Neisseria lactamica meningitis as the initial presentation of a rhino-cerebrospinal fluid fistula in an adult with remote cranial trauma

Background: Neisseria lactamica is a commensal of the upper respiratory tract in children and is rarely isolated in adults. Its genome shares approximately 60% homology with Neisseria meningitidis but lacks the virulence genomic islands essential for blood-brain barrier invasion. Meningitis due to Neisseria lactamica is exceptionally rare and few cases are reported in literature, mainly among pediatric patients. To date, only one case of Neisseria lactamica meningitis in an adult, associated with head trauma, has been reported in the literature (1991). Case presentation: We report the case of a 58-year-old woman with a remote history of cranial trauma who presented with acute-onset headache, nausea, vomiting, and photophobia. Cerebrospinal fluid (CSF) analysis was consistent with acute bacterial meningitis. Despite negative multiplex PCR testing for common neurotropic pathogens, Neisseria lactamica was isolated from both CSF and blood cultures. Imaging revealed a skull base defect of the sphenoid sinus with an associated rhino-CSF fistula. Antimicrobial therapy was de-escalated to ceftriaxone monotherapy based on susceptibility testing, followed by successful endoscopic surgical repair of the CSF leak. The patient made a full recovery. Conclusion: This case represents the only adult case of Neisseria lactamica meningitis reported since 1991, emphasizing the need to recognize this organism as a potential cause of meningitis in patients with anatomical defects, to differentiate it microbiologically from Neisseria meningitidis, and to search for anatomical defects when it is isolated.