Identification of mip-like genes in the genus Legionella

Molecular regulation of virulence in Legionella pneumophila

Legionella pneumophila is a gram-negative bacteria found in natural and anthropogenic aquatic environments such as evaporative cooling towers, where it reproduces as an intracellular parasite of cohabiting protozoa. If L. pneumophila is aerosolized and inhaled by a susceptible person, bacteria may colonize their alveolar macrophages causing the opportunistic pneumonia Legionnaires' disease. L. pneumophila utilizes an elaborate regulatory network to control virulence processes such as the Dot/Icm Type IV secretion system and effector repertoire, responding to changing nutritional cues as their host becomes depleted. The bacteria subsequently differentiate to a transmissive state that can survive in the environment until a replacement host is encountered and colonized. In this review, we discuss the lifecycle of L. pneumophila and the molecular regulatory network that senses nutritional depletion via the stringent response, a link to stationary phase-like metabolic changes via alternative sigma factors, and two-component systems that are homologous to stress sensors in other pathogens, to regulate differentiation between the intracellular replicative phase and more transmissible states. Together, we highlight how this prototypic intracellular pathogen offers enormous potential in understanding how molecular mechanisms enable intracellular parasitism and pathogenicity.

Legionella pneumophila PPIase Mip Interacts with the Bacterial Proteins SspB, Lpc2061, and FlaA and Promotes Flagellation

The peptidyl-prolyl- cis/trans -isomerase (PPIase) macrophage infectivity potentiator (Mip) contributes to the pathogenicity and fitness of L. pneumophila , the causative agent of Legionnaires’ disease. Here, we identified the stringent starvation protein SspB, hypothetical protein Lpc2061, and flagellin FlaA as bacterial interaction partners of Mip.

Legionellosis Caused by Non-Legionella pneumophila Species, with a Focus on Legionella longbeachae

Although known as causes of community-acquired pneumonia and Pontiac fever, the global burden of infection caused by Legionella species other than Legionella pneumophila is under-recognised. Non-L. pneumophila legionellae have a worldwide distribution, although common testing strategies for legionellosis favour detection of L. pneumophila over other Legionella species, leading to an inherent diagnostic bias and under-detection of cases. When systematically tested for in Australia and New Zealand, L. longbeachae was shown to be a leading cause of community-acquired pneumonia. Exposure to potting soils and compost is a particular risk for infection from L. longbeachae, and L. longbeachae may be better adapted to soil and composting plant material than other Legionella species. It is possible that the high rate of L. longbeachae reported in Australia and New Zealand is related to the composition of commercial potting soils which, unlike European products, contain pine bark and sawdust. Genetic studies have demonstrated that the Legionella genomes are highly plastic, with areas of the chromosome showing high levels of recombination as well as horizontal gene transfer both within and between species via plasmids. This, combined with various secretion systems and extensive effector repertoires that enable the bacterium to hijack host cell functions and resources, is instrumental in shaping its pathogenesis, survival and growth. Prevention of legionellosis is hampered by surveillance systems that are compromised by ascertainment bias, which limits commitment to an effective public health response. Current prevention strategies in Australia and New Zealand are directed at individual gardeners who use potting soils and compost. This consists of advice to avoid aerosols generated by the use of potting soils and use masks and gloves, but there is little evidence that this is effective. There is a need to better understand the epidemiology of L. longbeachae and other Legionella species in order to develop effective treatment and preventative strategies globally.

Structure and functional analysis of the Legionella pneumophila chitinase ChiA reveals a novel mechanism of metal-dependent mucin degradation

