Background: Infection increases the vulnerability of the neonatal brain to non-inflammatory insults, amplifying the risk of neurological sequelae. Experimentally, Toll-like receptor agonists have previously been used to mimic infections and induce inflammation prior to hypoxic-ischemic (HI) brain insult. Here, we present a clinically relevant model of combined effects of bacterial infection with subsequent HI brain insult on the neonatal brain. 

Methods: Live clinical isolate of Staphylococcus epidermidis 1457 was administered intraperitoneally (1.4´10⁷ CFU/g of body weight) into postnatal day (PND) 0 or 4 mice. Body weight, temperature and bacterial load were assessed. Cytokine production was determined by 23-plex inflammatory cytokine bead array. Ligation of one carotid artery and exposure to 10% O₂ was used to induce cerebral HI. Immunohistochemistry was used to determine brain injury.

Results: Infection of PND0 versus PND4 mice led to different patterns of bacteremia and clearance. Pro-inflammatory cytokines were significantly up-regulated in the blood, with higher responses in PND4 mice. Neutrophil and monocyte chemotactic cytokines, granulocyte-colony stimulating factor, and caspase-3 activity were transiently elevated in the brain of PND4 but not PND0 mice. Although no change in the gray and white matter volumes was detected in the brain of PND4 infected mice, when HI insult was induced 14 - 24 hours post infection, more severe brain damage was observed in infected animals compared to saline injected controls. Increased brain injury was not observed when HI was induced 5 days post infection, after resolution of the infection-induced inflammation.

Conclusion: Cytokine and caspase-3 responses to infection are age-dependent and infection increases vulnerability of the immature brain and potentiates subsequent HI brain injury.