Multilobulated frontal brain abscess caused by Gemella morbillorum and Kocuria rosea in an immunocompetent young adult: illustrative case.

backgroundBrain abscesses typically arise from streptococcal, staphylococcal, or gram-negative pathogens, but low-virulence commensals may occasionally cause severe intracranial infection when anatomical barriers are disrupted. Gemella morbillorum and Kocuria rosea are rare opportunistic organisms that are seldom implicated in CNS disease and are usually associated with immunosuppression, chronic otolaryngological pathology, or postoperative states. Their simultaneous isolation from a single intracranial abscess has not been previously reported. OBSERVATIONS: A previously healthy 23-year-old man presented with progressive somnolence and persistent headache following recent sinusitis. Neuroimaging revealed a multilobulated left frontal abscess with extensive vasogenic edema. Initial surgical aspiration yielded purulent material, and culture grew G. morbillorum and K. rosea. Despite 1 month of broad-spectrum intravenous antibiotics, follow-up MRI demonstrated recurrence, necessitating reoperation with complete excision of the abscess capsule. The patient recovered fully after 6 weeks of antimicrobial therapy. LESSONS: This case highlights that rare commensal organisms can cause aggressive intracranial infection even in immunocompetent individuals. Accurate microbiological diagnosis is essential when evaluating sinus-derived abscesses, as atypical pathogens may influence treatment decisions and risk of recurrence. Multiloculated abscesses may require complete excision when aspiration and antibiotic therapy are insufficient for durable infection control. https://thejns.org/doi/10.3171/CASE25950.