Chitinases are important enzymes that contribute to the generation of carbon and nitrogen from chitin, a long chain polymer of N-acetylglucosamine that is abundant in insects, fungi, invertebrates and fish. Although mammals do not produce chitin, chitinases have been identified in bacteria that are key virulence factors in severe respiratory, gastrointestinal and urinary diseases. However, it is unclear how these enzymes are able to carry out this dual function. Legionella pneumophila is the causative agent of Legionnaires' disease, an often-fatal pneumonia and its chitinase ChiA is essential for the survival of L. pneumophila in the lung. Here we report the first atomic resolution insight into the pathogenic mechanism of a bacterial chitinase. We derive an experimental model of intact ChiA and show how its N-terminal region targets ChiA to the bacterial surface after its secretion. We provide the first evidence that L. pneumophila can bind mucins on its surface, but this is not dependent on ChiA. This demonstrates that additional peripheral mucin binding proteins are also expressed in L. pneumophila. We also show that the ChiA C-terminal chitinase domain has novel Zn2+-dependent peptidase activity against mammalian mucin-like proteins, namely MUC5AC and the C1-esterase inhibitor, and that ChiA promotes bacterial penetration of mucin gels. Our findings suggest that ChiA can facilitate passage of L. pneumophila through the alveolar mucosa, can modulate the host complement system and that ChiA may be a promising target for vaccine development.

Isolation and Identification of Legionella spp. in environmental water sources based on macrophage infectivity potentiator (mip) gene sequencing in southwest Iran

Legionella species are widespread in natural water sources and man-made aqueous environments, as well as fresh-water. The present study was conducted owing to the lack of research regarding the prevalence of Legionella spp in the water sources of Ahvaz city in southwest Iran. In this study the macrophage infectivity potentiator (mip) gene sequencing was used for identification of various Legionella species isolated from different water sources. In this study, 144 water samples were collected and inoculated on the buffered charcoal-yeast extract (BCYE) agar and modified Wadowsky-Yee (MWY) medium. The DNA was extracted from positive cultures. The Legionella species were confirmed by amplifying a 654 bp fragment of the 16S rRNA gene. The mip gene of all isolates were amplified by PCR and purified for sequencing. The mip gene sequences were analyzed by jPHYDIT software version 1. The results showed a 13.9% (20/144) prevalence of Legionella spp. in water sources of Ahvaz city, southwest Iran. Analyzing of the mip gene sequences showed, out of 20 Legionella isolates, 13 isolates (54.1%) were positive for L. pneumophila, 5 isolates (20.8%) were positive for L. worsleinsis, one isolates for each one of L. dumoffi and L. fairfieldensis, (4.1%). According to our research, the occurrence of Legionella spp in water sources could be a hazard for the health systems especially in the hospitals. The regular monitoring of these water sources by health planners may therefore be useful for decreasing the risk for Legionella spp. infections.

Quantification of viable and non-viable Legionella spp. by heterogeneous asymmetric recombinase polymerase amplification (haRPA) on a flow-based chemiluminescence microarray

Increasing numbers of legionellosis outbreaks within the last years have shown that Legionella are a growing challenge for public health. Molecular biological detection methods capable of rapidly identifying viable Legionella are important for the control of engineered water systems. The current gold standard based on culture methods takes up to 10 days to show positive results. For this reason, a flow-based chemiluminescence (CL) DNA microarray was developed that is able to quantify viable and non-viable Legionella spp. as well as Legionella pneumophila in one hour. An isothermal heterogeneous asymmetric recombinase polymerase amplification (haRPA) was carried out on flow-based CL DNA microarrays. Detection limits of 87 genomic units (GU) µL^-1 and 26GUµL^-1 for Legionella spp. and Legionella pneumophila, respectively, were achieved. In this work, it was shown for the first time that the combination of a propidium monoazide (PMA) treatment with haRPA, the so-called viability haRPA, is able to identify viable Legionella on DNA microarrays. Different proportions of viable and non-viable Legionella, shown with the example of L. pneumophila, ranging in a total concentration between 10¹ to 10⁵GUµL^-1 were analyzed on the microarray analysis platform MCR 3. Recovery values for viable Legionella spp. were found between 81% and 133%. With the combination of these two methods, there is a chance to replace culture-based methods in the future for the monitoring of engineered water systems like condensation recooling plants.

