Entry and intracellular growth of Legionella dumoffii in alveolar epithelial cells

The intracellular growth of the vacuolar pathogen Legionella pneumophila is dependent on the acyl chain composition of host membranes

Legionella pneumophila is an accidental human bacterial pathogen that infects and replicates within alveolar macrophages causing a severe atypical pneumonia known as Legionnaires' disease. As a prototypical vacuolar pathogen L. pneumophila establishes a unique endoplasmic reticulum (ER)-derived organelle within which bacterial replication takes place. Bacteria-derived proteins are deposited in the host cytosol and in the lumen of the pathogen-occupied vacuole via a type IVb (T4bSS) and a type II (T2SS) secretion system respectively. These secretion system effector proteins manipulate multiple host functions to facilitate intracellular survival of the bacteria. Subversion of host membrane glycerophospholipids (GPLs) by the internalized bacteria via distinct mechanisms feature prominently in trafficking and biogenesis of the Legionella -containing vacuole (LCV). Conventional GPLs composed of a glycerol backbone linked to a polar headgroup and esterified with two fatty acids constitute the bulk of membrane lipids in eukaryotic cells. The acyl chain composition of GPLs dictates phase separation of the lipid bilayer and therefore determines the physiochemical properties of biological membranes - such as membrane disorder, fluidity and permeability. In mammalian cells, fatty acids esterified in membrane GPLs are sourced endogenously from de novo synthesis or via internalization from the exogenous pool of lipids present in serum and other interstitial fluids. Here, we exploited the preferential utilization of exogenous fatty acids for GPL synthesis by macrophages to reprogram the acyl chain composition of host membranes and investigated its impact on LCV homeostasis and L. pneumophila intracellular replication. Using saturated fatty acids as well as cis - and trans - isomers of monounsaturated fatty acids we discovered that under conditions promoting lipid packing and membrane rigidification L. pneumophila intracellular replication was significantly reduced. Palmitoleic acid - a C16:1 monounsaturated fatty acid - that promotes membrane disorder when enriched in GPLs significantly increased bacterial replication within human and murine macrophages but not in axenic growth assays. Lipidome analysis of infected macrophages showed that treatment with exogenous palmitoleic acid resulted in membrane acyl chain reprogramming in a manner that promotes membrane disorder and live-cell imaging revealed that the consequences of increasing membrane disorder impinge on several LCV homeostasis parameters. Collectively, we provide experimental evidence that L. pneumophila replication within its intracellular niche is a function of the lipid bilayer disorder and hydrophobic thickness.

A sequential utilization of the UV-A (365 nm) fluence rate for disinfection of water, contaminated with Legionella pneumophila and Legionelladumoffii

Legionella species are the etiological agent of Legionnaires' disease, a pathology easily contracted from water circuits and by the inhalation of aerosol droplets. This bacterium mainly proliferates in water: Legionella pneumophila is the most commonly isolated specie in water environments and consequently in water system, although further Legionella species have frequently been isolated, including Legionella dumoffii. The simultaneous presence of the two species in the water system can therefore lead to the simultaneous infection of several people, giving rise to harmful outbreaks. Ultraviolet inactivation of waterborne microorganisms offers a rapid and effective treatment technique and recently is getting more attention mostly to eliminate unsafe level of contamination. To tackle the issue, the inactivation of the two species of Legionella spp., namely L. pneumophila and L. dumoffii, by means of UV-A light emitting diodes (UV-A LED) system is explored. We used a commercially available UV-A LED at 365 nm wavelength, and the UV-A dose is given incrementally to the Legionellae with a concentration of 106 CFU/mL in 0.9% NaCl (aq) solution. In this study, with a UV-A-dose of 1700 mJ/cm2, the log-reduction of 3-log (99.9% inactivation) for L. pneumophila and 2.1-log (99.1% inactivation) for L. dumoffii of the contaminated water are achieved. The Electrical Energy per Order (EEO) is evaluated and showed this system is more economic and efficient in comparison with UV-C and UV-B LEDs. Following the support of this preliminary study with additional tests, aiming to validate the technology, we expect this device may be installed in water plants such as cooling systems or any water purification station in either industrial or home scales to reduce the risk of this infectious disease, preventing consumers' health.

Choline Supplementation Sensitizes Legionella dumoffii to Galleria mellonella Apolipophorin III

The growth of Legionella dumoffii can be inhibited by Galleria mellonella apolipophorin III (apoLp-III) which is an insect homologue of human apolipoprotein E., and choline-cultured L. dumoffii cells are considerably more susceptible to apoLp-III than bacteria grown without choline supplementation. In the present study, the interactions of apoLp-III with intact L. dumoffii cells cultured without and with exogenous choline were analyzed to explain the basis of this difference. Fluorescently labeled apoLp-III (FITC-apoLp-III) bound more efficiently to choline-grown L. dumoffii, as revealed by laser scanning confocal microscopy. The cell envelope of these bacteria was penetrated more deeply by FITC-apoLp-III, as demonstrated by fluorescence lifetime imaging microscopy analyses. The increased susceptibility of the choline-cultured L. dumoffii to apoLp-III was also accompanied by alterations in the cell surface topography and nanomechanical properties. A detailed analysis of the interaction of apoLp-III with components of the L. dumoffii cells was carried out using both purified lipopolysaccharide (LPS) and liposomes composed of L. dumoffii phospholipids and LPS. A single micelle of L. dumoffii LPS was formed from 12 to 29 monomeric LPS molecules and one L. dumoffii LPS micelle bound two molecules of apoLp-III. ApoLp-III exhibited the strongest interactions with liposomes with incorporated LPS formed of phospholipids isolated from bacteria cultured on exogenous choline. These results indicated that the differences in the phospholipid content in the cell membrane, especially PC, and LPS affected the interactions of apoLp-III with bacterial cells and suggested that these differences contributed to the increased susceptibility of the choline-cultured L. dumoffii to G. mellonella apoLp-III.

