Multiplication of different Legionella species in Mono Mac 6 cells and in Acanthamoeba castellanii

Intracellular Behaviour of Legionella Non-pneumophila Strains within Three Amoeba Strains, Including Willaertia magna C2c Maky

Legionellosis, an often-lethal pneumonia, is generally associated with contamination by Legionella pneumophila. This bacterium can persist in the environment and resist chemical treatment when it is internalized by amoebae. In addition, there is increasing medical proof that other Legionella species can be causative agents of Legionellosis. The objective of this study was to evaluate whether Legionella non-pneumophila (Lnp) strains were able to use the machinery of amoeba to multiply, or whether amoebae were able to limit their proliferation. Seven strains belonging to the species L. longbeachae, L. anisa, L. bozemanae, L. taurinensis, and L. dumoffii were cocultured with three amoebae, Acanthamoeba castellanii, Willaertia magna T5(S)44, and Willaertia magna C2c Maky, at two temperatures, 22 and 37 °C. We found that at 22 °C, all amoebae were able to phagocytose the seven Lnp strains and to avoid intracellular development, except for L. longbeachae, which was able to multiply inside W. magna T5(S)44. At 37 °C, four Lnp strains were able to hijack the machinery of one or two amoebae and to use it to proliferate, but none were able to multiply inside W. magna C2c Maky.

Are pathogenic Legionella non-pneumophila a common bacteria in Water Distribution Networks?

The present study analyzes at the national level, the presence of circulating Legionella in the artificial aquatic systems of different facilities of all of them state-owned centers throughout Spain for 12 months. 1754 water samples from various state-owned centers were collected from January to December 2014. Samples were collected from the cooling towers and evaporative condensers (CTC), and water distribution networks such as domestic hot water (DHW), cold water for human consumption (CW), sprinkler irrigation systems (SIS), fire sprinkler systems (FSS), and water from decorative fountains (DF). All these facilities are considered, according to current regulations, as potential amplifying systems for bacteria and possible sources of infection by the generation of droplets and aerosols. The isolation and counting of Legionella in water samples was carried out using microbiological culture following the international normative UNE-EN-ISO 11,731:2007 (ISO 11,731:1998) and UNE-EN ISO 8199:2008 (ISO 8199:2005).The quantification of Legionella colonization, the annual distribution, and the geographical distribution of the Legionella isolates recovered in the water were analyzed. Besides, molecular techniques were used for the characterization of the Legionella non-pneumophila isolates. Legionella was recovered from 15.79% of the analyzed water samples. High colonization was more frequently detected in water samples from CTC, DHW, CW, and DF. Regarding the geographic distribution, positive samples of Legionella were obtained in 14 of the 18 Spanish locations analyzed. Legionella non-pneumophila was the most prevalent and was isolated from water samples from 13 different geographical locations (72%). Legionella anisa and Legionella jordanis were the most frequently non-pneumophila species isolated. Legionella donaldsonii was isolated for the first time in the water distribution networks in Spain. Legionella pneumophila sg 2-14 was detected in 13 locations and Legionella pneumophila sg 1 in 11 locations. Therefore, our study concludes that the presence of Legionella pneumophila and Legionella non-pneumophila species in these systems can be a potential threat to public health and should be examined thoroughly with complementary techniques, such as molecular techniques as a screen for routine diagnosis.

Development of a Fluorescent Tool for Studying Legionella bozemanae Intracellular Infection

Legionnaires' disease incidence is on the rise, with the majority of cases attributed to the intracellular pathogen, Legionella pneumophila. Nominally a parasite of protozoa, L. pneumophila can also infect alveolar macrophages when bacteria-laden aerosols enter the lungs of immunocompromised individuals. L. pneumophila pathogenesis has been well characterized; however, little is known about the >25 different Legionella spp. that can cause disease in humans. Here, we report for the first time a study demonstrating the intracellular infection of an L. bozemanae clinical isolate using approaches previously established for L. pneumophila investigations. Specifically, we report on the modification and use of a green fluorescent protein (GFP)-expressing plasmid as a tool to monitor the L. bozemanae presence in the Acanthamoeba castellanii protozoan infection model. As comparative controls, L. pneumophila strains were also transformed with the GFP-expressing plasmid. In vitro and in vivo growth kinetics of the Legionella parental and GFP-expressing strains were conducted followed by confocal microscopy. Results suggest that the metabolic burden imposed by GFP expression did not impact cell viability, as growth kinetics were similar between the GFP-expressing Legionella spp. and their parental strains. This study demonstrates that the use of a GFP-expressing plasmid can serve as a viable approach for investigating Legionella non-pneumophila spp. in real time.

Isolation and identification of Legionella spp. from hot spring water in Algeria by culture and molecular methods

AIMS

Due to infectious risk associated with the presence of Legionella in warm water, we determined the prevalence of living Legionella spp. in hot spring water in Algeria.

METHODS AND RESULTS

Detection of Legionella by culture was done by using two methods, direct culture on agar plates and co-culture with amoeba. Fifty samples were taken from different hot springs in northern Algeria, including swimming pools, showers and thermal sources. Legionella pneumophila serotypes were predominant, accounting for 60% of positive samples. Direct method allowed the isolation of 13 L. pneumophila only of 50 samples (26%), whereas co-culture using a panel of three free living amoeba allowed the isolation of 119 Legionella species from the same samples (80%) CONCLUSIONS: Amoeba co-culture allowed the isolation of several Legionella sp., while direct culture allowed the isolation of L. pneumophila only. Remarkably, Legionella longbeachae, usually isolated from soil and compost, was isolated for the first time in thermal water in three samples using Vermamoeba vermiformis co-culture.

SIGNIFICANCE AND IMPACT OF THE STUDY

The presence of Legionella in the water of hot springs in Algeria, which are mainly frequented by individuals at risk of Legionellosis, requires urgent control measures.

Investigations on Contamination of Environmental Water Samples by Legionella using Real-Time Quantitative PCR Combined with Amoebic Co-Culturing

We analyzed the contamination of environmental water samples with Legionella spp. using a conventional culture method, real-time quantitative PCR (qPCR), and real-time qPCR combined with an amoebic co-culture method. Samples (n = 110) were collected from 19 cooling towers, 31 amenity water facilities, and 60 river water sources of tap water in Japan. Legionella was detected in only three samples (3/110, 2.7%) using the culture method. The rate of Legionella detection using amoebic co-culture followed by qPCR was 74.5%, while that using qPCR without amoebic co-culture was 75.5%. A higher than 10-fold bacterial count was observed in 19 samples (19/110, 17.3%) using real-time qPCR subsequent to amoebic co-culture, compared with identical samples analyzed without co-culture. Of these 19 samples, 13 were identified as Legionella spp., including L. pneumophila and L. anisa, and the non-culturable species were identified as L. lytica and L. rowbothamii. This study showed that the detection of Legionella spp., even in those samples where they were not detected by the culture method, was possible using real-time qPCR and an amoebic co-culture method. In addition, this analytical test combination is a useful tool to detect viable and virulent Legionella spp..