Rapid detection and typing of pathogenic nonpneumophila Legionella spp. isolates using a multiplex real-time PCR assay

We developed a single tube multiplex real-time PCR assay that allows for the rapid detection and typing of 9 nonpneumophila Legionella spp. isolates that are clinically relevant. The multiplex assay is capable of simultaneously detecting and discriminating L. micdadei, L. bozemanii, L. dumoffii, L. longbeachae, L. feeleii, L. anisa, L. parisiensis, L. tucsonensis serogroup (sg) 1 and 3, and L. sainthelensis sg 1 and 2 isolates. Evaluation of the assay with nucleic acid from each of these species derived from both clinical and environmental isolates and typing strains demonstrated 100% sensitivity and 100% specificity when tested against 43 other Legionella spp. Typing of L. anisa, L. parisiensis, and L. tucsonensis sg 1 and 3 isolates was accomplished by developing a real-time PCR assay followed by high-resolution melt (HRM) analysis targeting the ssrA gene. Further typing of L. bozemanii, L. longbeachae, and L. feeleii isolates to the serogroup level was accomplished by developing a real-time PCR assay followed by HRM analysis targeting the mip gene. When used in conjunction with other currently available diagnostic tests, these assays may aid in rapidly identifying specific etiologies associated with Legionella outbreaks, clusters, sporadic cases, and potential environmental sources.

Novel Cycloheximide Derivatives Targeting the Moonlighting Protein Mip Exhibit Specific Antimicrobial Activity Against Legionella pneumophila

Macrophage infectivity potentiator (Mip) and Mip-like proteins are virulence factors in a wide range of pathogens including Legionella pneumophila. These proteins belong to the FK506 binding protein (FKBP) family of peptidyl-prolyl-cis/trans-isomerases (PPIases). In L. pneumophila, the PPIase activity of Mip is required for invasion of macrophages, transmigration through an in vitro lung–epithelial barrier, and full virulence in the guinea pig infection model. Additionally, Mip is a moonlighting protein that binds to collagen IV in the extracellular matrix. Here, we describe the development and synthesis of cycloheximide derivatives with adamantyl moieties as novel FKBP ligands, and analyze their effect on the viability of L. pneumophila and other bacteria. All compounds efficiently inhibited PPIase activity of the prototypic human FKBP12 as well as Mip with IC₅₀-values as low as 180 nM and 1.7 μM, respectively. Five of these derivatives inhibited the growth of L. pneumophila at concentrations of 30–40 μM, but exhibited no effect on other tested bacterial species indicating a specific spectrum of antibacterial activity. The derivatives carrying a 3,5-dimethyladamantan-1-[yl]acetamide substitution (MT_30.32), and a 3-ethyladamantan-1-[yl]acetamide substitution (MT_30.51) had the strongest effects in PPIase- and liquid growth assays. MT_30.32 and MT_30.51 were also inhibitory in macrophage infection studies without being cytotoxic. Accordingly, by applying a combinatorial approach, we were able to generate novel, hybrid inhibitors consisting of cycloheximide and adamantane, two known FKBP inhibitors that interact with different parts of the PPIase domain, respectively. Interestingly, despite the proven Mip-inhibitory activity, the viability of a Mip-deficient strain was affected to the same degree as its wild type. Hence, we also propose that cycloheximide derivatives with adamantyl moieties are potent PPIase inhibitors with multiple targets in L. pneumophila.

Pathogen-free screening of bacteria-specific hybridomas for selecting high-quality monoclonal antibodies against pathogen bacteria as illustrated for Legionella pneumophila

Antibodies are potent biological tools increasingly used as detection, diagnostic and therapeutic reagents. Many technological advances have optimized and facilitated production and screening of monoclonal antibodies. We report here an original method to screen for antibodies targeting biosafety level 2 or 3 pathogens without the fastidious handling inherent to pathogen use. A double ELISA screening was performed using as coated antigen transformed Escherichia coli expressing at its surface a protein specific to the pathogenic bacteria versus control untransformed E. coli. This method was applied to Legionella, using the surface-exposed Mip protein (macrophage infectivity potentiator). This screening proved to be an excellent means of selecting mAbs that bind Legionella pneumophila 1 surface-exposed Mip protein. This method also appears more biologically relevant than screening using the recombinant Mip protein alone and less tedious than a test performed directly on Legionella bacteria. We obtained 21 mAbs that bind strongly to L. pneumophila serogroups 1 to 13, and we validated their use in a rapid ELISA (performed in 4.5 h) and an immunochromatographic test (20 min).

