Neisseria meningitidis. Artwork of the bacteria Neisseria meningitidis. These bacteria are agents which cause bacterial meningitis. They are non- motile Gram-negative cocci (spherical) and occur as two cells (purple) in a capsule (green). Three such two-celled diplococci are shown. Neisseria meningitidis is an obligate human parasite. It causes meningococcal (cerebrospinal) meningitis, the inflammation of connective tissue membranes lining the brain and spinal cord. Symptoms include headache, fever, stiff neck, vomiting, delirium, sometimes death. Treatment is with antibiotics.