Mechanisms of Epithelial Immunity Evasion by Respiratory Bacterial Pathogens

Bacterial lung infections are major healthcare challenges killing millions of people worldwide and resulting in a huge economic burden. Both basic and clinical research have elucidated host mechanisms that contribute to the bacterial clearance where an indispensable role of immune cells has been established. However, the role of respiratory epithelial cells in bacterial clearance has garnered limited attention due to their weak inflammatory or phagocytic ability compared to immune cells such as macrophages and neutrophils. These studies often underappreciate the fact that epithelial cells are the most abundant cells in the lung, not only serving as building blocks but also providing immune protection throughout the lung. Epithelial cells function either independently to eradicate the pathogen or communicate with immune cells to orchestrate pathogen clearance. The epithelial cells have multiple mechanisms that include mucus production, antimicrobial peptide production, muco-ciliary clearance, and phagocytosis, all of which contribute to their direct antibacterial function. Secretion of cytokines to recruit immune cells and potentiate their antimicrobial activities is a pathway by which the epithelium contributes to bacterial clearance. Successful pathogens outsmart epithelial resistance and find a way to replicate in sufficient numbers to establish infections in the airway or lung epithelial surfaces. In this mini-review, we discuss evidences that establish important roles for epithelial host defense against invading respiratory bacterial pathogens and demonstrate how pathogens outsmart these epithelial immune mechanisms to successfully establish infection. Finally, we discuss briefly how to boost epithelial immunity to improve outcomes in bacterial lung infections.

Mechanisms of Epithelial Immunity Evasion by Respiratory Bacterial Pathogens

Bacterial lung infections are major healthcare challenges killing millions of people worldwide and resulting in a huge economic burden. Both basic and clinical research have elucidated host mechanisms that contribute to the bacterial clearance where an indispensable role of immune cells has been established. However, the role of respiratory epithelial cells in bacterial clearance has garnered limited attention due to their weak inflammatory or phagocytic ability compared to immune cells such as macrophages and neutrophils. These studies often underappreciate the fact that epithelial cells are the most abundant cells in the lung, not only serving as building blocks but also providing immune protection throughout the lung. Epithelial cells function either independently to eradicate the pathogen or communicate with immune cells to orchestrate pathogen clearance. The epithelial cells have multiple mechanisms that include mucus production, antimicrobial peptide production, muco-ciliary clearance, and phagocytosis, all of which contribute to their direct antibacterial function. Secretion of cytokines to recruit immune cells and potentiate their antimicrobial activities is a pathway by which the epithelium contributes to bacterial clearance. Successful pathogens outsmart epithelial resistance and find a way to replicate in sufficient numbers to establish infections in the airway or lung epithelial surfaces. In this mini-review, we discuss evidences that establish important roles for epithelial host defense against invading respiratory bacterial pathogens and demonstrate how pathogens outsmart these epithelial immune mechanisms to successfully establish infection. Finally, we discuss briefly how to boost epithelial immunity to improve outcomes in bacterial lung infections.

Assessing the impact, genomics and evolution of type II secretion across a large, medically important genus: the Legionella type II secretion paradigm

The type II secretion system (T2SS) plays a major role in promoting bacterial survival in the environment and in human hosts. One of the best characterized T2SS is that of Legionella pneumophila, the agent of Legionnaires’ disease. Secreting at least 25 proteins, including degradative enzymes, eukaryotic-like proteins and novel effectors, this T2SS contributes to the ability of L. pneumophila to grow at low temperatures, infect amoebal and macrophage hosts, damage lung tissue, evade the immune system, and undergo sliding motility. The genes encoding the T2SS are conserved across the genus Legionella, which includes 62 species and >30 pathogens in addition to L. pneumophila. The vast majority of effectors associated with L. pneumophila are shared by a large number of Legionella species, hinting at a critical role for them in the ecology of Legionella as a whole. However, no other species has the same repertoire as L. pneumophila, with, as a general rule, phylogenetically more closely related species sharing similar sets of effectors. T2SS effectors that are involved in infection of a eukaryotic host(s) are more prevalent throughout Legionella, indicating that they are under stronger selective pressure. The Legionella T2SS apparatus is closest to that of Aquicella (another parasite of amoebae), and a significant number of L. pneumophila effectors have their closest homologues in Aquicella. Thus, the T2SS of L. pneumophila probably originated within the order Legionellales, with some of its effectors having arisen within that Aquicella-like progenitor, while other effectors derived from the amoebal host, mimiviruses, fungi and less closely related bacteria.

Evasion of phagotrophic predation by protist hosts and innate immunity of metazoan hosts by Legionella pneumophila

Legionella pneumophila is a ubiquitous environmental bacterium that has evolved to infect and proliferate within amoebae and other protists. It is thought that accidental inhalation of contaminated water particles by humans is what has enabled this pathogen to proliferate within alveolar macrophages and cause pneumonia. However, the highly evolved macrophages are equipped with more sophisticated innate defence mechanisms than are protists, such as the evolution of phagotrophic feeding into phagocytosis with more evolved innate defence processes. Not surprisingly, the majority of proteins involved in phagosome biogenesis (~80%) have origins in the phagotrophy stage of evolution. There are a plethora of highly evolved cellular and innate metazoan processes, not represented in protist biology, that are modulated by L. pneumophila, including TLR2 signalling, NF-κB, apoptotic and inflammatory processes, histone modification, caspases, and the NLRC-Naip5 inflammasomes. Importantly, L. pneumophila infects haemocytes of the invertebrate Galleria mellonella, kill G. mellonella larvae, and proliferate in and kill Drosophila adult flies and Caenorhabditis elegans. Although coevolution with protist hosts has provided a substantial blueprint for L. pneumophila to infect macrophages, we discuss the further evolutionary aspects of coevolution of L. pneumophila and its adaptation to modulate various highly evolved innate metazoan processes prior to becoming a human pathogen.

