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Brucellae
are facultative intracellular Gram-negative coccobacilli that chronically infect various mammals and cause brucellosis. Human brucellosis is among the most common bacterial zoonoses and the vast majority of cases are attributed to
B
.
melitensis
. Using transposon sequencing (Tn-seq) analysis, we showed that among 3369 predicted genes of the
B
.
melitensis
genome, 861 are required for optimal growth in rich medium and 186 additional genes appeared necessary for survival of
B
.
melitensis
in RAW 264.7 macrophages
in vitro
. As the mucosal immune system represents the first defense against
Brucella
infection, we investigated the early phase of pulmonary infection in mice.
In situ
analysis at the single cell level indicates a succession of killing and growth phases, followed by heterogenous proliferation of
B
.
melitensis
in alveolar macrophages during the first 48 hours of infection. Tn-seq analysis identified 94 additional genes that are required for survival in the lung at 48 hours post infection. Among them, 42 genes are common to RAW 264.7 macrophages and the lung conditions, including the T4SS and purine synthesis genes. But 52 genes are not identified in RAW 264.7 macrophages, including genes implicated in lipopolysaccharide (LPS) biosynthesis, methionine transport, tryptophan synthesis as well as fatty acid and carbohydrate metabolism. Interestingly, genes implicated in LPS synthesis and β oxidation of fatty acids are no longer required in Interleukin (IL)-17RA
-/-
mice and asthmatic mice, respectively. This demonstrates that the immune status determines which genes are required for optimal survival and growth of
B
.
melitensis in vivo
.