Intracellular trafficking of Brucella abortus in J774 macrophages

Brucella abortus is a facultative intracellular bacterium capable of surviv ing inside professional and nonprofessional phagocytes. The microorganism r emains in membrane-bound compartments that in several cell types resemble m odified endoplasmic reticulum structures. To monitor the intracellular tran sport of B, abortus in macrophages, the kinetics of fusion of phagosomes wi th preformed lysosomes labeled with colloidal gold particles was observed b y electron microscopy, The results indicated that phagosomes containing liv e B. abortus were reluctant to Fuse with lysosomes. Furthermore, newly endo cytosed material was not incorporated into these phagosomes. These observat ions indicate that the bacteria strongly affect the normal maturation proce ss of macrophage phagosomes, However, after overnight incubation, a signifi cant percentage of the microorganisms were found in large phagosomes contai ning gold particles, resembling phagolysosomes. Most of the Brucella bacter ia present in phagolysosomes were not morphologically altered, suggesting t hat they can also resist the harsh conditions prevalent in this compartment . About 50% colocalization of B, abortus with LysoSensor, a weak base that accumulates in acidic compartments, was observed, indicating that the B, ab ortus bacteria do not prevent phagosome acidification. In contrast to what has been described for HeLa cells, only a minor percentage of the microorga nisms were found in compartments Labeled with monodansylcadaverine, a marke r for autophagosomes, and with DiOC6 (3,3'-dihexyloxacarbocyanine iodide), a marker for the endoplasmic reticulum, These results indicate that B, abor tus bacteria alter phagosome maturation in macrophages, However, acidificat ion does occur in these phagosomes, and some of them can eventually mature to phagolysosomes.