Characterization of Legionella pneumophila pmiA, a gene essential for infectivity of protozoa and macrophages

Concept about the Virulence Factor of Legionella

Pathogenic species of Legionella can infect human alveolar macrophages through Legionella-containing aerosols to cause a disease called Legionellosis, which has two forms: a flu-like Pontiac fever and severe pneumonia named Legionnaires' disease (LD). Legionella is an opportunistic pathogen that frequently presents in aquatic environments as a biofilm or protozoa parasite. Long-term interaction and extensive co-evolution with various genera of amoebae render Legionellae pathogenic to infect humans and also generate virulence differentiation and heterogeneity. Conventionally, the proteins involved in initiating replication processes and human macrophage infections have been regarded as virulence factors and linked to pathogenicity. However, because some of the virulence factors are associated with the infection of protozoa and macrophages, it would be more accurate to classify them as survival factors rather than virulence factors. Given that the molecular basis of virulence variations among non-pathogenic, pathogenic, and highly pathogenic Legionella has not yet been elaborated from the perspective of virulence factors, a comprehensive explanation of how Legionella infects its natural hosts, protozoans, and accidental hosts, humans is essential to show a novel concept regarding the virulence factor of Legionella. In this review, we overviewed the pathogenic development of Legionella from protozoa, the function of conventional virulence factors in the infections of protozoa and macrophages, the host's innate immune system, and factors involved in regulating the host immune response, before discussing a probably new definition for the virulence factors of Legionella.

Structure and function of REP34 implicates carboxypeptidase activity in Francisella tularensis host cell invasion

Francisella tularensis is the etiological agent of tularemia, or rabbit fever. Although F. tularensis is a recognized biothreat agent with broad and expanding geographical range, its mechanism of infection and environmental persistence remain poorly understood. Previously, we identified seven F. tularensis proteins that induce a rapid encystment phenotype (REP) in the free-living amoeba, Acanthamoeba castellanii. Encystment is essential to the pathogen's long term intracellular survival in the amoeba. Here, we characterize the cellular and molecular function of REP34, a REP protein with a mass of 34 kDa. A REP34 knock-out strain of F. tularensis has a reduced ability to both induce encystment in A. castellanii and invade human macrophages. We determined the crystal structure of REP34 to 2.05-Å resolution and demonstrate robust carboxypeptidase B-like activity for the enzyme. REP34 is a zinc-containing monomeric protein with close structural homology to the metallocarboxypeptidase family of peptidases. REP34 possesses a novel topology and substrate binding pocket that deviates from the canonical funnelin structure of carboxypeptidases, putatively resulting in a catalytic role for a conserved tyrosine and distinct S1' recognition site. Taken together, these results identify REP34 as an active carboxypeptidase, implicate the enzyme as a potential key F. tularensis effector protein, and may help elucidate a mechanistic understanding of F. tularensis infection of phagocytic cells.

Ciliates expel environmental Legionella-laden pellets to stockpile food

When Tetrahymena ciliates are cultured with Legionella pneumophila, the ciliates expel bacteria packaged in free spherical pellets. Why the ciliates expel these pellets remains unclear. Hence, we determined the optimal conditions for pellet expulsion and assessed whether pellet expulsion contributes to the maintenance of growth and the survival of ciliates. When incubated with environmental L. pneumophila, the ciliates expelled the pellets maximally at 2 days after infection. Heat-killed bacteria failed to produce pellets from ciliates, and there was no obvious difference in pellet production among the ciliates or bacterial strains. Morphological studies assessing lipid accumulation showed that pellets contained tightly packed bacteria with rapid lipid accumulation and were composed of the layers of membranes; bacterial culturability in the pellets rapidly decreased, in contrast to what was seen in ciliate-free culture, although the bacteria maintained membrane integrity in the pellets. Furthermore, ciliates newly cultured with pellets were maintained and grew vigorously compared with those without pellets. In contrast, a human L. pneumophila isolate killed ciliates 7 days postinfection in a Dot/Icm-dependent manner, and pellets harboring this strain did not support ciliate growth. Also, pellets harboring the human isolate were resuscitated by coculturing with amoebae, depending on Dot/Icm expression. Thus, while ciliates expel pellet-packaged environmental L. pneumophila for stockpiling food, the pellets packaging the human isolate are harmful to ciliate survival, which may be of clinical significance.