Clinical application of a multiplex real-time PCR assay for simultaneous detection of Legionella species, Legionella pneumophila, and Legionella pneumophila serogroup 1

We developed a single-tube multiplex real-time PCR assay capable of simultaneously detecting and discriminating Legionella spp., Legionella pneumophila, and Legionella pneumophila serogroup 1 in primary specimens. Evaluation of 21 clinical specimens and 115 clinical isolates demonstrated this assay to be a rapid, high-throughput diagnostic test with 100% specificity that may aid during legionellosis outbreaks and epidemiologic investigations.

The major facilitator superfamily-type protein LbtC promotes the utilization of the legiobactin siderophore by Legionella pneumophila

The Gram-negative bacterium Legionella pneumophila elaborates the siderophore legiobactin. We previously showed that cytoplasmic LbtA helps mediate legiobactin synthesis, inner-membrane LbtB promotes export of legiobactin, and outer-membrane LbtU acts as the ferrisiderophore receptor. RT-PCR analyses now identified lbtC as an iron-repressed gene that is the final gene in an operon containing lbtA and lbtB. In silico analysis predicted that LbtC is an inner-membrane protein that belongs to the major facilitator superfamily (MFS). Although capable of normal growth in standard media, lbtC mutants were defective for growth on iron-depleted agar media. While producing normal levels of legiobactin, lbtC mutants were unable to utilize supplied legiobactin to stimulate growth on iron-depleted media and displayed an impaired ability to take up radiolabelled iron. All lbtC mutant phenotypes were complemented by reintroduction of an intact copy of lbtC. When a cloned copy of both lbtC and lbtU was introduced into a heterologous bacterium (Legionella longbeachae), the organism acquired the ability to utilize legiobactin to grow better on low-iron media. Together, these data indicate that LbtC is involved in the uptake of legiobactin, and based upon its predicted location is most likely the mediator of ferrilegiobactin transport across the inner membrane. The data are also a unique documentation of how an MFS protein can promote bacterial iron-siderophore import, standing in contrast to the vast majority of studies which have defined ABC-type permeases as the mediators of siderophore import across the Gram-negative inner membrane or the Gram-positive cytoplasmic membrane.

Molecular Detection of Legionella: Moving on From mip

The detection of Legionella pneumophila in environmental and clinical samples is frequently performed by PCR amplification of the mip and/or 16S rRNA genes. Combined with DNA sequencing, these two genetic loci can be used to distinguish different species of Legionella and identify L. pneumophila. However, the recent Legionella genome sequences have opened up hundreds of possibilities for the development of new molecular targets for detection and diagnosis. Ongoing comparative genomics has the potential to fine tune the identification of Legionella species and serogroups by combining specific and general genetic targets. For example, the coincident detection of LPS biosynthesis genes and virulence genes may allow the differentiation of both pathogen and serogroup without the need for nucleotide sequencing. We tested this idea using data derived from a previous genomic subtractive hybridization we performed between L. pneumophila serogroup 1 and L. micdadei. Although not yet formally tested, these targets serve as an example of how comparative genomics has the potential to improve the scope and accuracy of Legionella molecular detection if embraced by laboratories undertaking Legionella surveillance.

Molecular pathogenesis of infections caused by Legionella pneumophila

The genus Legionella contains more than 50 species, of which at least 24 have been associated with human infection. The best-characterized member of the genus, Legionella pneumophila, is the major causative agent of Legionnaires' disease, a severe form of acute pneumonia. L. pneumophila is an intracellular pathogen, and as part of its pathogenesis, the bacteria avoid phagolysosome fusion and replicate within alveolar macrophages and epithelial cells in a vacuole that exhibits many characteristics of the endoplasmic reticulum (ER). The formation of the unusual L. pneumophila vacuole is a feature of its interaction with the host, yet the mechanisms by which the bacteria avoid classical endosome fusion and recruit markers of the ER are incompletely understood. Here we review the factors that contribute to the ability of L. pneumophila to infect and replicate in human cells and amoebae with an emphasis on proteins that are secreted by the bacteria into the Legionella vacuole and/or the host cell. Many of these factors undermine eukaryotic trafficking and signaling pathways by acting as functional and, in some cases, structural mimics of eukaryotic proteins. We discuss the consequences of this mimicry for the biology of the infected cell and also for immune responses to L. pneumophila infection.