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.

Adaptation of Amoeba Plate Test To Recover Legionella Strains from Clinical Samples

The isolation of Legionella from respiratory samples is the gold standard for diagnosis of Legionnaires' disease (LD) and enables epidemiological studies and outbreak investigations. The purpose of this work was to adapt and to evaluate the performance of an amoebic coculture procedure (the amoeba plate test [APT]) for the recovery of Legionella strains from respiratory samples, in comparison with axenic culture and liquid-based amoebic coculture (LAC). Axenic culture, LAC, and APT were prospectively performed with 133 respiratory samples from patients with LD. The sensitivities and times to results for the three techniques were compared. Using the three techniques, Legionella strains were isolated in 46.6% (n = 62) of the 133 respiratory samples. The sensitivity of axenic culture was 42.9% (n = 57), that of LAC was 30.1% (n = 40), and that of APT was 36.1% (n = 48). Seven samples were positive by axenic culture only; for those samples, there were <10 colonies in total. Five samples, all sputum samples, were positive by an amoebic procedure only (5/5 samples by APT and 2/5 samples by LAC); all had overgrowth by oropharyngeal flora with axenic culture. The combination of axenic culture with APT yielded a maximal isolation rate (i.e., 46.6%). Overall, the APT significantly reduced the median time for Legionella identification to 4 days, compared with 7 days for LAC (P < 0.0001). The results of this study support the substitution of LAC by APT, which could be implemented as a second-line technique for culture-negative samples and samples with microbial overgrowth, especially sputum samples. The findings provide a logical basis for further studies in both clinical and environmental settings.

From Many Hosts, One Accidental Pathogen: The Diverse Protozoan Hosts of Legionella

The 1976 outbreak of Legionnaires' disease led to the discovery of the intracellular bacterial pathogen Legionella pneumophila. Given their impact on human health, Legionella species and the mechanisms responsible for their replication within host cells are often studied in alveolar macrophages, the primary human cell type associated with disease. Despite the potential severity of individual cases of disease, Legionella are not spread from person-to-person. Thus, from the pathogen's perspective, interactions with human cells are accidents of time and space—evolutionary dead ends with no impact on Legionella's long-term survival or pathogenic trajectory. To understand Legionella as a pathogen is to understand its interaction with its natural hosts: the polyphyletic protozoa, a group of unicellular eukaryotes with a staggering amount of evolutionary diversity. While much remains to be understood about these enigmatic hosts, we summarize the current state of knowledge concerning Legionella's natural host range, the diversity of Legionella-protozoa interactions, the factors influencing these interactions, the importance of avoiding the generalization of protozoan-bacterial interactions based on a limited number of model hosts and the central role of protozoa to the biology, evolution, and persistence of Legionella in the environment.

The Flagellar Regulon of Legionella-A Review

The Legionella genus comprises more than 60 species. In particular, Legionella pneumophila is known to cause severe illnesses in humans. Legionellaceae are ubiquitous inhabitants of aquatic environments. Some Legionellaceae are motile and their motility is important to move around in habitats. Motility can be considered as a potential virulence factor as already shown for various human pathogens. The genes of the flagellar system, regulator and structural genes, are structured in hierarchical levels described as the flagellar regulon. Their expression is modulated by various environmental factors. For L. pneumophila it was shown that the expression of genes of the flagellar regulon is modulated by the actual growth phase and temperature. Especially, flagellated Legionella are known to express genes during the transmissive phase of growth that are involved in the expression of virulence traits. It has been demonstrated that the alternative sigma-28 factor is part of the link between virulence expression and motility. In the following review, the structure of the flagellar regulon of L. pneumophila is discussed and compared to other flagellar systems of different Legionella species. Recently, it has been described that Legionella micdadei and Legionella fallonii contain a second putative partial flagellar system. Hence, the report will focus on flagellated and non-flagellated Legionella strains, phylogenetic relationships, the role and function of the alternative sigma factor (FliA) and its anti-sigma-28 factor (FlgM).

Resistance of Legionella and Acanthamoeba mauritaniensis to heat treatment as determined by relative and quantitative polymerase chain reactions

Legionella and Acanthamoeba spp. persist in harvested rainwater pasteurized at high temperatures (> 72°C) and the interaction mechanisms exhibited between these organisms need to be elucidated. The resistance of two Legionella reference strains (Legionella pneumophila ATCC 33152 and Legionella longbeachae ATCC 33462), three environmental strains [Legionella longbeachae (env.), Legionella norrlandica (env.) and Legionella rowbothamii (env.)] and Acanthamoeba mauritaniensis ATCC 50676 to heat treatment (50-90°C) was determined by monitoring culturability and viability [ethidium monoazide quantitative polymerase chain reaction (EMA-qPCR)]. The expression of metabolic and virulence genes of L. pneumophila ATCC 33152 (lolA, sidF, csrA) and L. longbeachae (env.) (lolA) in co-culture with A. mauritaniensis ATCC 50676 during heat treatment (50-90°C) was monitored using relative qPCR. While the culturability (CFU/mL) and viability (gene copies/mL) of the Legionella strains reduced significantly (p < 0.05) following heat treatment (60-90°C), L. longbeachae (env.) and L. pneumophila ATCC 33152 were culturable following heat treatment at 50-60°C. Metabolically active trophozoites and dormant cysts of A. mauritaniensis ATCC 50676 were detected at 50°C and 60-90°C, respectively. For L. pneumophila ATCC 33152, lolA expression remained constant, sidF expression increased and the expression of csrA decreased during co-culture with A. mauritaniensis ATCC 50676. For L. longbeachae (env.), while lolA was up-regulated at 50-70°C, expression was not detected at 80-90°C and in co-culture. In conclusion, while heat treatment may reduce the number of viable Legionella spp. in monoculture, results indicate that the presence of A. mauritaniensis increases the virulence of L. pneumophila during heat treatment. The virulence of Legionella spp. in co-culture with Acanthamoeba spp. should thus be monitored in water distribution systems where temperature (heat) is utilized for treatment.

