Intravenous mouse infection model for studying the pathology of Enterococcus faecalis infections

Abstract

An intravenous mouse infection model was used to compare the virulence of Enterococcus faecalis strains, to study bacterial localization and organ histopathology, and to examine the effects of Nramp1 and gamma interferon (IFN-gamma) on the course of infection. Infection of BALB/c mice with 5 x 10(8) CFU of E. faecalis JH2-2, MGH-2, 418, DS16C2, or OG1X revealed the following virulence ranking (from highest to lowest): MGH-2, 418, DS16C2, JH2-2, and OG1X. Discernible differences in the number of MGH-2 and JH2-2 bacteria were observed at 7 days (168 h) in the blood (P = 0.037), at 72 h in the liver (P = 0.002), and at 8 h in the spleen (P = 0.036). At these time points, the number of MGH-2 bacteria was higher in the blood and liver while the number of JH2-2 bacteria was higher in the spleen. At 72 h, livers from MGH-2-infected mice had higher numbers of coalescing aggregates of leukocytes and a greater degree of caseous necrosis than those from JH2-2-infected mice. These results indicate a correlation between the virulence of the E. faecalis strain, the number of bacteria in the liver, and the degree of histopathology of the liver at 72 h postinfection. IFN-gamma was important in E. faecalis infection, since IFN-gamma gene knockout mice had reduced mortality and massive coagulative necrosis was observed in wild-type mice. The contribution of Nramp1 was unclear, since Nramp1(-/-) mice and the respective control mice were innately resistant to E. faecalis. The mortality of mice in this model is probably due to induction of cytokine release and massive coagulative necrosis.

FIG. 1.

Mortality of BALB/c, IFN-γ GKO, Nramp1^−/−, and 129/svEvTac mice after intravenous infection with E. faecalis strains. Each bar represents the cumulative percent mortality for day 0, 5, 10, 15, or 20 postinfection. The medium used for bacterial growth (rabbit serum or BHI broth) is shown below the graph. In initial infections with BALB/c mice, strains MGH-2, JH2-2, and 418 were grown in rabbit serum while E. faecalis strains DS16C2 and OG1X were grown in BHI prior to injection for the reasons given in Materials and Methods. In subsequent experiments with IFN-γ GKO, Nramp1^−/−, and 129/svEvTac mice, E. faecalis MGH-2 and JH2-2 were grown in rabbit serum before injection.

FIG. 2.

Scattergraph of viable bacteria recovered from organs of BALB/c mice at 8, 24, 72, 168, or 240 h postinfection. Organs from which bacteria were recovered include the blood (A), liver (B), kidney (C), spleen (D), and heart (E). Mice were infected intravenously with either E. faecalis MGH-2 (red circles) or JH2-2 (blue circles). Each circle represents the number of viable bacteria (CFU per organ or CFU per milliliter of blood) in an individual infected mouse. In some cases, where the numbers of viable bacteria in organs from different mice are almost identical, the values are not visible as individual circles. Circles below 10⁰ on the y axis represent values below the level of detection. The numbers of mice infected with MGH-2 or JH2-2 and necropsied at each time point are given in parentheses (MGH-2/JH2-2) above the graph.

FIG. 3.

Liver sections from BALB/c mice at 72 h after infection with E. faecalis MGH-2 (A) or JH2-2 (B). Note numerous multifocal coalescing aggregates of leukocytes and caseous necrosis in livers of mice infected with E. faecalis MGH-2. Slides were stained with hematoxylin-eosin.