Model of Differential Susceptibility to Mucosal <em>Burkholderia pseudomallei</em> Infection

Boping Liu

Ghee Koo

Eu Yap

Kim Chua

Yunn Gan

Department of Biochemistry,^{Department of Microbiology, National University of Singapore, Singapore 119260}²

^{Corresponding author. Mailing address: Department of Biochemistry, National University of Singapore, 10, Kent Ridge Crescent, Singapore 119260. Phone: 65-874-3678. Fax: 65-779-1453. E-mail:}gs.ude.sun@hynaghcb.

Received 2001 May 8; Revised 2001 Jul 10; Accepted 2001 Oct 31.

Abstract

Burkholderia pseudomallei is the causative agent of melioidosis, an infectious disease with protean clinical manifestations. The major route of infection is thought to be through subcutaneous inoculation of contaminated soil and water, although ingestion and inhalation of contaminated aerosols are also possible. This study examines infection through the intranasal route in a murine model to mimic infection through inhalation. Two strains of mice, C57BL/6 and BALB/c, exhibit differential susceptibilities to the infection, with the C57BL/6 mice being considerably more resistant. To examine host factors that could contribute to this difference, bacterial loads and cytokine profiles in the two strains of mice were compared. We found that infected BALB/c mice exhibited higher bacterial loads in the lung and spleen and that they produced significantly higher levels of gamma interferon (IFN-γ) in the serum than C57BL/6 mice. Although tumor necrosis factor alpha and interleukin-1 could be detected in the nasal washes and sera of both strains of mice, the production in serum was transient and much lower than that of IFN-γ. C57BL/6 mice also exhibited memory responses to bacteria upon reinfection, with the production of serum immunoglobulin G (IgG) and mucosal IgA antibodies. Thus, it is possible that the production of systemic and mucosal antibodies is important for protection against disease in C57BL/6 mice.

Abstract

Melioidosis is an infectious disease caused by the gram-negative bacterium Burkholderia pseudomallei. The disease is endemic in southeast Asia and northern Australia, and there is evidence that it may be endemic in Africa, the Indian subcontinent, and Central and South America (11). The clinical manifestations of the disease vary and include an asymptomatic state, benign pneumonitis, acute or chronic pneumonia, or fulminant septicemias (10). Severe septic melioidosis is usually associated with underlying diseases such as diabetes and chronic renal failure, although it sometimes occurs in previously healthy individuals (8). Asymptomatic seroconversion has been observed in areas of endemicity such as northeast Thailand, where antibodies were found in about 80% of children by the time they were 4 years old (15). In most cases, infection is thought to occur through subcutaneous inoculation of contaminated soil and water, although it is thought to also occur via ingestion or inhalation of contaminated aerosols (17). B. pseudomallei can cause infection in many organs, although pathology occurs mainly in the lung, spleen, and liver (20, 25).

In order to understand the pathogenesis of the disease and the mechanism of host resistance, a murine model that mimics the acute and chronic forms of human melioidosis has been established (16). It was demonstrated that BALB/c mice were highly susceptible to infection with virulent B. pseudomallei, while C57BL/6 mice were relatively resistant. Following intravenous infection with as few as 37 CFU of virulent bacteria, substantial bacterial growth occurred in the livers and spleens of BALB/c mice, followed by overwhelming bacteremia to which the mice succumbed within 72 to 96 h. In contrast, C57BL/6 mice did not develop bacteremia, although they finally succumbed with apparent bacterial growth in the liver and spleen, displaying incomplete resistance (16).

The intraperitoneal and intravenous routes of infections used in animal models are thought to mimic systemic melioidosis. However, the first exposure to many microorganisms is often through the mucosal surfaces in the nasal passages and the gut. The route of infection can lead to different disease outcomes due to differences in the local immune environment. As melioidosis has such a wide spectrum of clinical presentations, it is possible that the route of infection is one of several factors that influence disease outcome. In this study, we examined the outcome of intranasal infection of BALB/c and C57BL/6 mice with virulent B. pseudomallei to simulate natural infection through inhalation and to determine if the murine model of differential susceptibility is still valid. The establishment of an intranasal infection model with differential outcomes would allow us to investigate the factors that could potentially contribute to host resistance and to our understanding of mucosal immunity. The examination of correlates of immunity in animals that were protected from infection would contribute to our understanding of protective immunity and to vaccine development.

Acknowledgments

We thank Joseph Thong and Soh Chan Lim for technical assistance with this project.

This work was funded by grant R-183-000-030-112 from the Academic Research Fund of the National University of Singapore.

Acknowledgments

Notes

Editor: R. N. Moore

Notes

Editor: R. N. Moore

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