VirB3 to VirB6 and VirB8 to VirB11, but Not VirB7, Are Essential for Mediating Persistence ofBrucellain the Reticuloendothelial System

Abstract

The Brucella abortus virB locus contains 12 open reading frames, termed virB1 through virB12, which encode a type IV secretion system. Polar mutations in the virB locus markedly reduce the ability of B. abortus to survive in cultured macrophages or to persist in organs of mice. While a nonpolar deletion of the virB2 gene reduces survival in cultured macrophages and in organs of mice, a nonpolar deletion of virB1 only reduces survival in macrophages, whereas virB12 is dispensable for either virulence trait. Here we investigated the role of the remaining genes in the virB locus during survival in macrophages and virulence in mice. Mutants carrying nonpolar deletions of the virB3, virB4, virB5, virB6, virB7, virB8, virB9, virB10, or virB11 gene were constructed and characterized. All mutations reduced the ability of B. abortus to survive in J774A.1 mouse macrophage-like cells to a degree similar to that caused by a deletion of the entire virB locus. Deletion of virB3, virB4, virB5, virB6, virB8, virB9, virB10, or virB11 markedly reduced the ability of B. abortus to persist in the spleens of mice at 8 weeks after infection. Interestingly, deletion of virB7 did not reduce the ability of B. abortus to persist in spleens of mice. We conclude that virB2, virB3, virB4, virB5, virB6, virB8, virB9, virB10, and virB11 are essential for virulence of B. abortus in mice, while functions encoded by the virB1, virB7, and virB12 genes are not required for persistence in organs with this animal model. Brucellosis, also known as Malta fever or undulant fever, is the most frequent anthropozoonosis in the world, with over half a million new cases each year (26, 44). The disease is caused by gram-negative bacteria that are members of the genus Brucella. These pathogens cause abortion and sterility in animals, while in humans the disease is characterized by recurrent episodes of fever. In both cases, the pathogens are capable of surviving for extended periods of time in the organs of the reticulo-endothelial system, such as liver, spleen, lymph nodes, and bone marrow. While Brucella species lack many of the classical virulence factors present in other Proteobacteria, such as fimbriae, toxins, or virulence plasmids, their chromosomes carry the virB locus, which encodes a type IV secretion system (T4SS) that is required for infection and modulation of the host immune system (24,29,42,56). T4SSs are ancestrally related to conjugation systems and have been identified in a large number of gram-negative pathogens that cause disease in both mammals and plants (1,4,5,20,43,52,54,62,64). Based on sequence comparison of their structural components, T4SSs can be divided into two groups, the virB-like T4SSs (T4A) and the dot/icm-like T4SSs (T4B) (14). Both T4A and T4B secretion systems have been shown to translocate or secrete a range of different substrates from DNA bound to protein to single proteins to multisubunit toxins (4, 8, 11, 12, 16, 36, 37, 39-41, 43, 53, 55, 61, 64). The closest homologues of the virB locus are T4A systems encoded by ...

The Dimer Interface of Agrobacterium tumefaciens VirB8 Is Important for Type IV Secretion System Function, Stability, and Association of VirB2 with the Core Complex

“…This is consistent with similar observations on reduced amounts of several VirB proteins in Brucella carrying a deletion of virB8 (17). VirB8 was previously shown to interact with itself, VirB1, VirB4, VirB5, VirB9, VirB10, and VirB11 (16,18,24,39,48,50).…”

Nondividing but Metabolically Active Gamma-IrradiatedBrucella melitensisIs Protective against VirulentB. melitensisChallenge in Mice

“…Others have monitored the interaction of phagosomes containing dead and live B. abortus and observed that live bacteria divert its intracellular fate and preserve the morphological structure after infection (3). More recent findings established the critical role of the T4SS machinery in this process (9,12,15). Like previous reports, heat-killed bacteria were completely digested shortly after exposure to dendritic cells (3).…”

“…Recent evidence demonstrated that T4SS is critical for Brucella intracellular persistence and trafficking (9,12,15). Furthermore, secreted bacterial antigens are reported to be more protective than cytosolic antigens (47).…”

Biological Diversity of Prokaryotic Type IV Secretion Systems

“…VirB7 At localizes predominantly at the outer membrane, but cytoplasmic membrane, pilus-associated, and other extracellular forms also exist (96). In B. pertussis and Brucella abortus, VirB7 homologs also stabilize VirB9-like partners as well as other VirB subunits (78,93). In B. abortus, mature VirB7 lacks an N-terminal Cys residue and thus is not lipid modified.…”

“…In Brucella spp., a VirB T4SS related to that of A. tumefaciens contributes to virulence, although the identification of translocated effector proteins has proven challenging (34,78). Recently, two substrates, VceA and VceC, were identified through the use of CyaA or TEM1 ␤-lactamase fusion assays (Table 1) (76).…”