Human isolates of Bartonella tamiae induce pathology in experimentally inoculated immunocompetent mice

Abstract

Background: Bartonella tamiae, a newly described bacterial species, was isolated from the blood of three hospitalized patients in Thailand. These patients presented with headache, myalgia, anemia, and mild liver function abnormalities. Since B. tamiae was presumed to be the cause of their illness, these isolates were inoculated into immunocompetent mice to determine their relative pathogenicity in inducing manifestations of disease and pathology similar to that observed in humans.

Methods: Three groups of four Swiss Webster female mice aged 15-18 months were each inoculated with 10(6-7) colony forming units of one of three B. tamiae isolates [Th239, Th307, and Th339]. A mouse from each experimental group was sampled at 3, 4, 5 and 6 weeks post-inoculation. Two saline inoculated age-matched controls were included in the study. Samples collected at necropsy were evaluated for the presence of B. tamiae DNA, and tissues were formalin-fixed, stained with hematoxylin and eosin, and examined for histopathology.

Results: Following inoculation with B. tamiae, mice developed ulcerative skin lesions and subcutaneous masses on the lateral thorax, as well as axillary and inguinal lymphadenopathy. B. tamiae DNA was found in subcutaneous masses, lymph node, and liver of inoculated mice. Histopathological changes were observed in tissues of inoculated mice, and severity of lesions correlated with the isolate inoculated, with the most severe pathology induced by B. tamiae Th239. Mice inoculated with Th239 and Th339 demonstrated myocarditis, lymphadenitis with associated vascular necrosis, and granulomatous hepatitis and nephritis with associated hepatocellular and renal necrosis. Mice inoculated with Th307 developed a deep dermatitis and granulomas within the kidneys.

Conclusions: The three isolates of B. tamiae evaluated in this study induce disease in immunocompetent Swiss Webster mice up to 6 weeks after inoculation. The human patients from whom these isolates were obtained had clinical presentations consistent with the multi-organ pathology observed in mice in this study. This mouse model for B. tamiae induced disease not only strengthens the causal link between this pathogen and clinical illness in humans, but provides a model to further study the pathological processes induced by these bacteria.

Figure 1

Photomicrographs of hematoxylin and eosin stained heart sections of mice sampled during the study. Mononuclear cell infiltration of the ventricle (A, 10×; B, 40×) and atrium (C, 10×) of a Swiss Webster mouse 5 weeks post-inoculation with B. tamiae Th339. The white arrow indicates a myocyte with a pyknotic nucleus. (D) Ventricle of a Swiss Webster mouse inoculated with saline (10×).

Figure 2

Photomicrographs of hematoxylin and eosin stained liver sections from mice sampled during the study. Granulomatous cell infiltration in liver tissue (A, 10×; B, 40×) of a Swiss Webster mouse inoculated with B. tamiae Th239. The black arrow indicates a necrotic hepatocyte. (C) Section of liver from a Swiss Webster mouse inoculated with saline (20×).

Figure 3

Photomicrographs of hematoxylin and eosin stained kidney and lymph node sections from mice sampled during the study. Granulomatous cell infiltration in the kidney (A, 10×) of a Swiss Webster mouse inoculated with B. tamiae Th239. Black arrows indicate degenerative glomeruli; the white arrow indicates a degenerating proximal tubule. (B) Necrotizing vasculitis in an axillary lymph node recovered from a Swiss Webster mouse 6 weeks after inoculation with B. tamiae Th339 (40×).