A 65-year-old man presents with fever, severe headache, and nuchal rigidity. Physical examination in the emergency depament shows a Glasgow coma score of 7. Lumbar puncture reveals cloudy cerebrospinal fluid (CSF) with 1200 neutrophils/mm3, elevated protein, and decreased glucose. Which of the following is the most probable etiologic agent of this condition?

The clinical manifestations (fever, headache, nuchal rigidity, and low Glasgow coma score), along with the CSF findings (increased neutrophils, elevated protein, and reduced glucose), strongly indicate acute pyogenic (bacterial) meningitis as the underlying condition. Of the microorganisms either Neisseria meningitidis or Streptococcus pneumoniae can cause this form of meningitis; however, Streptococcus pneumoniae is by far the most frequent organism causing acute meningitis in elderly patients. Arboviruses and herpesviruses can cause an encephalitis characterized by lymphocytic infiltration of the brain parenchyma and leptomeninges. In this case, CSF findings would include an increased number of lymphocytes and a normal glucose concentration. Mycobacterium tuberculosis may cause a chronic meningoencephalitis, with a prolonged clinical course. It is characterized pathologically by a dense granulomatous infiltrate of the base of the brain. Associated CSF findings include increased lymphocytes and normal or slightly decreased glucose.