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Infection and Immunity, October 2003, p. 5871-5880, Vol. 71, No. 10
0019-9567/03/$08.00+0     DOI: 10.1128/IAI.71.10.5871-5880.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

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

Department of Pathology,¹ Department of Radiology, Childrens Hospital Los Angeles,² University of Southern California Keck School of Medicine, Los Angeles, California 90027³

Received 7 March 2003/ Returned for modification 4 June 2003/ Accepted 3 July 2003

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 Citrobacter-filled 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. FcRI 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.

Editor: B. B. Finlay

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