Functional Analysis of the Alternative Sigma-28 Factor FliA and Its Anti-Sigma Factor FlgM of the Nonflagellated Legionella Species L. oakridgensis

Legionella oakridgensis causes Legionnaires' disease but is known to be less virulent than Legionella pneumophilaL. oakridgensis is one of the Legionella species that is nonflagellated. The genes of the flagellar regulon are absent, except those encoding the alternative sigma-28 factor (FliA) and its anti-sigma-28 factor (FlgM). Similar to L. oakridgensis, Legionella adelaidensis and Legionella londiniensis, located in the same phylogenetic clade, have no flagellar regulon, although both are positive for fliA and flgM Here, we investigated the role and function of both genes to better understand the role of FliA, the positive regulator of flagellin expression, in nonflagellated strains. We demonstrated that the FliA gene of L. oakridgensis encodes a functional sigma-28 factor that enables the transcription start from the sigma-28-dependent promoter site. The investigations have shown that FliA is necessary for full fitness of L. oakridgensis Interestingly, expression of FliA-dependent genes depends on the growth phase and temperature, as already shown for L. pneumophila strains that are flagellated. In addition, we demonstrated that FlgM is a negative regulator of FliA-dependent gene expression. FlgM seems to be degraded in a growth-phase- and temperature-dependent manner, instead of being exported into the medium as reported for most bacteria. The degradation of FlgM leads to an increase of FliA activity.IMPORTANCE A less virulent Legionella species, L. oakridgensis, causes Legionnaires' disease and is known to not have flagella, even though L. oakridgensis has the regulator of flagellin expression (FliA). This protein has been shown to be involved in the expression of virulence factors. Thus, the strain was chosen for use in this investigation to search for FliA target genes and to identify putative virulence factors of L. oakridgensis One of the five major target genes of FliA identified here encodes the anti-FliA sigma factor FlgM. Interestingly, in contrast to most homologs in other bacteria, FlgM in L. oakridgensis seems not to be transported from the cell so that FliA gets activated. In L. oakridgensis, FlgM seems to be degraded by protease activities.

Environmental Legionella spp. collected in urban test sites of South East Queensland, Australia, are virulent to human macrophages in vitro

Legionellae are frequent contaminants of potable water supplies, resulting in sporadic infections and occasional outbreaks. Isolates of Legionella were collected from urban test sites within South East Queensland and evaluated for their virulence potential in vitro. Two strains (from the species Legionella londiniensis and Legionella quinlivanii) were demonstrated to have the ability to infect human macrophages, while a strain from the species Legionella anisa did not maintain an infection over the same time course. This suggests that the spectrum of urban environmentally associated Legionella with potential to cause human disease might be greater than currently considered.

Investigation of Legionella Contamination in Bath Water Samples by Culture, Amoebic Co-Culture, and Real-Time Quantitative PCR Methods

We investigated Legionella contamination in bath water samples, collected from 68 bathing facilities in Japan, by culture, culture with amoebic co-culture, real-time quantitative PCR (qPCR), and real-time qPCR with amoebic co-culture. Using the conventional culture method, Legionella pneumophila was detected in 11 samples (11/68, 16.2%). Contrary to our expectation, the culture method with the amoebic co-culture technique did not increase the detection rate of Legionella (4/68, 5.9%). In contrast, a combination of the amoebic co-culture technique followed by qPCR successfully increased the detection rate (57/68, 83.8%) compared with real-time qPCR alone (46/68, 67.6%). Using real-time qPCR after culture with amoebic co-culture, more than 10-fold higher bacterial numbers were observed in 30 samples (30/68, 44.1%) compared with the same samples without co-culture. On the other hand, higher bacterial numbers were not observed after propagation by amoebae in 32 samples (32/68, 47.1%). Legionella was not detected in the remaining six samples (6/68, 8.8%), irrespective of the method. These results suggest that application of the amoebic co-culture technique prior to real-time qPCR may be useful for the sensitive detection of Legionella from bath water samples. Furthermore, a combination of amoebic co-culture and real-time qPCR might be useful to detect viable and virulent Legionella because their ability to invade and multiply within free-living amoebae is considered to correlate with their pathogenicity for humans. This is the first report evaluating the efficacy of the amoebic co-culture technique for detecting Legionella in bath water samples.

Legionella oakridgensis ATCC 33761 genome sequence and phenotypic characterization reveals its replication capacity in amoebae

Legionella oakridgensis is able to cause Legionnaires' disease, but is less virulent compared to L. pneumophila strains and very rarely associated with human disease. L. oakridgensis is the only species of the family legionellae which is able to grow on media without additional cysteine. In contrast to earlier publications, we found that L. oakridgensis is able to multiply in amoebae. We sequenced the genome of L. oakridgensis type strain OR-10 (ATCC 33761). The genome is smaller than the other yet sequenced Legionella genomes and has a higher G+C-content of 40.9%. L. oakridgensis lacks a flagellum and it also lacks all genes of the flagellar regulon except of the alternative sigma-28 factor FliA and the anti-sigma-28 factor FlgM. Genes encoding structural components of type I, type II, type IV Lvh and type IV Dot/Icm, Sec- and Tat-secretion systems could be identified. Only a limited set of Dot/Icm effector proteins have been recognized within the genome sequence of L. oakridgensis. Like in L. pneumophila strains, various proteins with eukaryotic motifs and eukaryote-like proteins were detected. We could demonstrate that the Dot/Icm system is essential for intracellular replication of L. oakridgensis. Furthermore, we identified new putative virulence factors of Legionella.

Regulation, integrase-dependent excision, and horizontal transfer of genomic islands in Legionella pneumophila

Legionella pneumophila is a Gram-negative freshwater agent which multiplies in specialized nutrient-rich vacuoles of amoebae. When replicating in human alveolar macrophages, Legionella can cause Legionnaires' disease. Recently, we identified a new type of conjugation/type IVA secretion system (T4ASS) in L. pneumophila Corby (named trb-tra). Analogous versions of trb-tra are localized on the genomic islands Trb-1 and Trb-2. Both can exist as an episomal circular form, and Trb-1 can be transferred horizontally to other Legionella strains by conjugation. In our current work, we discovered the importance of a site-specific integrase (Int-1, lpc2818) for the excision and conjugation process of Trb-1. Furthermore, we identified the genes lvrRABC (lpc2813 to lpc2816) to be involved in the regulation of Trb-1 excision. In addition, we demonstrated for the first time that a Legionella genomic island (LGI) of L. pneumophila Corby (LpcGI-2) encodes a functional type IV secretion system. The island can be transferred horizontally by conjugation and is integrated site specifically into the genome of the transconjugants. LpcGI-2 generates three different episomal forms. The predominant episomal form, form A, is generated integrase dependently (Lpc1833) and transferred by conjugation in a pilT-dependent manner. Therefore, the genomic islands Trb-1 and LpcGI-2 should be classified as integrative and conjugative elements (ICEs). Coculture studies of L. pneumophila wild-type and mutant strains revealed that the int-1 and lvrRABC genes (located on Trb-1) as well as lpc1833 and pilT (located on LpcGI-2) do not influence the in vivo fitness of L. pneumophila in Acanthamoeba castellanii.

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.

