The road less traveled: transport of Legionella to the endoplasmic reticulum

Phagosomes containing the bacterial pathogen Legionella pneumophila are transported to the ER after macrophage internalization. To modulate phagosome transport, Legionella use a specialized secretion system that injects bacterial proteins into eukaryotic cells. This review will focus on recent studies that have identified bacterial proteins and host processes that play a concerted role in transporting Legionella to the ER.

Porphyromonas gingivalis virulence factors and invasion of cells of the cardiovascular system

Our laboratory is interested in the genes and gene products involved in the interactions between Porphyromonas gingivalis (Pg) and the host. These interactions may occur in either the periodontal tissues or other non-oral host tissues such as those of the cardiovascular system. We have previously reported the cloning of several genes encoding hemagglutinins, surface proteins that interact with the host tissues, and are investigating their roles in the disease process. Primary among these is HagA, a very large protein with multiple functional groups that have significant sequence homology to protease genes of this species. Preliminary evidence indicates that an avirulent Salmonella typhimurium strain containing hagA is virulent in mice. These data indicate that HagA may be a key virulence factor of Pg. Additionally, we are investigating the invasion of primary human coronary artery endothelial cells (HCAEC) by Pg because of the recent epidemiological studies indicating a correlation between periodontal disease (PD) and coronary heart disease (CHD). We found that some, but not all, strains of Pg are able to invade these cells. Scanning electron microsopy of the infected HCAEC demonstrated that the invading organisms initially attached to the host cell surface as aggregates and by a "pedestal"-like structure. By transmission electronmicroscopy it could be seen that internalized bacteria were present within multimembranous compartments localized with rough endoplasmic reticulum. In addition, invasion of the HCAEC by Pg resulted in an increase in the degradation of long-lived cellular proteins. These data indicate that Pg are present within autophagosomes and may use components of the autophagic pathway as a means to survive intracellularly. However, Pg presence within autophagosomes in KB cells could not be observed or detected. It is therefore likely that Pg uses different invasive mechanisms for different host cells. This and the role of HagA in invasion is currently being investigated further.

Is Brucella abortus a facultative intracellular pathogen with mitochondria-like activity?

Brucella abortus is the agent of bovine brucellosis, a zoonotic disease of worldwide importance. In latently infected humans and animals, acute disease may recur under conditions that decrease the host resistance. This bacterium is considered to be a facultative intracellular pathogen. However, its pathogenic attributes appear reduced in comparison with other Gram-negative pathogens. It has been recognized that B. abortus and other Brucella species reach their intracellular location inside the rough endoplasmic reticulum (RER) of placental trophoblasts and other nonphagocytic epithelial cells. This location is the limiting step for their replication and is in contrast to their intraphagosomal survival and growth in macrophages. To reach the RER, Brucella may be handled as another cellular organelle, like mitochondria. Furthermore, because of its inherent morphological and physiological characteristics, this alpha Proteobacteria may display here some mitochondria-like functions. Finally, external signals mediated by the host hormones and/or cytokines may turn this weak endosymbiotic relationship into a pathological one.

Heterogeneity in the attachment and uptake mechanisms of the Legionnaires' disease bacterium, Legionella pneumophila, by protozoan hosts

Invasion and intracellular replication of Legionella pneumophila within protozoa in the environment plays a major role in the transmission of Legionnaires' disease. Intracellular replication of L. pneumophila within protozoa occurs in a rough endoplasmic reticulum (RER)-surrounded phagosome (Y. Abu Kwaik, Appl. Environ. Microbiol. 62:2022-2028, 1996). Since the subsequent fate of many intracellular pathogens is determined by the route of entry, we compared the mechanisms of attachment and subsequent uptake of L. pneumophila by the two protozoa Hartmannella vermiformis and Acanthamoeba polyphaga. Our data provide biochemical and genetic evidence that the mechanisms of attachment and subsequent uptake of L. pneumophila by the two protozoan hosts are, in part, different. First, uptake of L. pneumophila by H. vermiformis is completely blocked by the monovalent sugars galactose and N-acetyl-D-galactosamine, but these sugars partially blocked A. polyphaga. Second, attachment of L. pneumophila to H. vermiformis is associated with a time-dependent and reversible tyrosine dephosphorylation of multiple host proteins. In contrast, only a slight dephosphorylation of a 170-kDa protein of A. polyphaga is detected upon infection. Third, synthesis of H. vermiformis proteins but not of A. polyphaga proteins is required for uptake of L. pneumophila. Fourth, we have identified L. pneumophila mutants that are severely defective in attachment to A. polyphaga but which exhibit minor reductions in attachment to H. vermiformis and, thus, provide a genetic basis for the difference in mechanisms of attachment to both protozoa. The data indicate a remarkable adaptation of L. pneumophila to attach and invade different protozoan hosts by different mechanisms, yet invasion is followed by a remarkably similar intracellular replication within a RER-surrounded phagosome and subsequent killing of the host cell.