Invasive Listeriosis in an Otherwise Immunocompetent Elderly Patient.

Listeria monocytogenes is a foodborne, intracellular Gram-positive bacillus that typically causes mild gastrointestinal illness or flu-like symptoms in healthy hosts but can lead to invasive disease, including bacteremia and central nervous system infection. Although classically associated with immunocompromised states, advanced age has emerged as an independent risk factor for invasive listeriosis. We report a case of an otherwise healthy elderly male patient who presented with nonspecific systemic symptoms and was subsequently found to have Listeria monocytogenes bacteremia and meningitis. Initial presentation was atypical for bacterial meningitis, contributing to diagnostic uncertainty. Blood cultures and cerebrospinal fluid analysis ultimately confirmed the diagnosis of invasive listeriosis. The patient was treated with targeted antimicrobial therapy, including ampicillin-based treatment, with clinical management guided by the susceptibility of Listeria species. This case report highlights the importance of having a high clinical suspicion of invasive listeriosis in the geriatric population given atypical meningitis symptoms, high mortality rate, and need for appropriate and time-sensitive antibiotic coverage.