Bacteraemia with meningitis caused by Listeria innocua of unknown source in an elderly patient with type 2 diabetes mellitus: a case report and literature review

Background: Listeria innocua is widely distributed in the natural environment and food products. Human infections caused by L. innocua are extremely rare, with only approximately 5 cases reported globally over the past three decades (1990-2025). Case report: We report a case of a 74-year-old female with a history of poorly controlled type 2 diabetes mellitus (HbA1c 9.88%) and hypertension, who was admitted with headache, low-grade fever, and progressive altered mental status over 5 days. Cerebrospinal fluid analysis revealed pleocytosis. Both blood cultures (two sets) and the cerebrospinal fluid culture were positive for L. innocua, identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Despite an epidemiological investigation, the source of infection could not be determined. Initial treatment with ceftriaxone for 3 days failed with no clinical improvement. The patient was successfully treated with ampicillin for 49 days after culture results and antibiotic susceptibility testing became available. Cerebrospinal fluid parameters were normalized, and the patient was discharged after a 49-day treatment. Conclusion: This is a rare case of bacteraemia with meningitis caused by L. innocua in an elderly diabetic patient. The case emphasizes the importance of awareness of the pathogenic potential of L. innocua in immunocompromised patients, and the need to add ampicillin to empiric antibiotic regimens for meningitis in high-risk groups (age >65, diabetes mellitus, cancer, or immunosuppression).