Legionella feeleii: pneumonia or Pontiac fever? Bacterial virulence traits and host immune response

Gram-negative bacterium Legionella is able to proliferate intracellularly in mammalian host cells and amoeba, which became known in 1976 since they caused a large outbreak of pneumonia. It had been reported that different strains of Legionella pneumophila, Legionella micdadei, Legionella longbeachae, and Legionella feeleii caused human respiratory diseases, which were known as Pontiac fever or Legionnaires' disease. However, the differences of the virulence traits among the strains of the single species and the pathogenesis of the two diseases that were due to the bacterial virulence factors had not been well elucidated. L. feeleii is an important pathogenic organism in Legionellae, which attracted attention due to cause an outbreak of Pontiac fever in 1981 in Canada. In published researches, it has been found that L. feeleii serogroup 2 (ATCC 35849, LfLD) possess mono-polar flagellum, and L. feeleii serogroup 1 (ATCC 35072, WRLf) could secrete some exopolysaccharide (EPS) materials to the surrounding. Although the virulence of the L. feeleii strain was evidenced that could be promoted, the EPS might be dispensable for the bacteria that caused Pontiac fever. Based on the current knowledge, we focused on bacterial infection in human and murine host cells, intracellular growth, cytopathogenicity, stimulatory capacity of cytokines secretion, and pathogenic effects of the EPS of L. feeleii in this review.

Colony types and virulence traits of Legionella feeleii determined by exopolysaccharide materials

Legionella feeleii is a Gram-negative pathogenic bacterium that causes Pontiac fever and pneumonia in humans. When L. feeleii serogroup 1 (ATCC 35072) was cultured on BCYE agar plates, two types of colonies were observed and exhibited differences in color, opacity and morphology. Since the two colony types are white rugose and brown translucent, they were termed as white rugose L. feeleii (WRLf) and brown translucent L. feeleii (BTLf), respectively. They exhibited different growth capacities in BYE broth in vitro, and it was found that WRLf could transform to BTLf. Under the electron microscope, it was observed that WRLf secreted materials which could be stained with ruthenium red, which was absent in BTLf. When U937 macrophages and HeLa cells were infected with the bacteria, WRLf manifested stronger internalization ability than BTLf. Intracellular growth in murine macrophages and Acanthamoeba cells was affected by the level of initial phagocytosis. WRLf was more resistant to human serum bactericidal action than BTLf. After being inoculated to guinea pigs, both organisms caused fever in the animals. These results suggest that ruthenium red-stained materials secreted in the surroundings may play a crucial role in determining L. feeleii colony morphology and virulence traits.

Comparative analysis of virulence traits between a Legionella feeleii strain implicated in Pontiac fever and a strain that caused Legionnaires' disease

Legionella strains of the same species and serogroup are known to cause Legionnaires' disease (a potentially fatal atypical pneumonia) or Pontiac fever (a mild, flu-like disease), but the bacterial factors that define these dramatic differences in pathology have not been elucidated. To gain a better understanding of these factors, we compared the characteristics of Legionella feeleii strains that were isolated from either a sample of freshwater implicated in an outbreak of Pontiac fever (ATCC 35072, serogroup 1, LfPF), or a patient with Legionnaires' disease (ATCC 38549, serogroup 2, LfLD). Growth of LfPF and LfLD in BYE broth was slower than the positive control, Legionella pneumophila strain JR32. However, LfLD grew faster than LfPF at 42 °C. After in vitro infection to J774 murine or U937 human macrophage cell lines and A549 human lung epithelial cell line, LfLD showed a higher cell infection rate, stronger internalization by host cells, and greater cytotoxicity than that of LfPF. Large amounts of IL-6 and IL-8 were secreted by human host cells after infection with LfLD, but not with LfPF. LfLD possessed mono-polar flagellum while LfPF was unflagellated. When LfLD was cultured at 25, 30 and 37 °C, the bacteria had higher motility rate at lower temperatures. Based on our results, this is the first study that showed distinct characteristics between LfPF and LfLD, which may give important leads in elucidating differences in their virulence.

Viable but not culturable forms of Legionella pneumophila generated after heat shock treatment are infectious for macrophage-like and alveolar epithelial cells after resuscitation on Acanthamoeba polyphaga

Legionella pneumophila, the causative agent of legionellosis is transmitted to human through aerosols from environmental sources and invades lung's macrophages. It also can invade and replicate within various protozoan species in environmental reservoirs. Following exposures to various stresses, L. pneumophila enters a non-replicative viable but non-culturable (VBNC) state. Here, we evaluated whether VBNC forms of three L. pneumophila serogroup 1 strains (Philadelphia GFP 008, clinical 044 and environmental RNN) infect differentiated macrophage-like cell lines (U937 and HL-60), A549 alveolar cells and Acanthamoeba polyphaga. VBNC forms obtained following shocks at temperatures ranging from 50 to 70 °C for 5 to 60 min were quantified using a flow cytometric assay (FCA). Their loss of culturability was checked on BCYE agar medium. VBNC forms were systematically detected upon a 70 °C heat shock for 30 min. When testing their potential to resuscitate upon amoebal infection, VBNC forms obtained after 30 min at 70 °C were re-cultivated except for the clinical strain. No resuscitation or cell lysis was evidenced when using U937, HL-60, or A549 cells despite the use of various contact times and culture media. None of the strains tested could infect A. polyphaga, macrophage-like or alveolar epithelial cells after a 60-min treatment at 70 °C. However, heat-treated VBNC forms were able to infect macrophage-like or alveolar epithelial cells following their resuscitation on A. polyphaga. These results suggest that heat-generated VBNC forms of L. pneumophila (i) are not infectious for macrophage-like or alveolar epithelial cells in vitro although resuscitation is still possible using amoeba, and (ii) may become infectious for human cell lines following a previous interaction with A. polyphaga.

