Comparison by electron microscopy of intracellular events and survival of Burkholderia pseudomallei in monocytes from normal subjects and patients with melioidosis

INTRODUCTION: Burkholderia pseudomallei (B. pseudomallei) has been shown to persist intracellularly in patients with melioidosis, until reactivated by decreasing immunocompetence. We have previously demonstrated by transmission electron microscopy, the internalisation of B. pseudomallei by human macrophages and the occurrence of phagosome-lysosome fusion. METHODS: Phagocytosis and electron microscopy were used to compare the rate of phagosome-lysosome fusion and the intracellular survival of B. pseudomallei using monocytes obtained from five patients with melioidosis and five normal healthy adults. RESULTS: Ingested bacilli were seen in various stages of degradation, with a few remaining viable within phagolysosomes, and the proliferation of these viable bacteria was observed. Phagocytosis of B. pseudomallei by normal macrophages was two-fold higher than uptake by the melioidosis macrophages (p-value is less than 0.001). Three times more phagolysosomes were present in the normal macrophages, indicating that fusion occurred slowly and inefficiently in the melioidosis macrophages (p-value is less than 0.001), resulting in higher number of organisms within the melioidosis macrophages (p-value is less than 0.001). Both variables were inversely related to each other. CONCLUSION: Our observations suggest that phagolysosome fusion occurred slowly and inefficiently in monocytes of patients with melioidosis, leading to an increased number of intracellular organisms compared to monocytes obtained from healthy donors