Lateral gene exchanges shape the genomes of amoeba-resisting microorganisms

Based on Darwin's concept of the tree of life, vertical inheritance was thought to be dominant, and mutations, deletions, and duplication were streaming the genomes of living organisms. In the current genomic era, increasing data indicated that both vertical and lateral gene inheritance interact in space and time to trigger genome evolution, particularly among microorganisms sharing a given ecological niche. As a paradigm to their diversity and their survival in a variety of cell types, intracellular microorganisms, and notably intracellular bacteria, were considered as less prone to lateral genetic exchanges. Such specialized microorganisms generally have a smaller gene repertoire because they do rely on their host's factors for some basic regulatory and metabolic functions. Here we review events of lateral gene transfer (LGT) that illustrate the genetic exchanges among intra-amoebal microorganisms or between the microorganism and its amoebal host. We tentatively investigate the functions of laterally transferred genes in the light of the interaction with their host as they should confer a selective advantage and success to the amoeba-resisting microorganisms (ARMs).

Isolation of Legionella pneumophila from pluvial floods by amoebal coculture

Viable Legionella pneumophila bacteria were isolated by amoebal coculture from pluvial floods after intense rainfall and from water collected at sewage treatment plants. Several isolated L. pneumophila strains belonged to sequence types that have been previously identified in patients.

Interaction of Acinetobacter baumannii 19606 and 1656-2 with Acanthamoeba castellanii

Acinetobacter baumannii is virtually avirulent for healthy people but maintains a high virulence among critically ill patients or immuno-compromised individuals. The ability of A. baumannii to adhere to cells and persist on surfaces as biofilms could be central to its pathogenicity. In the present study, we compared the virulence of the A. baumannii 1656-2 clinical strain, which is able to form a thick biofilm, with the virulence of the A. baumannii type strain (ATCC 19606(T)). Acanthamoeba castellanii, a single-celled organism, was used as the host model system to study the virulence of A. baumannii. Compared to A. baumannii ATCC 19606(T), A. baumannii 1656-2 exhibited a higher ability to adhere and invade A. castellanii cells and had a higher killing rate of A. castellanii cells. Furthermore, co-incubation of the amoeba cells and the cell-free supernatant of A. baumannii resulted in the cell death of the amoebae. Heat inactivation or proteinase K treatment of the supernatant did not eliminate its cytotoxicity, suggesting heat stable non-protein factors are responsible for its cytotoxicity to A. castellanii cells. In conclusion, this study for the first time has revealed the capacity of the A. baumannii strain and/or its metabolic products to induce cytotoxicity in A. castellanii cells.

Increased persistence of Salmonella enterica serovar Typhi in the presence of Acanthamoeba castellanii

Salmonella enterica serovar Typhi (S. Typhi) is the etiological agent of the systemic disease typhoid fever. Transmission occurs via ingestion of contaminated food or water. S. Typhi is specific to humans, and no animal or environmental reservoirs are known. As the free-living amoeba Acanthamoeba castellanii is an environmental host for many pathogenic bacteria, this study investigates interactions between S. Typhi and A. castellanii by using cocultures. Growth of both organisms was estimated by cell count, viable count, flow cytometry, and fluorescence microscopy. Results indicate that S. Typhi can survive at least 3 weeks when grown with A. castellanii, as opposed to less than 10 days when grown as singly cultured bacteria under the same conditions. Interestingly, growth rates of amoebae after 14 days were similar in cocultures or when amoebae were singly cultured, suggesting that S. Typhi is not cytotoxic to A. castellanii. Bacteria surviving in coculture were not intracellular and did not require a physical contact with amoebae for their survival. These results suggest that S. Typhi may have a selective advantage when it is associated with A. castellanii and that amoebae may contribute to S. Typhi persistence in the environment.

Legionella longbeachae and legionellosis

Reported cases of legionellosis attributable to Legionella longbeachae infection have increased worldwide. In Australia and New Zealand, L. longbeachae has been a known cause of legionellosis since the late 1980s. All cases for which a source was confirmed were associated with potting mixes and composts. Unlike the situation with other Legionella spp., L. longbeachae-contaminated water systems in the built environment that cause disease have not been reported. Spatially and temporally linked outbreaks of legionellosis associated with this organism also have not been reported. Sporadic cases of disease seem to be limited to persons who have had direct contact with potting soil or compost. Long-distance travel of the organism resulting in infection has not been reported. These factors indicate emergence of an agent of legionellosis that differs in etiology from other species and possibly in route of disease transmission.

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.

Stability of Chlamydophila pneumoniae in a harsh environment without a requirement for acanthamoebae

The effect of actual interactions between Chlamydophila pneumoniae and amoebae (Acanthamoeba) on the survival of C. pneumoniae was investigated. C. pneumoniae and amoebae were detected in 75 soil samples by IFU assay and PCR. Although C. pneumoniae could not be cultured, the DNA prevalence of C. pneumoniae and amoebae in natural soil was 20% and 92% (no correlation between the prevalence of DNA was observed). The viability of C. pneumoniae spiked in autoclaved soil was assessed by IFU assay and RT-PCR. Although the number of infective progeny decreased for three days, transcripts of C. pneumoniae were detected for up to 98 days independently of amoebae. The stability of C. pneumoniae in liquid medium was also assessed by IFU assay and transmission electron microscopy. The bacteria could survive at 15 degrees C for 14 days independently of amoebae. Bacteria cultured without amoebae were confirmed to have normal structures. Thus, the presence of amoebae has no effect on C. pneumoniae survival, and the bacteria can survive in the absence of host cells for an extended period of time.

Amoebal pathogens as emerging causal agents of pneumonia

Despite using modern microbiological diagnostic approaches, the aetiological agents of pneumonia remain unidentified in about 50% of cases. Some bacteria that grow poorly or not at all in axenic media used in routine clinical bacteriology laboratory but which can develop inside amoebae may be the agents of these lower respiratory tract infections (RTIs) of unexplained aetiology. Such amoebae-resisting bacteria, which coevolved with amoebae to resist their microbicidal machinery, may have developed virulence traits that help them survive within human macrophages, i.e. the first line of innate immune defence in the lung. We review here the current evidence for the emerging pathogenic role of various amoebae-resisting microorganisms as agents of RTIs in humans. Specifically, we discuss the emerging pathogenic roles of Legionella-like amoebal pathogens, novel Chlamydiae (Parachlamydia acanthamoebae, Simkania negevensis), waterborne mycobacteria and Bradyrhizobiaceae (Bosea and Afipia spp.).

Experimental Legionella longbeachae infection in intratracheally inoculated mice

This study established an experimental model of replicative Legionella longbeachae infection in A/J mice. The animals were infected by intratracheal inoculation of 10(3)-10(9) c.f.u. L. longbeachae serogroup 1 (USA clinical isolates D4968, D4969 and D4973). The inocula of 10(9), 10(8), 10(7) and 10(6) c.f.u. of all tested L. longbeachae serogroup 1 isolates were lethal for A/J mice. Inoculation of 10(5) c.f.u. L. longbeachae caused death in 90 % of the animals within 5 days, whilst inoculation of 10(4) c.f.u. caused sporadic death of mice. All animals that received 10(3) c.f.u. bacteria developed acute lower respiratory disease, but were able to clear Legionella from the lungs within 3 weeks. The kinetics of bacterial growth in the lungs was independent of inoculum size and reached a growth peak about 3 logarithms above the initial inoculum at 72 h after inoculation. The most prominent histological changes in the lungs were observed at 48-72 h after inoculation in the form of a focal, neutrophil-dominant, peribronchiolar infiltration. The inflammatory process did not progress towards the interstitial or alveolar spaces. Immunohistological analyses revealed L. longbeachae serogroup 1 during the early phase of infection near the bronchiolar epithelia and later co-localized with inflammatory cells. BALB/c and C57BL/6 mice strains were also susceptible to infection with all L. longbeachae serogroup 1 strains tested and very similar changes were observed in the lungs of infected animals. These results underline the infection potential of L. longbeachae serogroup 1, which is associated with high morbidity and lethality in mice.