Alveolar macrophages and neutrophils are the primary reservoirs for Legionella pneumophila and mediate cytosolic surveillance of type IV secretion

Legionella pneumophila, an intracellular pathogen responsible for the severe pneumonia Legionnaires' disease, uses its dot/icm-encoded type IV secretion system (T4SS) to translocate effector proteins that promote its survival and replication into the host cell cytosol. However, by introducing bacterial products into the host cytosol, L. pneumophila also activates cytosolic immunosurveillance pathways, thereby triggering robust proinflammatory responses that mediate the control of infection. Thus, the pulmonary cell types that L. pneumophila infects not only may act as an intracellular niche that facilitates its pathogenesis but also may contribute to the immune response against L. pneumophila. The identity of these host cells remains poorly understood. Here, we developed a strain of L. pneumophila producing a fusion protein consisting of β-lactamase fused to the T4SS-translocated effector RalF, which allowed us to track cells injected by the T4SS. Our data reveal that alveolar macrophages and neutrophils both are the primary recipients of T4SS-translocated effectors and harbor viable L. pneumophila during pulmonary infection of mice. Moreover, both alveolar macrophages and neutrophils from infected mice produced tumor necrosis factor and interleukin-1α in response to T4SS-sufficient, but not T4SS-deficient, L. pneumophila. Collectively, our data suggest that alveolar macrophages and neutrophils are both an intracellular reservoir for L. pneumophila and a source of proinflammatory cytokines that contribute to the host immune response against L. pneumophila during pulmonary infection.

An unclassified microorganism: novel pathogen candidate lurking in human airways

During the assessments of the correlation of the diseases and the microbiota of various clinical specimens, unique 16S ribosomal RNA (rRNA) gene sequences (less than 80% similarity to known bacterial type strains) were predominantly detected in a bronchoalveolar lavage fluid (BALF) specimen from a patient with chronic lower respiratory tract infection. The origin of this unique sequence is suspected to be the causative agent of the infection. We temporarily named the owner organism of this sequence "IOLA" (Infectious Organism Lurking in Airways). In order to evaluate the significance of IOLA in human lung disorders, we performed several experiments. IOLA-16S rRNA genes were detected in 6 of 386 clone libraries constructed from clinical specimens of patients with respiratory diseases (in our study series). The gene sequences (1,427 bp) are identical, and no significantly similar sequence was found in public databases (using NCBI blastn) except for the 8 shorter sequences detected from patients with respiratory diseases in other studies from 2 other countries. Phylogenetic analyses revealed that the 16S rRNA gene of IOLA is more closely related to eukaryotic mitochondria than bacteria. However, the size and shape of IOLA seen by fluorescent in-situ hybridization are similar to small bacteria (approximately 1 µm with a spherical shape). Furthermore, features of both bacteria and mitochondria were observed in the genomic fragment (about 19 kb) of IOLA, and the GC ratio of the sequence was extremely low (20.5%). Two main conclusions were reached: (1) IOLA is a novel bacteria-like microorganism that, interestingly, possesses features of eukaryotic mitochondria. (2) IOLA is a novel pathogen candidate, and it may be the causative agent of human lung or airway disease. IOLA exists in BALF specimens from patients with remarkable symptoms; this information is an important piece for helping solve the elusive etiology of chronic respiratory disorders.

Anti-Legionella dumoffii activity of Galleria mellonella defensin and apolipophorin III

The gram-negative bacterium Legionella dumoffii is, beside Legionella pneumophila, an etiological agent of Legionnaires’ disease, an atypical form of pneumonia. The aim of this study was to determine the antimicrobial activity of Galleria mellonella defense polypeptides against L. dumoffii. The extract of immune hemolymph, containing a mixture of defense peptides and proteins, exhibited a dose-dependent bactericidal effect on L. dumoffii. The bacterium appeared sensitive to a main component of the hemolymph extract, apolipophorin III, as well as to a defense peptide, Galleria defensin, used at the concentrations 0.4 mg/mL and 40 μg/mL, respectively. L. dumoffii cells cultured in the presence of choline were more susceptible to both defense factors analyzed. A transmission electron microscopy study of bacterial cells demonstrated that Galleria defensin and apolipophorin III induced irreversible cell wall damage and strong intracellular alterations, i.e., increased vacuolization, cytoplasm condensation and the appearance of electron-white spaces in electron micrographs. Our findings suggest that insects, such as G. mellonella, with their great diversity of antimicrobial factors, can serve as a rich source of compounds for the testing of Legionella susceptibility to defense-related peptides and proteins.

Draft genome sequences of two Legionella dumoffii strains, TEX-KL and NY-23

Legionella (Fluoribacter) dumoffii is one of the agents causing Legionnaires' disease. Here, we used Illumina second-generation sequencing technology to decipher for the first time the whole-genome sequences of two strains of this species, TEX-KL and NY-23. The assembly results for both strains consist of one chromosome and two plasmids.

