Burkholderia pseudomallei, a facultative intracellular bacterium, is the causative agent of a broad spectrum of diseases collectively known as melioidosis. Its ability to survive inside phagocytic and nonphagocytic cells and to induce multinucleated giant cell (MNGC) formation has been demonstrated. This study was designed to assess a possible mechanism(s) leading to this cellular change, using virulent and nonvirulent strains of B. pseudomallei to infect both phagocytic and nonphagocytic cell lines. We demonstrated that when the cells were labeled with two different cell markers (CMFDA or CMTMR), mixed, and then infected with B. pseudomallei, direct cell-to-cell fusion could be observed, leading to MNGC formation. Staining of the infected cells with rhodamine-conjugated phalloidin indicated that immediately after the infection, actin rearrangement into a comet tail appearance occurred, similar to that described earlier for other bacteria. The latter rearrangement led to the formation of bacterium-containing, actin-associated membrane protrusions which could lead to a direct cell-to-cell spreading of B. pseudomallei in the infected hosts. Results from 4,6-diamidine-2-phenylindole dihydrochloride (DAPI) nuclear staining, poly-ADP ribose polymerase cleavage, staining of infected cells for phosphatidylserine exposure with annexin V, and electrophoresis of the DNA extracted from these infected cells showed that B. pseudomallei could kill the host cells by inducing apoptosis in both phagocytic and nonphagocytic cells.

Burkholderia pseudomallei is a causative agent of melioidosis, a life threatening disease which affects humans and animals in tropical and subtropical areas. This bacterium is known to survive and multiply inside cells such as macrophages. The mechanism of host defense against this bacterium is still unknown.In this study, we demonstrated that B. pseudomallei exhibited unique macrophage activation activity compared with Escherichia coli and Salmonella typhi, The mouse macrophage cell line (RAW 264.7) infected with B. pseudomallei at MOl of 0.1:1, 1:1 and 10:1 did not express a detectable level of inducible nitric oxide synthase (iNOS). Moreover, the B. pseudomallei infected cells released TNF-a only when they were infected with high MOl (10:1). Unlike the cells infected with B. pseudomallei, the cells infected with E. coli, and S. typhi expressed iNOS even at MOl of 0.1:1. These infected cells also released a significantly higher level of TNF-a at the low MOl ratio. The cells that were preactivated with IFN-oy prior to being infected with B. pseudomallei exhibited an enhanced production of iNOS and TNF-a release. The increased macrophage activation activity in the presence of IFN -oy also correlated with the restriction of the intracellular bacteria survival. Moreover, IFN-oy also prevented cell fusion and multinucleated cell formation induced by B. pseudomallei, a phenomenon recently described by our group. Altogether, these results indicate that internalization of B. pseudomallei failed to trigger substantial macrophage activation, a phenomenon which could prolong their survival inside the phagocytic cells and facilitate a direct cell to cell spreading of B. pseudomallei to neighboring cells.

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Although CpG oligodeoxynucleotides (CpG ODNs) are known to enhance resistance against infection in a number of animal models, little is known about the CpG-induced protection against acute fatal sepsis such as that associated with the highly virulent bacterium Burkholderia pseudomallei. We previously demonstrated in an in vitro study that immunostimulatory CpG ODN 1826 enhances phagocytosis of B. pseudomallei and induces nitric oxide synthase and nitric oxide production by mouse macrophages. In the present study, CpG ODN 1826 given intramuscularly to BALB/c mice 2 to 10 days prior to B. pseudomallei challenge conferred better than 90% protection. CpG ODN 1826 given 2 days before the bacterial challenge rapidly enhanced the innate immunity of these animals, judging from the elevated serum levels of interleukin-12 (IL-12)p70 and gamma interferon (IFN-␥) over the baseline values. No bacteremia was detected on day 2 in 85 to 90% of the CpG-treated animals, whereas more than 80% of the untreated animals exhibited heavy bacterial loads. Although marked elevation of IFN-␥ was found consistently in the infected animals 2 days after the bacterial challenge, it was ameliorated by the CpG ODN 1826 pretreatment (P ‫؍‬ 0.0002). Taken together, the kinetics of bacteremia and cytokine profiles presented are compatible with the possibility that protection by CpG ODN 1826 against acute fatal septicemic melioidosis in this animal model is associated with a reduction of bacterial load and interference with the potential detrimental effect of the robust production of proinflammatory cytokines associated with B. pseudomallei multiplication.