Summary Intracellular replication of Brucella requires the VirB complex, which is highly similar to conjugative DNA transfer systems. In this study, we show that Brucella internalizes into macrophages by swimming on the cell surface with generalized membrane ruffling for several minutes, after which the bacteria are enclosed by macropinosomes. Lipid raft‐associated molecules such as glycosylphosphatidylinositol (GPI)‐anchored proteins, GM1 gangliosides and cholesterol were selectively incorporated into macropinosomes containing Brucella. In contrast, lysosomal glycoprotein LAMP‐1 and host cell transmembrane protein CD44 were excluded from the macropinosomes. Removing GPI‐anchored proteins from the macrophage surface and cholesterol sequestration markedly inhibited the VirB‐dependent macropinocytosis and intracellular replication. Our results suggest that the entry route of Brucella into the macrophage determines the intracellular fate of the bacteria that is modulated by lipid raft microdomains.

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The products of the Brucella abortus virB gene locus, which are highly similar to conjugative DNA transfer system, enable the bacterium to replicate within macrophage vacuoles. The replicative phagosome is thought to be established by the interaction of a substrate of the VirB complex with macrophages, although the substrate and its host cellular target have not yet been identified. We report here that Hsp60, a member of the GroEL family of chaperonins, of B. abortus is capable of interacting directly or indirectly with cellular prion protein (PrPC) on host cells. Aggregation of PrPC tail-like formation was observed during bacterial swimming internalization into macrophages and PrPC was selectively incorporated into macropinosomes containing B. abortus. Hsp60 reacted strongly with serum from human brucellosis patients and was exposed on the bacterial surface via a VirB complex–associated process. Under in vitro and in vivo conditions, Hsp60 of B. abortus bound to PrPC. Hsp60 of B. abortus, expressed on the surface of Lactococcus lactis, promoted the aggregation of PrPC but not PrPC tail formation on macrophages. The PrPC deficiency prevented swimming internalization and intracellular replication of B. abortus, with the result that phagosomes bearing the bacteria were targeted into the endocytic network. These results indicate that signal transduction induced by the interaction between bacterial Hsp60 and PrPC on macrophages contributes to the establishment of B. abortus infection.

Bacillus anthracis enters the body as an endospore, and encapsulation and toxin production occur after germination. Capsule is proposed to be an antiphagocytic factor, and toxin induces cytokine production for systemic shock. The dep gene, adjacent to the cap region for the encapsulation, degrades the high-molecular weight capsule (H-capsule) to the lower-molecular weight capsule (L-capsule), which releases into the culture supernatant. This study analyzed the biological function of the cap-dep region. The dep null mutant Sm-1, which formed H-capsule but not L-capsule, was avirulent. However, Sm-1 with an intact dep gene or with purified L-capsule recovered its pathogenicity. Sm-1 was subjected to phagocytosis by macrophages more easily than its parent strain, Sm, in vitro; in vivo, it cleared without L-capsule and grew well with L-capsule, which suggests that L-capsule is essential for in vivo multiplication. Moreover, a new name, capD, might be appropriate, because of the part of the cap operon involved in both polymerization and depolymerization of the capsule.

ABSTRACT. Brucella spp. are facultative intracellular pathogens that have the ability to survive and multiply in professional and nonprofessional phagocytes, and cause abortion in domestic animals and undulant fever in humans. The mechanism and factors of virulence are not fully understood. High-affinity zinc uptake system protein mutant (znuA mutant) showed reduced growth in zinc chelated medium, and failed to replicate in HeLa cells and mouse bone marrow-derived macrophages. Transformation of znuA mutant with a shuttle vector pBBR1MCS-4 containing znuA gene restored the growth in zinc chelated medium and intracellular replication in HeLa cells and macrophages to a level comparable to that of wild-type strain. Bacterial internalization into HeLa cells and macrophages and co-localization with either late endosomes or lysosomes of znuA mutant were not different from those of wild-type strain. These results suggest that znuA does not contribute to intracellular trafficking of B. abortus, but contributes to utilization of zinc required for intracellular growth. KEY WORDS: Brucella abortus, intracellular growth, zinc, znuA.J. Vet. Med. Sci. 66(9): 1059-1063, 2004 Brucella abortus, a facultative, gram-negative, intracellular pathogen, is the etiologic agent of brucellosis, a widely distributed zoonosis [22]. The establishment of chronic infection depends on the ability of brucellae to survive within phagocytes [9]. In contrast to other intracellular pathogens, Brucella species do not produce exotoxins, antiphagocytic capsules or thick cell walls, resistance forms or fimbriae and do not show antigenic variation [7]. Intracellular survival and replication are key virulence features of Brucella because mutants defective for these attributes are also avirulent [10,14]. A key aspect of the virulence of Brucella successfully bypasses the bactericidal effects of phagocytes, and their virulence and chronic infections are thought to be due to their ability to avoid the killing mechanisms within host cells [5,20].The genetic basis of Brucella virulence is still poorly understood. The VirB type IV secretion system of Brucella has been identified recently [16]. This operon is composed of 13 open reading frames (ORFs) that share homology with other bacterial type IV secretion systems in the intracellular trafficking of pathogens. Deletion or polar and non-polar mutations of these ORFs were not able to replicate and survive within phagocytes [19,21], and internalization and uptake pathway in phagosome trafficking between wildtype and virB mutant showed different patterns [11]. Thus, the VirB proteins of B. abortus are thought to be constituents of the secretion apparatus. The genus Brucella does not contain plasmids naturally, and it is therefore possible that the proteins encoded by virB genes are involved in protein secretion rather than conjugation. A possible role in virulence is to inject effector molecules, which help in the establishment of replication niche into the host cell.Zinc plays an important role in living organism...