Pore formation triggered by Legionella spp. is an Nlrc4 inflammasome-dependent host cell response that precedes pyroptosis

Legionella pneumophila, the etiological agent of Legionnaires disease, is known to trigger pore formation in bone marrow-derived macrophages (BMMs) by mechanisms dependent on the type IVB secretion system known as Dot/Icm. Here, we used several mutants of L. pneumophila in combination with knockout mice to assess the host and bacterial factors involved in pore formation in BMMs. We found that regardless of Dot/Icm activity, pore formation does not occur in BMMs deficient in caspase-1 and Nlrc4/Ipaf. Pore formation was temporally associated with interleukin-1beta secretion and preceded host cell lysis and pyroptosis. Pore-forming ability was dependent on bacterial Dot/Icm but independent of several effector proteins, multiplication, and de novo protein synthesis. Flagellin, which is known to trigger the Nlrc4 inflammasome, was required for pore formation as flaA mutant bacteria failed to induce cell permeabilization. Accordingly, transfection of purified flagellin was sufficient to trigger pore formation independent of infection. By using 11 different Legionella species, we found robust pore formation in response to L. micdadei, L. bozemanii, L. gratiana, L. jordanis, and L. rubrilucens, and this trait correlated with flagellin expression by these species. Together, the results suggest that pore formation is neither L. pneumophila specific nor the result of membrane damage induced by Dot/Icm activity; instead, it is a highly coordinated host cell response dependent on host Nlrc4 and caspase-1 and on bacterial flagellin and type IV secretion system.

Surface translocation by Legionella pneumophila: a form of sliding motility that is dependent upon type II protein secretion

Legionella pneumophila exhibits surface translocation when it is grown on a buffered charcoal yeast extract (BCYE) containing 0.5 to 1.0% agar. After 7 to 22 days of incubation, spreading legionellae appear in an amorphous, lobed pattern that is most manifest at 25 to 30 degrees C. All nine L. pneumophila strains examined displayed the phenotype. Surface translocation was also exhibited by some, but not all, other Legionella species examined. L. pneumophila mutants that were lacking flagella and/or type IV pili behaved as the wild type did when plated on low-percentage agar, indicating that the surface translocation is not swarming or twitching motility. A translucent film was visible atop the BCYE agar, advancing ahead of the spreading legionellae. Based on its abilities to disperse water droplets and to promote the spreading of heterologous bacteria, the film appeared to manipulate surface tension and, as such, acted like a surfactant. Indeed, a sample obtained from the film rapidly dispersed when it was spotted onto a plastic surface. L. pneumophila type II secretion (Lsp) mutants, but not their complemented derivatives, were defective for both surface translocation and film production. In contrast, mutants defective for type IV secretion exhibited normal surface translocation. When lsp mutants were spotted onto film produced by the wild type, they were able to spread, suggesting that type II secretion promotes the elaboration of the Legionella surfactant. Together, these data indicate that L. pneumophila exhibits a form of surface translocation that is most akin to "sliding motility" and uniquely dependent upon type II secretion.

Legionella pneumophila Mip, a surface-exposed peptidylproline cis-trans-isomerase, promotes the presence of phospholipase C-like activity in culture supernatants

The type II secretion system of Legionella pneumophila promotes pathogenesis. Among the Legionella type II-dependent exoenzymes is a p-nitrophenol phosphorylcholine (p-NPPC) hydrolase whose activity is only partially explained by the PlcA phospholipase C. In a screen to identify other factors that promote secreted hydrolase activity, we isolated a mip mutant. L. pneumophila Mip is a surface-exposed, FK506-binding protein that is needed for optimal infection and has peptidylproline cis-trans-isomerase (PPIase) activity. Since the molecular target of Mip was undefined, we investigated a possible relationship between Mip and the secreted p-NPPC hydrolase activity. In the mip mutant there was a 40 to 70% reduction in secreted activity that was successfully complemented by providing mip on a plasmid. A similar phenotype was observed when we examined four other independently derived mip mutants, and in all cases the defect was complemented by reintroduction of mip. Thus, mip promotes the presence of a p-NPPC hydrolase activity in culture supernatants. We also found that the C terminus of Mip is required for this effect. When supernatants were examined by anion-exchange chromatography, the p-NPPC hydrolase activity associated with Mip proved to be type II dependent but distinct from PlcA. This conclusion was supported by the phenotype of a newly constructed mip plcA double mutant. Thus, Mip promotes the elaboration of a new type II exoprotein. These data provide both the first evidence for a target for Mip and the first indication that a surface PPIase is involved in the secretion or activation of proteins beyond the outer membrane.