Minimization of the Legionella pneumophila genome reveals chromosomal regions involved in host range expansion

Legionella pneumophila is a bacterial pathogen of amoebae and humans. Intracellular growth requires a type IVB secretion system that translocates at least 200 different proteins into host cells. To distinguish between proteins necessary for growth in culture and those specifically required for intracellular replication, a screen was performed to identify genes necessary for optimal growth in nutrient-rich medium. Mapping of these genes revealed that the L. pneumophila chromosome has a modular architecture consisting of several large genomic islands that are dispensable for growth in bacteriological culture. Strains lacking six of these regions, and thus 18.5% of the genome, were viable but required secondary point mutations for optimal growth. The simultaneous deletion of five of these genomic loci had no adverse effect on growth of the bacterium in nutrient-rich media. Remarkably, this minimal genome strain, which lacked 31% of the known substrates of the type IVB system, caused only marginal defects in intracellular growth within mouse macrophages. In contrast, deletion of single regions reduced growth within amoebae. The importance of individual islands, however, differed among amoebal species. The host-specific requirements of these genomic islands support a model in which the acquisition of foreign DNA has broadened the L. pneumophila host range.

Proteomic analysis of growth phase-dependent expression of Legionella pneumophila proteins which involves regulation of bacterial virulence traits

Legionella pneumophila, which is a causative pathogen of Legionnaires' disease, expresses its virulent traits in response to growth conditions. In particular, it is known to become virulent at a post-exponential phase in vitro culture. In this study, we performed a proteomic analysis of differences in expression between the exponential phase and post-exponential phase to identify candidates associated with L. pneumophila virulence using 2-Dimentional Fluorescence Difference Gel Electrophoresis (2D-DIGE) combined with Matrix-Assisted Laser Desorption/Ionization–Mass Spectrometry (MALDI-TOF-MS). Of 68 identified proteins that significantly differed in expression between the two growth phases, 64 were up-regulated at a post-exponential phase. The up-regulated proteins included enzymes related to glycolysis, ketone body biogenesis and poly-3-hydroxybutyrate (PHB) biogenesis, suggesting that L. pneumophila may utilize sugars and lipids as energy sources, when amino acids become scarce. Proteins related to motility (flagella components and twitching motility-associated proteins) were also up-regulated, predicting that they enhance infectivity of the bacteria in host cells under certain conditions. Furthermore, 9 up-regulated proteins of unknown function were found. Two of them were identified as novel bacterial factors associated with hemolysis of sheep red blood cells (SRBCs). Another 2 were found to be translocated into macrophages via the Icm/Dot type IV secretion apparatus as effector candidates in a reporter assay with Bordetella pertussis adenylate cyclase. The study will be helpful for virulent analysis of L. pneumophila from the viewpoint of physiological or metabolic modulation dependent on growth phase.

The Legionella pneumophila Dps homolog is regulated by iron and involved in multiple stress tolerance

Iron homeostasis is essential to almost all organisms. In this study, we identified the putative homolog of the iron-storage protein-encoding gene, dpsL, in the intracellular pathogen Legionella pneumophila and demonstrated its expression under iron-limited conditions and its responses to multiple stresses. Quantitative real-time PCR analysis indicated that the expression of dpsL was enhanced under iron limitation regardless of the growth phase. Compared with the wild-type cells, the cells devoid of dpsL were heat and H(2)O(2)-sensitive. In contrast to the dps mutants of other bacteria, the growth of the dpsL mutant in an iron-deprived medium was delayed but finally reached the same cell density as wild-type cells during the stationary phase of growth. The finding that the dpsL mutant is salt resistant suggested the involvement of DpsL in virulence.

[Intracellular survival and replication of legionella pneumophila within host cells]

Legionella pneumophila is a facultative intracellular pathogen which replicates within macrophages and monocytes and finally cause a severe pneumonia known as Legionnaires' disease. An important hallmark of the pathogenesis of this bacterium is their ability to manipulate host cell processes, creating a specified replicative niche within host cells. An L. pneumophila-containing phagosome (LCP) is allowed to associate sequentially with smooth vesicles, mitochondria, and the rough endoplasmic reticulum (RER) to form a compartment called a replicative phagosome. LCPs are biologically characterized by delayed acidification and a low tendency to fuse with lysosomes. The establishment of these specialized phagosomes is mediated by the Icm/Dot Type IV secretion system, which is essential for the intracellular growth of L. pneumophila. L. pneumophila utilizes the Icm/Dot system to inject bacterial effector molecules into the host cell cytosol to survive and replicate in the intracellular compartment through modulation of phagosome biogenesis. This review focuses on our studies on specific aspects of L. pneumophila infection to host cells and bacterial factors which regulates its intracellular growth. We found several characteristic phenomena leading to L. pneumophila infection, which is dependent on LCP formation: active bacterial protein synthesis in L. pneumophila within macrophages, specific exclusion of actin-binding protein p57/Coronin-1 from LCP, and suppression of reactive oxygen species (ROS) production by macrophages upon infection with L. pneumophila. Furthermore, we identified a novel bacterial factor, PmiA, which is involved in multiplication within both protozoa and macrophages. Our recent study has begun to reveal that the biological function of PmiA is closely associated with that of the Icm/Dot type IV secretion system.