Permissivity of Vero cells, human pneumocytes and human endometrial cells to Waddlia chondrophila

Growing evidence suggests that the bacterium Waddlia chondrophila, a novel member of the Chlamydiales order, is an agent of miscarriage in humans and abortion in ruminants. We thus investigated the permissivity of three epithelial cell lines, primate Vero kidney cells, human A549 pneumocytes and human Ishikawa endometrial cells to this strict intracellular bacteria. Bacterial growth kinetics in these cell lines was assessed by quantitative PCR and immunofluorescence and our results demonstrated that W. chondrophila enters and efficiently multiplies in these epithelial cell lines. Additionally, confocal and electron microscopy indicated that the bacteria co-localize with host cell mitochondria. Within Vero and A549 cells, intracellular growth of W. chondrophila was associated with a significant decrease in host cell viability while no such cytophatic effect was detected in Ishikawa cells. Bacterial cell growth in this endometrial cell line stopped 48 h after infection. This stop in the replication of W. chondrophila coincided with the appearance of large aberrant bodies, a form of the bacteria also observed in Chlamydiaceae and associated with persistence. This persistent state of W. chondrophila may explain recurrent episodes of miscarriage in vivo, since the bacteria might reactivate within endometrial cells following hormonal changes that occur during pregnancy.

Replication of Legionella Pneumophila in Human Cells: Why are We Susceptible?

Legionella pneumophila is the causative agent of Legionnaires’ disease, a serious and often fatal form of pneumonia. The susceptibility to L. pneumophila arises from the ability of this intracellular pathogen to multiply in human alveolar macrophages and monocytes. L. pneumophila also replicates in several professional and non-professional phagocytic human-derived cell lines. With the exception of the A/J mouse strain, most mice strains are restrictive, thus they do not support L. pneumophila replication. Mice lacking the NOD-like receptor Nlrc4 or caspase-1 are also susceptible to L. pneumophila. On the other hand, in the susceptible human hosts, L. pneumophila utilizes several strategies to ensure intracellular replication and protect itself against the host immune system. Most of these strategies converge to prevent the fusion of the L. pneumophila phagosome with the lysosome, inhibiting host cell apoptosis, activating survival pathways, and sequestering essential nutrients for replication and pathogenesis. In this review, we summarize survival mechanisms employed by L. pneumophila to maintain its replication in human cells. In addition, we highlight different human-derived cell lines that support the multiplication of this intracellular bacterium. Therefore, these in vitro models can be applicable and are reproducible when investigating L. pneumophila/phagocyte interactions at the molecular and cellular levels in the human host.

Glucose metabolism in Legionella pneumophila: dependence on the Entner-Doudoroff pathway and connection with intracellular bacterial growth

Glucose metabolism in Legionella pneumophila was studied by focusing on the Entner-Doudoroff (ED) pathway with a combined genetic and biochemical approach. The bacterium utilized exogenous glucose for synthesis of acid-insoluble cell components but manifested no discernible increase in the growth rate. Assays with permeabilized cell preparations revealed the activities of three enzymes involved in the pathway, i.e., glucokinase, phosphogluconate dehydratase, and 2-dehydro-3-deoxy-phosphogluconate aldolase, presumed to be encoded by the glk, edd, and eda genes, respectively. Gene-disrupted mutants for the three genes and the ywtG gene encoding a putative sugar transporter were devoid of the ability to metabolize exogenous glucose, indicating that the pathway is almost exclusively responsible for glucose metabolism and that the ywtG gene product is the glucose transporter. It was also established that these four genes formed part of an operon in which the gene order was edd-glk-eda-ywtG, as predicted by genomic information. Intriguingly, while the mutants exhibited no appreciable change in growth characteristics in vitro, they were defective in multiplication within eukaryotic cells, strongly indicating that the ED pathway must be functional for the intracellular growth of the bacterium to occur. Curiously, while the deficient glucose metabolism of the ywtG mutant was successfully complemented by the ywtG(+) gene supplied in trans via plasmid, its defect in intracellular growth was not. However, the latter defect was also manifested in wild-type cells when a plasmid carrying the mutant ywtG gene was introduced. This phenomenon, resembling so-called dominant negativity, awaits further investigation.

Histone acetylation and flagellin are essential for Legionella pneumophila-induced cytokine expression

Legionella pneumophila causes severe pneumonia. Acetylation of histones is thought to be an important regulator of gene transcription, but its impact on L. pneumophila-induced expression of proinflammatory cytokines is unknown. L. pneumophila strain 130b induced the expression of the important chemoattractant IL-8 and genome-wide histone modifications in human lung epithelial A549 cells. We analyzed the IL-8-promoter and found that histone H4 was acetylated and H3 was phosphorylated at Ser(10) and acetylated at Lys(14), followed by transcription factor NF-kappaB. Recruitment of RNA polymerase II to the IL-8 promoter corresponded with increases in gene transcription. Histone modification and IL-8 release were dependent on p38 kinase and NF-kappaB pathways. Legionella-induced IL-8 expression was decreased by histone acetylase (HAT) inhibitor anacardic acid and enhanced by histone deacetylase (HDAC) inhibitor trichostatin A. After Legionella infection, HATs p300 and CREB-binding protein were time-dependently recruited to the IL-8 promoter, whereas HDAC1 and HDAC5 first decreased and later reappeared at the promoter. Legionella specifically induced expression of HDAC5 but not of other HDACs in lung epithelial cells, but knockdown of HDAC1 or 5 did not alter IL-8 release. Furthermore, Legionella-induced cytokine release, promoter-specific histone modifications, and RNA polymerase II recruitment were reduced in infection with flagellin-deletion mutants. Legionella-induced histone modification as well as HAT-/HDAC-dependent IL-8 release could also be shown in primary lung epithelial cells. In summary, histone acetylation seems to be important for the regulation of proinflammatory gene expression in L. pneumophila infected lung epithelial cells. These pathways may contribute to the host response in Legionnaires' disease.

Legionella pneumophila infection induces programmed cell death, caspase activation, and release of high-mobility group box 1 protein in A549 alveolar epithelial cells: inhibition by methyl prednisolone

Background

Legionella pneumophila pneumonia often exacerbates acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Apoptosis of alveolar epithelial cells is considered to play an important role in the pathogenesis of ALI and ARDS. In this study, we investigated the precise mechanism by which A549 alveolar epithelial cells induced by L. pneumophila undergo apoptosis. We also studied the effect of methyl prednisolone on apoptosis in these cells.