Clinical and environmental isolates of Legionella pneumophila serogroup 1 cannot be distinguished by sequence analysis of two surface protein genes and three housekeeping genes

We used gene sequencing to determine whether clinical (sporadic, epidemic, and endemic) and environmental isolates of Legionella pneumophila serogroup (sg) 1 belong to specific lineages. A total of 178 clinical and environmental L. pneumophila sg 1 isolates, defined by pulsed-field gel electrophoresis and epidemiological data as sporadic, epidemic, or endemic, were analyzed for polymorphisms in five gene fragments. The fragments belonged to three housekeeping genes (coding for aconitase [acn], aspartate-beta-semialdehyde dehydrogenase [asd], and RNA polymerase beta subunit [rpoB]) and two surface protein genes (coding for the macrophage infectivity potentiator [mip] and the major outer membrane protein [mompS]). The phylogenetic tree inferred from sequence polymorphisms of the five genes identified two large clusters, one consisting of 133 poorly differentiated strains and containing two smaller clusters (10 and 2 strains) unrelated to each other and the other consisting of 42 strains. Clinical and environmental isolates could not be distinguished on this basis, and no link between genetic background and epidemiological type was found, suggesting that other factors are responsible for differences in pathogenicity.

ST64B is a defective bacteriophage in Salmonella enterica serovar Typhimurium DT64 that encodes a functional immunity region capable of mediating phage-type conversion

The Salmonella enterica subsp. enterica serovar Typhimurium (S. Typhimurium) defective bacteriophage ST64B has a putative immunity (immC) region consisting of cI, cro and cII-like genes. Since ST64B is widespread in S. Typhimurium, studies were undertaken to determine whether this region might be functional and influence phage typing results. Cloning of ST64B immC-like genes and their subsequent expression in S. Typhimurium DTs showed that this region is able to mediate phage-type conversion in DTs 41 and 44. This confirms the functionality of the immC region and the patterns of lysis produced by phage typing are consistent with the predicted mechanism of action of the encoded protein products.

Detection of Legionella pneumophila by real-time PCR for the mip gene

A real-time PCR assay for the mip gene of Legionella pneumophila was tested with 27 isolates of L. pneumophila, 20 isolates of 14 other Legionella species, and 103 non-Legionella bacteria. Eight culture-positive and 40 culture-negative clinical specimens were tested. This assay was 100% sensitive and 100% specific for L. pneumophila.

The PPIase active site of Legionella pneumophila Mip protein is involved in the infection of eukaryotic host cells

We analysed eight monoclonal antibodies (mAbs) directed against the Mip (macrophage infectivity potentiator) protein, a virulence factor of the intracellular pathogen Legionella pneumophila. Mip belongs to the FK506-binding proteins (FKBPs) and exhibits peptidyl prolyl cis/trans isomerase (PPIase) activity. Five of the mAbs recognised epitopes in the C-terminal, FKBP-homologous domain of Mip, which is highly conserved among all Legionella species. Upon immunological binding to Mip, all but one of these mAbs caused inhibition of the PPIase activity in vitro. mAb binding to the N-terminal domain of Mip did not influence its enzymatic activity. All but one of the PPIase inhibiting mAbs were able to significantly inhibit the early establishment and initiation of an intracellular infection of the bacteria in Acanthamoeba castellanii, the natural host, and in the human phagocytic cell line U937. These data demonstrate for the first time that for the virulence-enhancing property of the L. pneumophila Mip protein, an intact active site of the enzyme is an essential requirement.

