Study on the pathophysiology of experimental Burkholderia pseudomallei infection in mice

Burkholderia pseudomallei is the etiological agent of melioidosis, a potent ially fatal disease occurring in man and animals. The aim of this study was to investigate the pathophysiological course of experimental melioidosis, and to identify the target organs, in an animal model. For this purpose SWI SS mice were infected intraperitoneally with the virulent strain B. pseudom allei 6068. The bacterial load of various organs was quantified daily by ba cteriological analysis and by an enzyme-linked immunosorbent assay (ELISA) based on a monoclonal antibody specific to B. pseudomallei exopolysaccharid e (EPS). Electron microscopic investigation of the spleen was performed to locate the bacteria at the cellular level. In this model of acute melioidos is, B. pseudomallei had a marked organ tropism for liver and spleen, and sh owed evidence of in vivo growth with a bacterial burden of 1.6 x 10(9) colo ny forming units (CFU) per gram of spleen 5 days after infection with 200 C FU. The highest bacterial loads were detected in the spleen at all time poi nts, in a range from 2x10(6) to 2x10(9) CFU g(-1). They were still 50-80 ti mes greater than the load of the liver at the time of peak burden. Other in vestigated organs such as lungs, kidneys, and bone marrow were 10(2)-10(4)- fold less infected than the spleen, with loads ranging from 3x10(2) to 3x10 (6) CFU g(-1). The heart and the brain were sites of a delayed infection, w ith counts in a range from 10(3) to 10(7) times lower than bacterial counts in the spleen. The EPS-specific ELISA proved to be highly sensitive, parti cularly at the level of those tissues in which colony counting on agar reve aled low; contamination. In the blood, EPS was detected at concentrations c orresponding to bacterial loads ranging from 8x10(3) to 6x10(4) CFU ml(-1). Electron microscopic examination of the spleen revealed figures of phagocy tosis, and the presence of large numbers of intact bacteria, which occurred either as single cells or densely packed into vacuoles. Sparse figures sug gesting bacterial replication were also observed. In addition, some bacteri a could be seen in vacuoles that seemed to have lost their membrane. These observations provide a basis for further investigations on the pathogenesis of the disease. (C) 2001 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.