Parvimonas micra and central nervous system infections: mechanisms of transformation from oral commensal to intracranial pathogen and clinical evidence.

Parvimonas micra (P. micra), an anaerobic oral coccus long dismissed as a culture contaminant, is increasingly recognized as a causative agent of pyogenic central nervous system (CNS) infections, yet its neuropathogenesis remains poorly defined. This narrative review synthesizes 41 peer-reviewed studies identified via a structured comprehensive literature search of PubMed, Embase and Web of Science through June 2026. Epidemiologically, pooling available case reports and small series (n=64 confirmed CNS cases), P. micra is among the most frequently isolated Gram-positive anaerobic cocci in reported brain abscess series, with proportions ranging from approximately 4% to over 10% in individual cohorts; species-level data are inconsistently reported and a precise pooled estimate cannot be derived. A male predominance (approximately 2.7:1) is observed among reported cases, though this should be interpreted cautiously given the small sample size and potential reporting bias. Approximately two-thirds of patients have no documented immunosuppressive condition. Brain abscess represents ~80% of CNS manifestations, with 60-70% presenting as polymicrobial infection. We propose a five-step pathogenic cascade-periodontal dysbiosis, epithelial barrier breach, intravascular survival, blood-brain barrier (BBB) translocation, and intracranial suppuration-as a conceptual framework. Virulence factors including the TmpC-AppA axis and extracellular vesicle DnaK have been characterized in non-CNS models (oral carcinoma, periodontitis, oral lichen planus); their relevance to CNS infection is inferred, and BBB translocation mechanisms remain entirely hypothetical. Metagenomic next-generation sequencing (mNGS) outperforms conventional culture in selected brain abscess cohorts (~85% vs ~50% in pus; ~84% vs ~8% in CSF across CNS infections), though performance varies by specimen type, prior antibiotic exposure, and sequencing platform. All-cause mortality is 5-10% and 20-25% of survivors sustain neurological sequelae, though these estimates may overestimate true population rates due to publication bias. P. micra shows consistently low in vitro minimum inhibitory concentrations to metronidazole and β-lactams. Early empirical anaerobic coverage and consideration of mNGS as a complementary diagnostic tool are recommended for suspected odontogenic brain abscess.