If infection of cat brain cells is any indication, Bartonella effects in the human brain could be through these cells.
Bartonella henselae is known to cause central nervous system (CNS) disease in humans, and neurological signs have been observed in experimentally infected cats. However, the pathogenesis of CNS disease remains unclear.
This study was undertaken to determine whether B. henselae infects feline fetal brain cells in vitro. Microglial cell and astrocyte-enriched cultures were inoculated with B. henselae. Giménez staining identified bacteria within microglial cells by day 7 post-inoculation. The viability of the intracellular bacteria was demonstrated by incubating cultures with gentamicin and plating cell lysate on agar.
Electron microscopy identified intracellular organisms with characteristic Bartonella morphology but identified no ultrastructural abnormalities within infected microglial cells. No evidence of infection was seen in Bartonella-inoculated astrocyte cultures.
These findings suggest a role for microglia in the pathogenesis of B. henselae-associated neurological disease.
Reference
Muñana KR, Vitek SM, Hegarty BC, Kordick DL, Breitschwerdt EB. Infection of fetal feline brain cells in culture with Bartonella henselae. Infect Immun. 2001 Jan;69(1):564-569. doi:10.1128/IAI.69.1.564-569.2001. PMID: 11119554; PMCID: PMC97920.
Microglia Function in Humans
Microglial cells are the primary resident immune cells of the central nervous system (CNS), acting as the brain’s first responders. Their essential roles include immune surveillance, clearing cellular debris and pathogens through phagocytosis, and actively remodeling synaptic connections to support brain development and learning.
1. Immune Defense and Surveillance
Microglia are highly dynamic sentinels. Under normal conditions, they extend and retract branching processes to continuously sweep and monitor the brain microenvironment for pathogens, toxins, or injury.
If they detect a threat or cellular damage, they rapidly transform into an active, amoebic state to engulf and destroy invaders or damaged cells.
2. Synaptic Pruning and Brain Development
Beyond immunity, microglia are heavily involved in shaping neural networks.
During brain development and throughout adulthood, they monitor synapses and engulf redundant or weakened connections. This process, known as synaptic pruning, is critical for optimizing learning, memory formation, and functional brain connectivity.
3. Debris Clearance
Microglia act as the “janitors” of the CNS. They constantly clear away metabolic waste products, dying or dead cells (apoptosis), and harmful protein aggregates, such as amyloid plaques.
This clean-up process is vital to maintaining a healthy neuronal environment and preventing neurotoxicity.
4. Inflammation and Repair
Upon encountering injury or disease, microglia can trigger a neuroinflammatory response. They release cytokines, chemokines, and reactive oxygen species to fight infections and recruit other immune cells to the site of damage.
Once the threat is neutralized, they switch to a repair mode, releasing anti-inflammatory molecules and growth factors to promote tissue regeneration.
Clinical Relevance
While this microglial activation is highly protective in the short term, chronic or dysregulated microglial activation can lead to excessive neuroinflammation. This ongoing inflammation damages surrounding neurons and is a major contributing factor in the progression of neurodegenerative conditions.