When 'Clean' is not clean: a diagnostic pitfall in Gram staining leading to pseudobacterial meningitis.

Introduction. Gram staining is a fundamental and widely used technique in clinical microbiology because of its simplicity and diagnostic value, particularly in the diagnosis of bacterial meningitis. However, its reliability may be compromised by contamination and staining artefacts. Case Presentation. This report describes a case of pseudobacterial meningitis in a previously healthy 25-year-old woman who presented with fever, headache and nuchal rigidity. Cerebrospinal fluid (CSF) analysis revealed pleocytosis with a predominance of mononuclear cells. Gram staining revealed findings consistent with Gram-negative bacilli, prompting empiric antibacterial therapy with concurrent antiviral treatment. Comprehensive microbiological investigations, including CSF culture and broad-range 16S rRNA PCR testing, yielded negative results. Residual material or microscopic debris on supposedly 'clean' slides was suspected to produce artefact-like appearances that mimicked micro-organisms, leading to false-positive interpretations. Conclusion. This case highlights a potential laboratory pitfall in the interpretation of CSF Gram stains and underscores the importance of considering staining artefacts, particularly when Gram stain findings are discordant with the overall clinical and laboratory findings.