The role of biofilms and protozoa in Legionella pathogenesis: implications for drinking water

Current models to study Legionella pathogenesis include the use of primary macrophages and monocyte cell lines, various free-living protozoan species and murine models of pneumonia. However, there are very few studies of Legionella spp. pathogenesis aimed at associating the role of biofilm colonization and parasitization of biofilm microbiota and release of virulent bacterial cell/vacuoles in drinking water distribution systems. Moreover, the implications of these environmental niches for drinking water exposure to pathogenic legionellae are poorly understood. This review summarizes the known mechanisms of Legionella spp. proliferation within Acanthamoeba and mammalian cells and advocates the use of the amoeba model to study Legionella pathogenicity because of their close association with Legionella spp. in the aquatic environment. The putative role of biofilms and amoebae in the proliferation, development and dissemination of potentially pathogenic Legionella spp. is also discussed. Elucidating the mechanisms of Legionella pathogenicity development in our drinking water systems will aid in elimination strategies and procedural designs for drinking water systems and in controlling exposure to Legionella spp. and similar pathogens.

Acanthamoeba castellanii an environmental host for Shigella dysenteriae and Shigella sonnei

The interaction between Shigella dysenteriae or Shigella sonnei and Acanthamoeba castellanii was studied by viable counts, gentamicin assay and electron microscopy. The result showed that Shigella dysenteriae or Shigella sonnei grew and survived in the presence of amoebae for more than 3 weeks. Gentamicin assay showed that the Shigella were viable inside the Acanthamoeba castellanii which was confirmed by electron microscopy that showed the Shigella localized in the cytoplasm of the Acanthamoeba castellanii. In conclusion, the relationship between Shigella dysenteriae and Shigella sonnei with Acanthamoeba castellanii is symbiotic, and accordingly free-living amoebae may serve as a transmission reservoir for Shigella in water.

Cytopathogenicity and molecular subtyping ofLegionella pneumophilaenvironmental isolates from 17 hospitals

The cytopathogenicity of 22Legionella pneumophilaisolates from 17 hospitals was determined by assessing the dose of bacteria necessary to produce 50% cytopathic effect (CPED50) in U937 human-derived macrophages. All isolates were able to infect and grow in macrophage-like cells (range log10CPED50: 2·67–6·73 c.f.u./ml). Five groups were established and related to the serogroup, the number of PFGE patterns coexisting in the same hospital water distribution system, and the possible reporting of hospital-acquired Legionnaires' disease cases.L. pneumophilaserogroup 1 isolates had the highest cytopathogenicity (P=0·003). Moreover, a trend to more cytopathogenic groups (groups 1–3) in hospitals with more than one PFGE pattern ofL. pneumophilain the water distribution system (60%vs. 17%) and in hospitals reporting cases of hospital-acquired Legionnaires' disease (36·3%vs. 16·6%) was observed. We conclude that the cytopathogenicty of environmentalL. pneumophilashould be taken into account in evaluating the risk of a contaminated water reservoir in a hospital and hospital acquisition of Legionnaires' disease.

Early trafficking and intracellular replication of Legionella longbeachaea within an ER-derived late endosome-like phagosome

Legionella pneumophila is the predominant cause of Legionnaires' disease in the USA and Europe in contrast to Legionella longbeachaea, which is the leading cause of the disease in Western Australia. The ability of L. pneumophila to replicate intracellularly is triggered at the post-exponential phase along with expression of other virulence traits, such as motility. We show that while motility of L. longbeachaea is triggered upon growth transition into post-exponential phase, its ability to proliferate intracellularly is totally independent of the bacterial growth phase. Within macrophages, L. pneumophila replicates in a phagosome that excludes early and late endocytic markers and is surrounded by the rough endoplasmic reticulum (RER). In contrast, the L. longbeachaea phagosome colocalizes with the early endosomal marker early endosomal antigen 1 (EEA1) and the late endosomal markers lysosomal associated membrane glycoprotein 2 (LAMP-2) and mannose 6-phosphate receptor (M6PR), and is surrounded by the RER. The L. longbeachaea phagosome does not colocalize with the vacuolar ATPase (vATPase) proton pump, and the lysosomal luminal protease Cathepsin D, or the lysosomal tracer Texas red Ovalbumin (TROV). Intracellular proliferation of L. longbeachaea occurs in LAMP-2-positive phagosomes that are remodelled by the RER. Despite their distinct trafficking, both L. longbeachaea and L. pneumophila can replicate in communal phagosomes whose biogenesis is predominantly modulated by L. longbeachaea into LAMP-2-positive phagosomes. In addition, the L. pneumophila dotA mutant is rescued for intracellular replication if it co-inhabits the phagosome with L. longbeachaea. During late stages of infection, L. longbeachaea escape into the cytoplasm, prior to lysis of the macrophage, similar to L. pneumophila. We conclude that the L. longbeachaea phagosome matures to a non-acidified late endosome-like stage that is remodelled by the RER, indicating an idiosyncratic trafficking of L. longbeachaea compared with other intracellular pathogens, and a divergence in its intracellular lifestyle from L. pneumophila. In addition, re-routing biogenesis of the L. pneumophila phagosome into a late endosome controlled by L. longbeachaea has no effect on intracellular replication.

Acanthamoeba: could it be an environmental host of Shigella?

Shigellosis is a serious public health problem in Korea, because large outbreaks of Shigella sonnei infections were recorded in many parts of the country during the period 1998-2000. However, the epidemiological features of shigellosis are not well known. In this study, we devised conditions suitable for the growth and replication of Shigella in an amoebic intracellular environment, and investigate whether medium conditions affect the survival and replication of Shigella within Acanthamoeba. We evaluated the uptake rates of invasive and non invasive S. sonnei strains by three Acanthamoeba species, namely, A. castellanii Neff, A. astronyxis Ray & Hayes, and A. healyi OC-3A. When A. castellanii Neff was infected with S. sonnei 99OBS1 or 80DH248, shigellae was maintained for a longer time in cytoplasms than in other Acanthamoeba species. S. sonnei 99OBS1 strain (a virulent strain) was recovered in higher numbers than the non-virulent S. sonnei 80DH248 strain in all experiments. Moreover, S. sonnei was more easily engulfed by Acanthamoeba at 18 degrees C. The shigellae uptake rates of Neff strain, which was cultured in free-media (less nutrition), were higher (>10-fold) than those observed in original amoeba culture media (PYG medium) in all time points. S. sonnei 99OBS1 was localized, with an intact membrane, to the vacuoles of Acanthamoeba. We conclude that free-living amoebae more likely act as environmental hosts for shigellae, and thus, may have contributed to outbreaks of shigellosis in Korea.