Methods

Nuclear deoxyribonucleic acid (DNA) fragmentation and caspase activation in L. pneumophila-infected A549 alveolar epithelial cells were assessed using the terminal deoxyribonucleotidyl transferase-mediated triphosphate (dUTP)-biotin nick end labeling method (TUNEL method) and colorimetric caspase activity assays. The virulent L. pneumophila strain AA100jm and the avirulent dotO mutant were used and compared in this study. In addition, we investigated whether methyl prednisolone has any influence on nuclear DNA fragmentation and caspase activation in A549 alveolar epithelial cells infected with L. pneumophila.

Results

The virulent strain of L. pneumophila grew within A549 alveolar epithelial cells and induced subsequent cell death in a dose-dependent manner. The avirulent strain dotO mutant showed no such effect. The virulent strains of L. pneumophila induced DNA fragmentation (shown by TUNEL staining) and activation of caspases 3, 8, 9, and 1 in A549 cells, while the avirulent strain did not. High-mobility group box 1 (HMGB1) protein was released from A549 cells infected with virulent Legionella. Methyl prednisolone (53.4 μM) did not influence the intracellular growth of L. pneumophila within alveolar epithelial cells, but affected DNA fragmentation and caspase activation of infected A549 cells.

Conclusion

Infection of A549 alveolar epithelial cells with L. pneumophila caused programmed cell death, activation of various caspases, and release of HMGB1. The dot/icm system, a major virulence factor of L. pneumophila, is involved in the effects we measured in alveolar epithelial cells. Methyl prednisolone may modulate the interaction of Legionella and these cells.

Infection of cultured human endothelial cells by Legionella pneumophila

Legionella pneumophila is a gram-negative pathogen that causes a severe pneumonia known as Legionnaires' disease. Here, we demonstrate for the first time that L. pneumophila infects and grows within cultured human endothelial cells. Endothelial infection may contribute to lung damage observed during Legionnaires' disease and to systemic spread of this organism.

Mechanisms of Legionella pneumophila-induced interleukin-8 expression in human lung epithelial cells

BACKGROUND

Legionella pneumophila is a facultative intracellular bacterium, capable of replicating within the phagosomes of macrophages and monocytes, but little is known about its interaction with human lung epithelial cells. We investigated the effect of L. pneumophila on the expression of interleukin-8 (IL-8) in human A549 alveolar and NCI-H292 tracheal epithelial cell lines.

RESULTS

Infection of L. pneumophila strain, but not heat-killed strain, resulted in upregulation of IL-8. IL-8 mRNA expression was induced immediately after the infection and its signal became gradually stronger until 24 h after infection. On the other hand, IL-8 expression in A549 cells infected with L. pneumophila lacking a functional type IV secretion system was transient. The IL-8 expression was slightly induced at 16 h and increased at 24 h after infection with flagellin-deficient Legionella. Activation of the IL-8 promoter by L. pneumophila infection occurred through the action of nuclear factor-kappaB (NF-kappaB). Transfection of dominant negative mutants of NF-kappaB-inducing kinase, IkappaB kinase and IkappaB inhibited L. pneumophila-mediated activation of IL-8 promoter. Treatment with hsp90 inhibitor suppressed L. pneumophila-induced IL-8 mRNA due to deactivation of NF-kappaB.

CONCLUSION

Collectively, these results suggest that L. pneumophila induces activation of NF-kappaB through an intracellular signaling pathway that involves NF-kappaB-inducing kinase and IkappaB kinase, leading to IL-8 gene transcription, and that hsp90 acts as a crucial regulator in L. pneumophila-induced IL-8 expression, presumably contributing to immune response in L. pneumophila. The presence of flagellin and a type IV secretion system are critical for Legionella to induce IL-8 expression in lung epithelial cells.

Complete nucleotide sequence of pLD-TEX-KL, a 66-kb plasmid of Legionella dumoffii TEX-KL strain

The complete nucleotide sequence of a large (66 kb) plasmid pLD-TEX-KL of Legionella dumoffii TEX-KL strain was determined. Of the 57 predicted open reading frames (ORFs), 39 (68%) encoded proteins similar to previously known proteins, five (9%) were assigned with putative functions, three (5%) encoded conserved hypothetical proteins, and 10 (18%) had no homology to any genes present in the current open databases. The ORFs with similar functions were organized in a modular structure; thus, transfer region was identified, as well as a putative heavy-metal ion transporter system (hel). The transfer region encoded homologs of the Salmonella entrica serovar Typhi conjugative system components involved in conjugation. In addition, we also found a potential protein that was analogous to the DNA polymerase III epsilon subunit. It is rarely found that plasmid encode the DNA polymerase.

Update of Legionella infections

Legionella spp. are significant causes of both community-acquired pneumonia and nosocomial pneumonia. More than 40 species of Legionella have now been identified. The spectrum of disease ranges from asymptomatic infection to serious disease, with two specific syndromes identified: Legionnaire's disease and Pontiac fever. Hospital-acquired infection arises from the presence of Legionella in the hospital water supply. The optimal approach for the detection and prevention of nosocomial infection is debatable-whether or not periodic sampling of hospital water systems should be carried out in the absence of clinical cases is controversial. Newer macrolides or newer fluoroquinolone agents are the preferred therapy for serious diseases caused by Legionella.

