Citrobacter koseriBrain Abscess in the Neonatal Rat: Survival and Replication within Human and Rat Macrophages

Abstract

A unique feature of Citrobacter koseri is the extremely high propensity to initiate brain abscesses during neonatal meningitis. Previous clinical reports and studies on infant rats have documented many Citrobacterfilled macrophages within the ventricles and brain abscesses. It has been hypothesized that intracellular survival and replication within macrophages may be a mechanism by which C. koseri subverts the host response and elicits chronic infection, resulting in brain abscess formation. In this study, we showed that C. koseri causes meningitis and brain abscesses in the neonatal rat model, and we utilized histology and magnetic resonance imaging technology to visualize brain abscess formation. Histology and electron microscopy (EM) revealed that macrophages (and not fibroblasts, astrocytes, oligodendrocytes, or neurons) were the primary target for long-term C. koseri infection. To better understand C. koseri pathogenesis, we have characterized the interactions of C. koseri with human macrophages. We found that C. koseri survives and replicates within macrophages in vitro and that uptake of C. koseri increases in the presence of human pooled serum in a dose-dependent manner. EM studies lend support to the hypothesis that C. koseri uses morphologically different methods of uptake to enter macrophages. Fc␥RI blocking experiments show that this receptor primarily facilitates the entry of opsonized C. koseri into macrophages. Further, confocal fluorescence microscopy demonstrates that C. koseri survives phagolysosomal fusion and that more than 90% of intracellular C. koseri organisms are colocalized within phagolysosomes. The ability of C. koseri to survive phagolysosome fusion and replicate within macrophages may contribute to the establishment of chronic central nervous system infection including brain abscesses.

Neonatal Infectious Diseases

“…Its ability to survive intracellularly has been linked to the capacity of creating chronic CNS infections and abscesses. 66,67 …”

Clinical Microbiology of Bacterial and Fungal Sepsis in Very-Low-Birth-Weight Infants

“…In a report from the Neonatal Meningitis Cooperative Study Groups published in 1981, brain abscesses developed in 41 (77%) of 53 neonates with Citrobacter meningitis (165), and a more recent review noted a similar incidence of abscesses (117). The organism appears to have a tropism for the CNS, and recent research with a neonatal rat model demonstrated that C. koseri is able to survive phagolysosomal fusion and replicate within macrophages, causing chronic infection and brain abscess formation (474). Of note, however, brain abscess formation appears to be uncommon in vertically transmitted, Citrobacter EONS, possibly because of earlier institution of antibiotic therapy (117).…”

Microglial Activation by Citrobacter koseri Is Mediated by TLR4- and MyD88-Dependent Pathways

“…However, many pathogens, such as Mycobacterium tuberculosis (30) and Leishmania species (31), have evolved strategies to evade microbial killing mechanisms and survive intracellularly in phagocytes. It has been proposed that the infiltration of Citrobacter -harboring monocytes/macrophages into the brain ventricles and periventricular parenchyma is important in the pathogenesis of brain abscess formation by this organism (10, 14). Ancillary evidence to support this possibility was provided in studies where C. koseri intracellular survival and replication was demonstrated in the human monocyte cell line U937 (14).…”

“…It has been proposed that the infiltration of Citrobacter -harboring monocytes/macrophages into the brain ventricles and periventricular parenchyma is important in the pathogenesis of brain abscess formation by this organism (10, 14). Ancillary evidence to support this possibility was provided in studies where C. koseri intracellular survival and replication was demonstrated in the human monocyte cell line U937 (14). Although resident microglia are equipped with numerous phagocytic receptors (16, 32), to date, no studies have examined whether C. koseri is capable of intracellular survival and/or replication in this CNS phagocyte population.…”