Efficient intracellular multiplication of Legionella pneumophila in human monocytes requires functional host cell L-type calcium channels

The infectious agent of Legionnaires' disease, Legionella pneumophila, multiplies intracellularly in a variety of eukaryotic cells. Genistein, a tyrosine kinase inhibitor, has been shown to block intracellular replication of L. pneumophila without harming the infected host cell. The present study has been performed to investigate the underlying mechanism. We demonstrate that inhibition of intracellular bacterial growth by genistein is not mediated by its protein tyrosine kinase-modulating effect but by inhibition of L-type calcium channels of the infected host cell.

Effective proliferation of low level Legionella pneumophila serogroup 1 cells using coculture procedure with Acanthamoeba castellanii

The multiplications of low level Legionella pneumophila serogroup 1 cells by the coculture procedure with Acanthamoeba castellanii were tested in five strains. The cells in all strains proliferated effectively for isolating. This procedure might be a useful means of improving the successful isolation from environmental and clinical specimens of low level Legionella cells, and pursuing the source of infection.

Identification of Legionella pneumophila-specific genes by genomic subtractive hybridization with Legionella micdadei and identification of lpnE, a gene required for efficient host cell entry

Legionella pneumophila is a ubiquitous environmental organism and a facultative intracellular pathogen of humans. To identify genes that may contribute to the virulence of L. pneumophila, we performed genomic subtractive hybridization between L. pneumophila serogroup 1 strain 02/41 and L. micdadei strain 02/42. A total of 144 L. pneumophila-specific clones were sequenced, revealing 151 genes that were absent in L. micdadei strain 02/42. Low-stringency Southern hybridization was used to determine the distribution of 41 sequences, representing 40 open reading frames (ORFs) with a range of putative functions among L. pneumophila isolates of various serogroups as well as strains of Legionella longbeachae, L. micdadei, Legionella gormanii, and Legionella jordanis. Twelve predicted ORFs were L. pneumophila specific, including the gene encoding the dot/icm effector, lepB, as well as several genes predicted to play a role in lipopolysaccharide biosynthesis and cell wall synthesis and several sequences with similarity to virulence-associated determinants. A further nine predicted ORFs were in all L. pneumophila serotypes tested and an isolate of L. gormanii. These included icmD, the 5' end of a pilMNOPQ locus, and two genes known to be upregulated during growth within macrophages, cadA2 and ceaA. Disruption of an L. pneumophila-specific gene (lpg2222 locus tag) encoding a putative protein with eight tetratricopeptide repeats resulted in reduced entry into the macrophage-like cell line, THP-1, and the type II alveolar epithelial cell line, A549. The gene was subsequently renamed lpnE, for "L. pneumophila entry." In summary, this investigation has revealed important genetic differences between L. pneumophila and other Legionella species that may contribute to the phenotypic and clinical differences observed within this genus.

Legionella contamination in hot water of Italian hotels

A cross-sectional multicenter survey of Italian hotels was conducted to investigate Legionella spp. contamination of hot water. Chemical parameters (hardness, free chlorine concentration, and trace element concentrations), water systems, and building characteristics were evaluated to study risk factors for colonization. The hot water systems of Italian hotels were strongly colonized by Legionella; 75% of the buildings examined and 60% of the water samples were contaminated, mainly at levels of > or =10(3) CFU liter(-1), and Legionella pneumophila was the most frequently isolated species (87%). L. pneumophila serogroup 1 was isolated from 45.8% of the contaminated sites and from 32.5% of the hotels examined. When a multivariate logistic model was used, only hotel age was associated with contamination, but the risk factors differed depending on the contaminating species and serogroup. Soft water with higher chlorine levels and higher temperatures were associated with L. pneumophila serogroup 1 colonization, whereas the opposite was observed for serogroups 2 to 14. In conclusion, Italian hotels, particularly those located in old buildings, represent a major source of risk for Legionnaires' disease due to the high frequency of Legionella contamination, high germ concentration, and major L. pneumophila serogroup 1 colonization. The possible role of chlorine in favoring the survival of Legionella species is discussed.

Intracellular multiplication of Legionella pneumophila depends on host cell amino acid transporter SLC1A5

The infectious agent of Legionnaires' disease, Legionella (L) pneumophila, multiplies intracellularly in eukaryotic cells. This study has been performed to explore the nutrient requirements of L. pneumophila during intracellular replication. In human monocytes, bacterial replication rate was reduced by 76% in defined medium lacking L-cysteine, L-glutamine or L-serine. SLC1A5 (hATB(0,+)), a neutral amino acid transporter, was upregulated in the host cells after infection with L. pneumophila. Inhibition of SLC1A5 by BCH, a competitive inhibitor of amino acid uptake as well as siRNA silencing of the slc1a5 gene blocked intracellular multiplication of L. pneumophila without compromising viability of host cells. These observations suggest that replication of L. pneumophila depends on the function of host cell SLC1A5.

Pathogenic free-living amoebae in Korea

Acanthamoeba and Naegleria are widely distributed in fresh water, soil and dust throughout the world, and cause meningoencephalitis or keratoconjunctivitis in humans and other mammals. Korean isolates, namely, Naegleria sp. YM-1 and Acanthamoeba sp. YM-2, YM-3, YM-4, YM-5, YM-6 and YM-7, were collected from sewage, water puddles, a storage reservoir, the gills of a fresh water fish, and by corneal washing. These isolates were categorized into three groups based on the mortalities of infected mice namely, highly virulent (YM-4), moderately virulent (YM-2, YM-5 and YM-7) and nonpathogenic (YM-3). In addition, a new species of Acanthamoeba was isolated from a freshwater fish in Korea and tentatively named Korean isolate YM-4. The morphologic characters of its cysts were similar to those of A. culbertsoni and A. royreba, which were previously designated as Acanthamoeba group III. Based on experimentally infected mouse mortality, Acanthamoeba YM-4 was highly virulent. The isoenzymes profile of Acanthamoeba YM-4 was similar to that of A. royreba. Moreover, an anti-Acanthamoeba YM-4 monoclonal antibody reacted only with Acanthamoeba YM-4, and not with A. culbertsoni. Random amplified polymorphic DNA marker analysis and RFLP analysis of mitochondrial DNA and of a 18S small subunit ribosomal RNA, placed Acanthamoeba YM-4 in a separate cluster based on phylogenic distances. Thus Acanthamoeba YM-4 was identified as a new species, and assigned Acanthamoeba sohi. Up to the year 2002 in Korea, two clinical cases were found to be infected with Acanthamoeba spp. These patients died of meningoencephalitis. In addition, one case of Acanthamoeba pneumonia with an immunodeficient status was reported and Acanthamoeba was detected in several cases of chronic relapsing corneal ulcer, chronic conjunctivitis, and keratitis.