Legionella pneumophila-induced PKCα-, MAPK-, and NF-κB-dependent COX-2 expression in human lung epithelium

Legionella pneumophila causes community- and hospital-acquired pneumonia. Lung airway and alveolar epithelial cells comprise an important barrier against airborne pathogens. Cyclooxygenase (COX) and microsomal PGE2synthase-1 (mPGES-1)-derived prostaglandins like prostaglandin E2(PGE2) are considered as important regulators of lung function. Herein we tested the hypothesis that L. pneumophila induced COX-2 and mPGES-1-dependent PGE2production in pulmonary epithelial cells. Legionella induced the release of PGE2in primary human small airway epithelial cells and A549 cells. This was accompanied by an increased expression of COX-2 and mPGES-1 as well as an increased PLA2activity in infected cells. Deletion of the type IV secretion system Dot/Icm did not impair Legionella-related COX-2 expression or PGE2release in A549 cells. L. pneumophila induced the degradation of IκBα and activated NF-κB. Inhibition of IKK blocked L. pneumophila-induced PGE2release and COX-2 expression. We noted activation of p38 and p42/44 MAP kinase in Legionella-infected A549 cells. Moreover, membrane translocation and activation of PKCα was observed in infected cells. PKCα and p38 and p42/44 MAP kinase inhibitors reduced PGE2release and COX-2 expression. In summary, PKCα and p38 and p42/44 MAP kinase controlled COX-2 expression and subsequent PGE2release by Legionella-infected lung epithelial cells. These pathways may significantly contribute to the host response in Legionnaires' disease.

Parachlamydia acanthamoebae enters and multiplies within pneumocytes and lung fibroblasts

Parachlamydia acanthamoebae is a Chlamydia-like organism that naturally infects free-living amoebae. P. acanthamoebae is a putative emerging agent of community-acquired and inhalation pneumonia that may enter and multiply within human macrophages. However, since Parachlamydia induces their apoptosis, macrophages may not represent a perennial niche for this obligate intracellular bacterium. Therefore, we investigated whether pneumocytes and lung fibroblasts are permissive to Parachlamydia infection and might act as a replicative niche. Entry of Parachlamydia into pneumocytes (A549) and lung fibroblasts (HEL) was confirmed by confocal and electron microscopy. In A549 cells, the mean number of Parachlamydia per cell increased 7-fold from day 0 to day 7, independently of the technique used to label the bacteria. The proportion of infected A549 cells also increased over time, whereas cell viability remained unaffected by Parachlamydia infection. The sustained (3 weeks) viability of Parachlamydia when incubated in the presence of A549 cells contrasted with that observed in the absence of cells. HEL cells were also permissive to Parachlamydia infection, as we observed a 3- to 4-fold increase in the mean number of bacteria per cell. In HEL cells, Parachlamydia retained some viability for 2 weeks. These findings demonstrate that Parachlamydia is able to enter and multiply within pneumocytes and fibroblasts. The viability of both cell types was not compromised after Parachlamydia infection. We therefore conclude that these cells may remain infected for a prolonged time and may represent an intrapulmonary niche for the strictly intracellular Parachlamydia. This indirectly supports the role of Parachlamydia as an agent of pneumonia.

Expression of IL-6 and TNF-α in human alveolar epithelial cells is induced by invading, but not by adhering, Legionella pneumophila

Legionella pnemophila causes atypical pneumonia in humans, especially in patients with chronic pulmonary diseases and underlying immunosuppression, and in elderly people. Several previous studies have shown that L. pneumophila induced several inflammatory cytokines in murine macrophages, but little is known about cytokine induction by the bacterium in lung epithelial cells. In this study, we investigated the ability of L. pneumophila to stimulate the production of pro-inflammatory cytokines in the human A549 alveolar epithelial cell line during 24h exposure to 10(6), 10(7), and 10(8) microbes. Infection of the wild L. pneumophila strain to A549 resulted in increased levels of interleukin-8 (IL-8), IL-6, and tumor necrosis factor alpha (TNF-alpha) mRNA, and also the secretion of their production into culture medium. In contrast, the level of mRNAs and proteins of IL-1beta and gamma interferon (IFN-gamma) remained unchanged and undetected, respectively. Production of IL-8, IL-6, and TNF-alpha in A549 decreased when an icmE multiplication-less mutant and the heat-killed L. pneumophila strain were inoculated. The treatment of cytochalasin D, which effectively inhibited invasion of L. pneumophila into A549, significantly reduced the production of IL-6 and TNF-alpha, but not IL-8. These results suggested that the induction and expression of IL-6 and TNF-alpha in the human alveolar epithelial cells especially required intracellular signaling by L. pneumophila after invasion.

Legionella dumoffii DjlA, a member of the DnaJ family, is required for intracellular growth

Legionella dumoffii is one of the common causes of Legionnaires' disease and is capable of replicating in macrophages. To understand the mechanism of survival within macrophages, transposon mutagenesis was employed to isolate the genes necessary for intracellular growth. We identified four defective mutants after screening 790 transposon insertion mutants. Two transposon insertions were in genes homologous to icmB or dotC, within dot/icm loci, required for intracellular multiplication of L. pneumophila. The third was in a gene whose product is homologous to the 17-kDa antigen forming part of the VirB/VirD4 type IV secretion system of Bartonella henselae. The fourth was in the djlA (for "dnaj-like A") gene. DjlA is a member of the DnaJ/Hsp40 family. Transcomplementation of the djlA mutant restored the parental phenotype in J774 macrophages, A549 human alveolar epithelial cells, and the amoeba Acanthamoeba culbertsoni. Using confocal laser-scanning microscopy and transmission electron microscopy, we revealed that in contrast to the wild-type strain, L. dumoffii djlA mutant-containing phagosomes were unable to inhibit phagosome-lysosome fusion. Transmission electron microscopy also showed that in contrast to the virulent parental strain, the djlA mutant was not able to recruit host cell rough endoplasmic reticulum. Furthermore, the stationary-phase L. dumoffii djlA mutants were more susceptible to H2O2, high osmolarity, high temperature, and low pH than was their parental strain. These results indicate that DjlA is required for intracellular growth and organelle trafficking, as well as bacterial resistance to environmental stress. This is the first report demonstrating that a single DjlA-deficient mutant exhibits a distinct phenotype.

