Brucella abortus is a facultative intracellular bacterium capable of surviving inside professional and nonprofessional phagocytes. The microorganism remains in membrane-bound compartments that in several cell types resemble modified endoplasmic reticulum structures. To monitor the intracellular transport of B. abortus in macrophages, the kinetics of fusion of phagosomes with preformed lysosomes labeled with colloidal gold particles was observed by electron microscopy. The results indicated that phagosomes containing live B. abortus were reluctant to fuse with lysosomes. Furthermore, newly endocytosed material was not incorporated into these phagosomes. These observations indicate that the bacteria strongly affect the normal maturation process of macrophage phagosomes. However, after overnight incubation, a significant percentage of the microorganisms were found in large phagosomes containing gold particles, resembling phagolysosomes. Most of the Brucella bacteria present in phagolysosomes were not morphologically altered, suggesting that 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. abortus bacteria do not prevent phagosome acidification. In contrast to what has been described for HeLa cells, only a minor percentage of the microorganisms were found in compartments labeled with monodansylcadaverine, a marker for autophagosomes, and with DiOC6 (3,3-dihexyloxacarbocyanine iodide), a marker for the endoplasmic reticulum. These results indicate that B. abortus bacteria alter phagosome maturation in macrophages. However, acidification does occur in these phagosomes, and some of them can eventually mature to phagolysosomes.The facultative intracellular parasite Brucella abortus causes abortion and infertility in cattle and undulant fever in humans. The bacterium is endemic in many underdeveloped countries and responsible for large economic losses and chronic infections in human beings (30). Brucella infects its hosts through mucosae and wounds and initially is incorporated into professional phagocytes where it survives and reproduces (14). Afterwards, the bacterium infects several types of nonprofessional phagocytic cells including those of endocardium, brain, joints, and bones. Brucella has a special tropism for reproductive organs, causing a high rate of abortion in pregnant animals (28).The intracellular survival of Brucella has been documented for several cell types. According to multiple observations, B. abortus is incorporated into phagosomes and remains in membrane-bound compartments until the host cell dies. In nonprofessional phagocytes, Brucella is located in structures that resemble the endoplasmic reticulum (ER) (6). Recent evidence indicates that Brucella is transported through the autophagic pathway before accumulating in the ER (22, 23).Macrophages are particularly important for the survival and spreading of Brucella during infection ...

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