemophilus influenzae type b. Streptococcus agalactiae, also known as Group B streptococcus (GBS), is the most common cause of neonatal meningitis, particularly affecting newborns during or shortly after birth. Neisseria meningitidis, which is also responsible for epidemic forms of meningitis, affects children and young adults and is spread through respiratory secretions. Haemophilus influenzae type b, previously a primary cause of meningitis in children from 2 months to 5 years old, has become less common in countries employing the Hib vaccine. These bacteria are encapsulated, increasing their ability to evade the host's immune response, and can cause severe infections with high mortality and morbidity rates if not treated promptly with appropriate antibacterial therapy.