Morphological variety of intracellular microcolonies of Legionella species in Vero cells

Intracellular microcolonies of six Legionella species growing in Vero cells showed distinctly varied morphologies. The varieties were observed by light microscopy of Gimenez-stained, Legionella-infected Vero cells and by electron microscopy (EM). Legionella pneumophila Philadelphia-1 formed needle-shaped crystal-like microcolonies. Legionella bozemanii WIGA formed microcolonies like wool balls containing filamentous cells. In EM, these organisms proliferated in endosomes, which were adjacent to swollen rough endoplasmic reticula. Legionella oakridgensis OR-10 showed serpentine chains. Many mitochondria were observed around the microcolonies. Legionella jordanis BL-540 formed spherical moss-like microcolonies which were or were not surrounded by endoplasmic membranes. Legionella feeleii WO-44C spread throughout the cytoplasm without making clusters. Legionella dumoffii Tex-KL made big clusters that spread in the cytoplasm, a portion of which was outside the endosome membranes. These different morphologies imply diversity in modes of intracellular multiplication of Legionella spp.

Evaluation of the activity of antimicrobial agents against Legionella pneumophila multiplying in a human monocytic cell line, THP-1, and an alveolar epithelial cell line, A549

The intracellular activity of three antimicrobial agents, erythromycin, clarithromycin, and ciprofloxacin, against Legionella pneumophila was examined in the human monocyte-derived cell line, THP-1, and the human alveolar epithelial cell line, A549. L. pneumophila multiplied by three- to four-log in THP-1 and by two- to three-log in A549 after 48-h incubation. The activity of the two macrolides was markedly greater in A549 than in THP-1, while ciprofloxacin exhibited similar activity in both cell lines. The intracellular concentrations of the two macrolides were markedly higher in A549 than in THP-1, while those of ciprofloxacin were almost equal in both types of cell lines. The intracellular activity of antimicrobial agents and the intracellular growth of L. pneumophila vary with different types of host cells.

Legionella: from environmental habitats to disease pathology, detection and control

Studies on Legionella show a continuum from environment to human disease. Legionellosis is caused by Legionella species acquired from environmental sources, principally water sources such as cooling towers, where Legionella grows intracellularly in protozoa within biofilms. Aquatic biofilms, which are widespread not only in nature, but also in medical and dental devices, are ecological niches in which Legionella survives and proliferates and the ultimate sources to which outbreaks of legionellosis can be traced. Invasion and intracellular replication of L. pneumophila within protozoa in the environment play a major role in the transmission of Legionnaires' disease. Protozoa provide the habitats for the environmental survival and reproduction of Legionella species. L. pneumophila proliferates intracellularly in various species of protozoa within vacuoles studded with ribosomes, as it also does within macrophages. Growth within protozoa enhances the environmental survival capability and the pathogenicity (virulence) of Legionella. The growth requirements of Legionella, the ability of Legionella to enter a viable non-culturable state, the association of Legionella with protozoa and the occurrence of Legionella within biofilms complicates the detection of Legionella and epidemiological investigations of legionellosis. Polymerase chain reaction (PCR) methods have been developed for the molecular detection of Legionella and used in environmental and epidemiological studies. Various physical and chemical disinfection methods have been developed to eliminate Legionella from environmental sources, but gaining control of Legionella in environmental waters, where they are protected from disinfection by growing within protozoa and biofilms, remains a challenge, and one that must be overcome in order to eliminate sporadic outbreaks of legionellosis.

Defense mechanisms against infections

Respiratory infections remain common and may be more difficult to treat because new microbes have been identified; commensal flora can become pathogens; and antibiotic resistance is developing, especially to Streptococcus pneumoniae. Contributing to the severity of pneumonia is the increasing number of more susceptible hosts--immunodeficient patients, created as a by-product of modern medical therapy or from infection with HIV, and elderly patients who have associated illnesses. Better strategies to replete immunocompromised hosts are required. Vaccination to modify infection with S. pneumoniae must be given higher priority by health care providers. New means to immunize the respiratory mucosal surface are needed. Research approaches to consider involve 1) targeting (carrying) relevant antigens to appropriate antigen-processing cells in the airways, eg, dendritic cells; and 2) coupling the immunomodulatory effects of cytokines, eg, interleukin-12. Interleukin-12, for example, can stimulate mucosal antibody responses producing S-IgA and IgG and cause T lymphocytes to produce interferon-gamma that can enhance microbial killing by macrophages.

Legionella

Since the identification of Legionella two decades ago, a vast amount of information has accumulated concerning the microbiology, clinical manifestations, and therapy of infections due to these organisms. There are now more than 40 species of Legionella identified. The spectrum of legionellosis ranges from asymptomatic infection to serious pneumonia. Two clinical syndromes have been identified: Legionnaire's disease and Pontiac fever. Recent information suggests that the newer macrolides and newer fluoroquinolones are preferred therapy for serious disease.

Up-regulation of the expression of major histocompatibility complex class I antigens by plasmid DNA transfection in non-hematopoietic cells

The effect of DNA on the surface expression of major histocompatibility (MHC) class I antigens was examined in non-hematopoietic tumor cell lines. Transfection with plasmid DNA via liposome or electroporation significantly increased the surface expression of MHC class I molecules in a transient manner. Northern blot analysis showed that levels of MHC class I mRNA were increased by DNA transfection, probably via transcriptional activation. In contrast, the expression of the MHC class II and beta-actin genes was not affected, suggesting that the up-regulation of MHC class I expression by plasmid DNA works in a gene-specific manner.