The aggregation substance (AS) of Enterococcus faecalis, encoded on sex pheromone plasmids, is a surfacebound glycoprotein that mediates aggregation between bacteria thereby facilitating plasmid transfer. Sequencing of the pAD1-encoded Asa1 revealed that this surface protein contains two RGD motifs which are known to ligate integrins. Therefore, we investigated the influence of AS on the interaction of E. faecalis with human monocyte-derived macrophages which constitutively express ␤ 2 integrins (e.g., CD18). AS was found to cause a greater-than-fivefold increase in enterococcal adherence to macrophages and a greater-than-sevenfold increase in phagocytosis. Adherence was mediated by an interaction between the RGD motif and the integrin CD11b/CD18 (complement receptor type 3) as demonstrated by inhibition studies with monoclonal antibodies and RGD peptide. AS-bearing enterococci were significantly more resistant to macrophage killing during the first 3 h postinfection, probably due to inhibition of the respiratory burst as indicated by reduced concentrations of superoxide anion.Enterococci are gram-positive cocci which inhabit the gastrointestinal tract as well as the vagina and the oral cavity. Enterococcus faecalis accounts for 90% of human enterococcal infections, the most common being urinary tract infections, followed by abdominal infections, wound infections, bacteremia, and infective endocarditis (31, 39). Although infections due to E. faecalis have increased substantially during the last 10 years, the understanding of virulence mechanisms is still limited (24). One of the postulated virulence factors is the aggregation substance (AS), a sex pheromone plasmid-encoded surface protein which promotes the conjugative transfer of sex pheromone plasmids by formation of mating aggregates between donor and recipient cells (6,13,52). DNA sequencing of the structural gene for the pAD1-encoded AS revealed the presence of two Arg-Gly-Asp (RGD) sequences (16); RGD is a well-known motif recognized by a family of eukaryotic receptors, the integrins (38). Integrins consist of noncovalently linked ␣ and ␤ chains and are expressed on leukocytes, thrombocytes, endothelium, and various epithelial cells (21, 37, 42). Our group first suggested an interaction of AS with integrins, since we found that AS augmented adherence to porcine renal tubular cells which could be inhibited competitively by an RGD-Ser (RGDS) peptide (26). This hypothesis was corroborated by in vitro experiments with human polymorphonuclear leukocytes (PMN) which demonstrated that AS promotes opsonin-independent binding of E. faecalis via a ␤ 2 integrinmediated mechanism (46). It is assumed that many enterococcal infections are endogenous, originating from the intestinal tract (25, 51). Wells et al. speculated that macrophages may serve as a vehicle facilitating translocation from the intestinum into the lymph system and bloodstream (49, 50). However, this can occur only if enterococci are able to survive within macrophages. Indeed, Gentry-Weeks et al. demons...

The C-terminus of IcmT is essential for pore formation and for intracellular trafficking of Legionella pneumophila within Acanthamoeba polyphaga respectively, in intracellular growth in A. polyphaga, and the respective defects correlated with fusion of the bacterial phagosomes to lysosomes. Taken together, the data showed that the C-terminus domain of IcmT is essential for the pore-forming activity and is required for intracellular trafficking and replication within A. polyphaga, but not within mammalian cells. IntroductionLegionella pneumophila, the bacterium responsible for the acute pneumonia designated Legionnaire's disease, is able to grow within cells in the alveolar spaces . In the environment, this bacterium replicates within protozoa (Rowbotham, 1980;1986). Intracellular replication is culminated in pore formationmediated cytolysis of the protozoan host and subsequent bacterial egress (Gao and Abu Kwaik, 2000a). At least 15 species of protozoa support the intracellular growth of L. pneumophila, and bacterial replication within amoebae plays a major role in bacterial ecology and pathogenesis .Infection of both mammalian and protozoan cells by L. pneumophila shares similar mechanisms (Gao et al., 1997;. After entry into both host cells, L pneumophila modulates the biogenesis of the vacuole into a replicative niche, which does not fuse to lysosomal compartments and is subsequently surrounded by mitochondria and the rough endoplasmic reticulum Abu Kwaik et al., 1998a). The type IV Dot/Icm secretion system is essential for evasion of lysosomal fusion Vogel et al., 1998). After termination of intracellular replication, L. pneumophila induces expression of the pore-forming toxin, which is essential for cytolysis of the host cell and subsequent bacterial egress (Byrne and Swanson, 1998;Alli et al., 2000). Egress from host cells is a fundamental step in the life cycle of intracellular pathogens that allows the organism to spread to a new susceptible host cell and to search for a new replication niche.Mutants defective in egress from the host cell upon termination of intracellular replication have been isolated in our laboratory and designated rib (release of intracellular bacteria) (Alli et al., 2000;Gao and Abu Kwaik, 2000a SummaryWe have shown previously that the five rib (release of intracellular bacteria) mutants of Legionella pneumophila are competent for intracellular replication but defective in pore formation-mediated cytolysis and egress from protozoan and mammalian cells. The rib phenotype results from a point mutation (deletion) DG 544 in icmT that is predicted to result in the expression of a protein truncated by 32 amino acids from the C-terminus. In contrast to the rib mutants that are capable of intracellular replication, an icmT null mutant was completely defective in intracellular replication within mammalian and protozoan cells, in addition to its defect in pore formation-mediated cytolysis. The icmT wild-type allele complemented the icmT null mutant for both defects of intracellular replicati...

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Hog cholera, also known as classical or European swine fever, is caused by hog cholera virus, a member of the genus Pestivirus. It is shown here that the end stage of lethal infection in the natural host is associated with a dramatic depletion preferentially of B lymphocytes in the circulatory system as well as in lymphoid tissues. Already at the onset of disease, viral replication in lymphoid tissues demarcates the germinal centers, and the viral genome remains localized to that site as the disease progresses even after morphologic disintegration of the follicular structure. A block in B-lymphocyte maturation by infection and destruction of germinal centers is discussed as a key event in the pathogenesis of acute, lethal hog cholera.