ABSTRACT
The intracellular bacteria Brucella spp. replicate within their host cell, the macrophage. This relationship is slowly being elucidated, especially the characterization of the compartment containing the pathogen, and the adaptive response of the latter to this environment. Entry of brucellae via a LPS O chain-lipid raft interaction is critical and leads to inhibition of early phagosome-lysosome fusion. Rough strains, devoid of O antigen, do not use lipid rafts for entry and cannot avoid fusion. Major stress proteins allow the pathogen to resist at this stage of infection. In a second step, activation of specific genes such as virB in an acidic compartment deprived of amino acids results in the establishment of the replicative niche termed “brucellosome”. Escape into this final compartment allows the onset of multiplication and necessitates expression of genes we defined as the intramacrophagic “virulome”. Its analysis allows an indirect characterization of the brucellosome, resulting in the conclusion that the environment encountered by Brucella inside the macrophage is deprived in amino acids, poor in nutrients, and characterised by low oxygen tension. Brucellae live in a specific, isolated niche, and infection does not seem to affect the main functions of the cell. These observations contributed to their nickname, “stealthy bacteria”.