Uptake and replication of Salmonella enterica in Acanthamoeba rhysodes

The ability of salmonellae to become internalized and to survive and replicate in amoebae was evaluated by using three separate serovars of Salmonella enterica and five different isolates of axenic Acanthamoeba spp. In gentamicin protection assays, Salmonella enterica serovar Dublin was internalized more efficiently than Salmonella enterica serovar Enteritidis or Salmonella enterica serovar Typhimurium in all of the amoeba isolates tested. The bacteria appeared to be most efficiently internalized by Acanthamoeba rhysodes. Variations in bacterial growth conditions affected internalization efficiency, but this effect was not altered by inactivation of hilA, a key regulator in the expression of the invasion-associated Salmonella pathogenicity island 1. Microscopy of infected A. rhysodes revealed that S. enterica resided within vacuoles. Prolonged incubation resulted in a loss of intracellular bacteria associated with morphological changes and loss of amoebae. In part, these alterations were associated with hilA and the Salmonella virulence plasmid. The data show that Acanthamoeba spp. can differentiate between different serovars of salmonellae and that internalization is associated with cytotoxic effects mediated by defined Salmonella virulence loci.

Comparative assessment of virulence traits in Legionella spp

Legionella pneumophila is a facultative intracellular pathogen that accounts for the majority of cases of Legionnaires' disease in the USA and Europe, but other Legionella spp. have been shown to cause disease. In contrast, Legionella longbeachae is the leading cause of Legionnaires' disease in Australia. The hallmark of Legionnaires' disease caused by L. pneumophila is the intracellular replication within phagocytes in the alveolar spaces, and the Dot/Icm type IV secretion system is essential for intracellular replication. Although it has been presumed that intracellular replication within phagocytes is the hallmark of other virulent legionellae, the virulence traits of Legionella spp. apart from L. pneumophila are not well defined. In this study, 27 strains of Legionella spp. belonging to 16 species that have been isolated from humans or from the environment were examined for five virulence traits exhibited by L. pneumophila: cytopathogenicity, intracellular replication within macrophages, induction of apoptosis/DNA fragmentation, pore-formation-mediated cytolysis of the host cell, and the presence of the dot/icm loci. The strains were divided into two broad groups (low and high cytopathogenic groups) based on cytopathogenicity assays using U937 human-derived macrophages. The other four virulence traits were evaluated in the low and high cytopathogenic groups of Legionella species. Most L. pneumophila serogroup 1 strains were highly cytopathogenic after 72 h, manifested high levels of intracellular growth, induced apoptosis/DNA fragmentation, and exhibited pore-forming activity. The majority of the other species were the low cytopathogenic group that did not induce apoptosis, neither did they exhibit pore-forming activity. All the species of legionellae tested have all the dot/icm loci, when examined by DNA hybridization. No correlation was found between cytopathogenicity and the other four pathogenic traits.

Investigation of mechanisms involved in phagocytosis of Legionella pneumophila by human cells

Legionella pneumophila, the causative agent of Legionnaires' disease, is able to survive and multiply efficiently in a variety of mammalian cells. By using in vitro assays, the uptake of L. pneumophila into monocytes has shown to be mediated, at least in part, through attachment of complement-coated bacteria to complement receptors, but complement-independent phagocytosis could also be demonstrated. Since complement levels in the human lung are normally low, the role of complement-dependent phagocytosis in the pathogenesis of Legionnaires' disease is doubtful. However, the contribution of other potential phagocytosis-related host cell surface molecules to the phagocytosis of L. pneumophila has never been investigated. We therefore analyzed the role of complement receptors 1 (CD35) and 3 (CD11b/18), the lipopolysaccharide (LPS) receptor (CD14), the beta(1)-integrin chain of the fibronectin receptor (CD29), the intercellular adhesion molecule 1 (ICAM-1, CD54) and the transferrin receptor (CD71) in the complement-independent uptake of L. pneumophila. To exclude any influence of culture conditions onto phagocytosis rates, we compared a fresh clinical isolate with an agar-adapted isolate of L. pneumophila. In addition, we used three different host cell types (MM6, HeLa and Jurkat cells) expressing different rates of complement receptors. We could show that both strains of L. pneumophila were phagocytized by the three host cell lines to the same extent, but intracellular multiplication was only found in MM6 and, although to a much lesser degree, in Jurkat cells. Preincubation of MM6 cells with monoclonal antibodies directed against the above cited phagocytosis-related receptors did not result in inhibition of L. pneumophila uptake. We therefore conclude that typical phagocytosis-related cell surface receptors are not involved in the complement-independent phagocytosis of L. pneumophila.

Survival and growth of Francisella tularensis in Acanthamoeba castellanii

Francisella tularensis is a highly infectious, facultative intracellular bacterium which causes epidemics of tularemia in both humans and mammals at regular intervals. The natural reservoir of the bacterium is largely unknown, although it has been speculated that protozoa may harbor it. To test this hypothesis, Acanthamoeba castellanii was cocultured with a strain of F. tularensis engineered to produce green fluorescent protein (GFP) in a nutrient-rich medium. GFP fluorescence within A. castellanii was then monitored by flow cytometry and fluorescence microscopy. In addition, extracellular bacteria were distinguished from intracellular bacteria by targeting with monoclonal antibodies. Electron microscopy was used to determine the intracellular location of F. tularensis in A. castellanii, and viable counts were obtained for both extracellular and intracellular bacteria. The results showed that many F. tularensis cells were located intracellularly in A. castellanii cells. The bacteria multiplied within intracellular vacuoles and eventually killed many of the host cells. F. tularensis was found in intact trophozoites, excreted vesicles, and cysts. Furthermore, F. tularensis grew faster in cocultures with A. castellanii than it did when grown alone in the same medium. This increase in growth was accompanied by a decrease in the number of A. castellanii cells. The interaction between F. tularensis and amoebae demonstrated in this study indicates that ubiquitous protozoa might be an important environmental reservoir for F. tularensis.

Legionnaires' disease at a Dutch flower show: prognostic factors and impact of therapy

After a large outbreak of Legionnaires' disease in the Netherlands, we determined risk factors for intensive care unit (ICU) admission and death and the impact of adequate therapy on ICU-free survival among 141 hospitalized patients. Overall mortality rate was 13%, and ICU mortality rate was 36%. Smoking, temperature >38.5 degrees C, and bilateral infiltrates shown on chest x-ray were independent risk factors for ICU admission or death (all p<0.05). Starting adequate therapy within 24 hours after admission resulted in a higher ICU-free survival rate compared to therapy initiation after 24 hours: 78% versus 54%, respectively (p=0.005). However, delay in providing therapy to patients with urinary antigen tests with negative results did not influence outcome. These data suggest that by using the urinary antigen test on admission a more tailored approach to patients with community-acquired pneumonia may be applied.