Exclusion of actin-binding protein p57/coronin-1 from bacteria-containing phagosomes in macrophages infected with Legionella

Legionella pneumophila, the causative agent of Legionnaires' disease, is a human pathogen that multiplies within alveolar macrophages. L. pneumophila establishes specialized phagosomes in which it evades the host defense through largely unknown mechanisms. Here we analyzed the role of an actin-binding protein, p57/coronin-1, a member of the coronin protein family, during Legionella infection. On fluorescence microscopy, p57/coronin-1 and F-actin were found to be co-localized at the sites on the plasma membrane where L. pneumophila adhered to U937 human macrophage-like cells. The localization of p57/coronin-1 at the sites of bacterial adherence was inhibited by treatment with cytochalasin D (an inhibitor of actin polymerization), suggesting that p57/coronin-1 is involved in the actin-dependent uptake of L. pneumophila into U937 cells. In addition, we showed that p57/coronin-1 was excluded from phagosomes containing live L. pneumophila throughout the infection, whereas transient accumulation of p57/coronin-1 was observed on phagosomes containing Texas-Red-labeled opsonized zymosan (TROpZ) or heat-killed L. pneumophila at an early stage of phagocytosis. The exclusion of p57/coronin-1 from phagosomes containing live another Legionella species Legionella gratiana at an early stage of infection was also observed. Taken together, these results suggest that the endocytic pathways of live Legionella species are distinct from general phagocytic pathways, which lead to lysosomal degradation.

Environmental predators as models for bacterial pathogenesis

Environmental bacteria are constantly threatened by bacterivorous predators such as free-living protozoa and nematodes. In the course of their coevolution with environmental predators, some bacteria developed sophisticated defence mechanisms, including the secretion of toxins, or the capacity to avoid lysosomal killing and to replicate intracellularly within protozoa. To analyse the interactions with bacterial pathogens on a molecular, cellular or organismic level, protozoa and other non-mammalian hosts are increasingly used. These include amoebae, as well as genetically tractable hosts, such as the social amoeba Dictyostelium discoideum, the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. Using these hosts, the virulence mechanisms of opportunistic pathogenic bacteria such as Legionella, Mycobacterium, Pseudomonas or Vibrio were found to be not only relevant for the interactions of the bacteria with protozoa, nematodes and insect phagocytes, but also with mammalian hosts including humans. Thus, non-mammalian model hosts provide valuable insight into the pathogenesis of environmental bacteria.

Environmental mimics and the Lvh type IVA secretion system contribute to virulence-related phenotypes of Legionella pneumophila

Legionella pneumophila, the causative organism of Legionnaires' disease, is a fresh-water bacterium and intracellular parasite of amoebae. This study examined the effects of incubation in water and amoeba encystment on L. pneumophila strain JR32 and null mutants in dot/icm genes encoding a type IVB secretion system required for entry, delayed acidification of L. pneumophila-containing phagosomes, and intracellular multiplication when stationary-phase bacteria infect amoebae and macrophages. Following incubation of stationary-phase cultures in water, mutants in dotA and dotB, essential for function of the type IVB secretion system, exhibited entry and delay of phagosome acidification comparable to that of strain JR32. Following encystment in Acanthamoeba castellanii and reversion of cysts to amoeba trophozoites, dotA and dotB mutants exhibited intracellular multiplication in amoebae. The L. pneumophila Lvh locus, encoding a type IVA secretion system homologous to that in Agrobacterium tumefaciens, was required for restoration of entry and intracellular multiplication in dot/icm mutants following incubation in water and amoeba encystment and was required for delay of phagosome acidification in strain JR32. These data support a model in which the Dot/Icm type IVB secretion system is conditionally rather than absolutely required for L. pneumophila virulence-related phenotypes. The data suggest that the Lvh type IVA secretion system, previously thought to be dispensable, is involved in virulence-related phenotypes under conditions mimicking the spread of Legionnaires' disease from environmental niches. Since environmental amoebae are implicated as reservoirs for an increasing number of environmental pathogens and for drug-resistant bacteria, the environmental mimics developed here may be useful in virulence studies of other pathogens.