Anaerobic Bacterial Meningitis Complicating Intracranial Empyema in an Immunocompromised Patient

A 45-year-old man with recently diagnosed human immunodeficiency virus (HIV) infection, not yet receiving antiretroviral therapy, presented with pansinusitis complicated by an orbital abscess requiring surgical drainage. The clinical course was marked by the subsequent development of an intracranial empyema, confirmed by imaging and managed by craniotomy with evacuation of the purulent collection. Cerebrospinal fluid analysis revealed a clear bacterial meningitis profile, with marked leukocytosis predominantly composed of neutrophils. Direct examination following Gram staining demonstrated encapsulated Gram-negative bacilli, strongly suggestive of anaerobic bacteria of the genus Bacteroides. However, cultures, including those performed under anaerobic conditions, as well as multiplex meningitis/encephalitis PCR, remained negative. This discordant profile highlights the diagnostic limitations of conventional microbiological techniques in anaerobic infections, particularly due to their high sensitivity to oxygen and the critical impact of pre-analytical conditions. In addition, commonly used syndromic molecular panels do not target these organisms, potentially leading to underestimation of their role. Species of the genus Bacteroides, which are commensals of the oro-sinus flora, play a major role in polymicrobial intracranial infections of ENT origin. Their virulence is largely related to their polysaccharide capsule, which promotes abscess formation and immune evasion. This case therefore illustrates the fundamental value of direct microscopic examination in early diagnostic orientation, as well as the need for integrated interpretation of clinical and microbiological data, particularly in immunocompromised patients.