Infection due to Legionella species other than L. pneumophila

In addition to Legionella pneumophila, 19 Legionella species have been documented as human pathogens on the basis of their isolation from clinical material. Like L. pneumophila, other Legionella species are inhabitants of natural and man-made aqueous environments. The major clinical manifestation of infection due to Legionella species is pneumonia, although nonpneumonic legionellosis (Pontiac fever) and extrapulmonary infection may occur. The majority of confirmed infections involving non-pneumophila Legionella species have occurred in immunosuppressed patients. Definitive diagnosis requires culture on selective media. Fluoroquinolones and newer macrolides are effective therapy. A number of nosocomial cases have occurred in association with colonization of hospital water systems; elimination of Legionella species from such systems prevents their transmission to susceptible patients. It is likely that many cases of both community-acquired and nosocomial Legionella infection remain undiagnosed. Application of appropriate culture methodology to the etiologic diagnosis of pneumonia is needed to further define the role of these organisms in disease in humans.

Regulation of the Legionella mip-promotor during infection of human monocytes

The opportunistic pathogen Legionella pneumophila, the etiologic agent of Legionnaires disease, is able to invade and multiply intracellularly in human macrophages. This process is controlled by several bacterial virulence factors. As recently demonstrated, one of these virulence factors, the macrophage infectivity potentiator (Mip) protein, is important for invasion and proper intracellular establishment of L. pneumophila in macrophages and protozoa. Knockout mutants devoid of a functional mip-gene enter host cells much less effectively but intracellular replication is not affected. Using a P(mip)-green fluorescent protein reporter construct in L. pneumophila substrain Corby, P(mip) was recently shown to be constitutively active in replicating bacteria. A stringent regulation during the infection process could not be observed, neither in intracellular nor in BYE broth-grown bacteria. For enhanced temporal and quantitative resolution, we examined the activity of mip on RNA level in order to detect short transient regulatory events. Our results show that P(mip) of L. pneumophila is temporarily repressed directly after invasion of the monocytic human cell line MonoMac 6 and regains activity after 24 h of intracellular replication.

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.

In vitro secretion kinetics of proteins from Legionella pneumophila in comparison to proteins from non-pneumophila species

It has been shown that the loss of PilD, a prepilin peptidase necessary for type IV pilus biogenesis and establishment of the type II secretion apparatus is associated with loss of virulence in Legionella pneumophila. L. pneumophila is the species most frequently associated with Legionnaires' disease, but virulence factors unique to this species are not known, so the secretion kinetics of several pilD-dependent enzyme activities, including protease, acid phosphatase, phospholipase A (PLA) and lysophospholipase A (LPLA), of L. pneumophila and non-pneumophila species were compared during growth in BYE broth. Enzyme activity appeared during mid-exponential growth phase and reached maximal levels on entry into stationary growth phase. None of the enzyme activities were unique to L. pneumophila and it did not exclusively secrete the highest amounts of the hydrolytic proteins. However, the timing of PLA and LPLA secretion in L. pneumophila differed compared to other species. PLA activity was secreted prior to LPLA activity in L. pneumophila, which may lead to an accumulation of the cytotoxic agent lysophosphatidylcholine (LPC). In addition to L. pneumophila, several other Legionella species, including Legionella steigerwaltii and Legionella gormanii, were able to enrich for LPC due to a very potent PLA activity accompanied by only moderate LPLA activity. These species, in contrast to L. pneumophila, have not been shown to multiply within monocytic host cells. Thus none of the secreted enzymic activities investigated were unique to L. pneumophila, nor were they secreted at high concentrations. However, the timing of PLA and LPLA secretion may contribute to pathogenicity.

Comparison of virulence of Legionella longbeachae strains in guinea pigs and U937 macrophage-like cells

A guinea pig model of experimental legionellosis was established for assessment of virulence of isolates of Legionella longbeachae. The results showed that there were distinct virulence groupings of L. longbeachae serogroup 1 strains based on the severity of disease produced in this model. Statistical analysis of the animal model data suggests that Australian isolates of L. longbeachae may be inherently more virulent than non-Australian strains. Infection studies performed with U937 cells were consistent with the animal model studies and showed that isolates of this species were capable of multiplying within these phagocytic cells. Electron microscopy studies of infected lung tissue were also undertaken to determine the intracellular nature of L. longbeachae serogroup 1 infection. The data showed that phagosomes containing virulent L. longbeachae serogroup 1 appeared bloated, contained cellular debris and had an apparent rim of ribosomes while those containing avirulent L. longbeachae serogroup 1 were compact, clear and smooth.

Induction of iNOS in human monocytes infected with different Legionella species

The contribution of nitric oxide (NO) radicals to the suppression of intracellular replication of Legionella has been well established in rodents but remained questionable in humans. Considering the fact that human monocytes do not exhibit a high-output NO production, we used sensitive methods such as detection of inducible NO synthase (iNOS) mRNA by reverse transcription-PCR and demonstration of iNOS protein expression by means of flow cytometry and Western blot to compare the levels of iNOS induced by Legionella species which, in accordance to their human prevalence, show different multiplication rates within human monocytic cells. The expression of iNOS in Mono Mac 6 (MM6) cells showed an only moderate inverse correlation to the intracellular replication rate of a given Legionella species in the protein expression assays. However, stimulation of host cells with 1,25-dihydroxyvitamin D(3) to enhance NO production and inhibition of NO production by treatment of host cells with N(G)-methyl-L-arginine were not able to modify the intracellular multiplication of legionellae within MM6 cells. Therefore, NO production does not seem to play a crucial role for the restriction of intracellular replication of Legionella bacteria within human monocytic cells. Rodent models in investigations which are supposed to clarify the involvement of NO radicals in defense mechanisms against Legionella infections in humans are of doubtful significance.

Localization of Legionella bacteria within ribosome-studded phagosomes is not restricted to Legionella pneumophila

In this report, we investigate the intracellular fate of selected members of the genus Legionella within the monocytic cell line Mono Mac 6 cells. By means of electron microscopy and immunocytochemistry, we could show that Legionella pneumophila as well as Legionella longbeachae are able to induce ribosome-studded phagosomes which associate with the rough endoplasmic reticulum (RER), whereas Legionella micdadei remains to be located within smooth phagosomes but also shows signs of RER association. In addition, we could demonstrate a remarkable correlation between the phagosome type and the morphological phenotype of intracellular bacteria: within ribosome-studded phagosomes, bacteria generally lacked the outer coat of low electron density whereas bacteria within the smooth phagosomes still possessed this outer coat. The virulence factors responsible for inhibition of phagosome maturation and their distribution within the genus Legionella as well as the biological significance of the morphological difference of bacteria within smooth and ER-associated phagosomes remain to be investigated.