Legionella and Legionnaires' disease: 25 years of investigation

There is still a low level of clinical awareness regarding Legionnaires' disease 25 years after it was first detected. The causative agents, legionellae, are freshwater bacteria with a fascinating ecology. These bacteria are intracellular pathogens of freshwater protozoa and utilize a similar mechanism to infect human phagocytic cells. There have been major advances in delineating the pathogenesis of legionellae through the identification of genes which allow the organism to bypass the endocytic pathways of both protozoan and human cells. Other bacteria that may share this novel infectious process are Coxiella burnetti and Brucella spp. More than 40 species and numerous serogroups of legionellae have been identified. Most diagnostic tests are directed at the species that causes most of the reported human cases of legionellosis, L. pneumophila serogroup 1. For this reason, information on the incidence of human respiratory disease attributable to other species and serogroups of legionellae is lacking. Improvements in diagnostic tests such as the urine antigen assay have inadvertently caused a decrease in the use of culture to detect infection, resulting in incomplete surveillance for legionellosis. Large, focal outbreaks of Legionnaires' disease continue to occur worldwide, and there is a critical need for surveillance for travel-related legionellosis in the United States. There is optimism that newly developed guidelines and water treatment practices can greatly reduce the incidence of this preventable illness.

Legionella and Legionnaires' disease: 25 years of investigation

There is still a low level of clinical awareness regarding Legionnaires' disease 25 years after it was first detected. The causative agents, legionellae, are freshwater bacteria with a fascinating ecology. These bacteria are intracellular pathogens of freshwater protozoa and utilize a similar mechanism to infect human phagocytic cells. There have been major advances in delineating the pathogenesis of legionellae through the identification of genes which allow the organism to bypass the endocytic pathways of both protozoan and human cells. Other bacteria that may share this novel infectious process are Coxiella burnetti and Brucella spp. More than 40 species and numerous serogroups of legionellae have been identified. Most diagnostic tests are directed at the species that causes most of the reported human cases of legionellosis, L. pneumophila serogroup 1. For this reason, information on the incidence of human respiratory disease attributable to other species and serogroups of legionellae is lacking. Improvements in diagnostic tests such as the urine antigen assay have inadvertently caused a decrease in the use of culture to detect infection, resulting in incomplete surveillance for legionellosis. Large, focal outbreaks of Legionnaires' disease continue to occur worldwide, and there is a critical need for surveillance for travel-related legionellosis in the United States. There is optimism that newly developed guidelines and water treatment practices can greatly reduce the incidence of this preventable illness.

Identification and subcellular localization of the Legionella pneumophila IcmX protein: a factor essential for establishment of a replicative organelle in eukaryotic host cells

The gram-negative respiratory pathogen Legionella pneumophila infects and grows within mammalian macrophages and protozoan host cells. Upon uptake into macrophages, L. pneumophila establishes a replicative organelle that avoids fusion with endocytic vesicles. There are 24 dot/icm genes on the L. pneumophila chromosome required for biogenesis of this vacuole. Many of the Dot/Icm proteins are predicted to be components of a membrane-bound secretion apparatus similar to type IV conjugal transfer systems. We have been investigating the function of L. pneumophila dot/icm gene products that do not have obvious orthologs in other type IV transfer systems, since these determinants could govern processes unique to phagosome biogenesis. The icmX gene product falls into this category. To understand the role of the IcmX protein in pathogenesis, we have detailed interactions between an L. pneumophila icmX deletion mutant and murine bone marrow-derived macrophages. These data demonstrate that icmX is required for biogenesis of the L. pneumophila replicative organelle. Immunoblot analysis indicates that the icmX gene product is a polypeptide with an estimated molecular mass of 50 kDa. The IcmX protein was localized to the bacterial periplasm, and periplasmic translocation was mediated by an N-terminal sec-dependent leader peptide. A truncated IcmX product was secreted into culture supernatants by wild-type L. pneumophila growing extracellularly in liquid media; however, transport of the IcmX protein into eukaryotic host cells was not detected. Proteins similar in molecular weight to IcmX were identified in other Legionella species by immunoblot analysis using a monoclonal antibody specific for L. pneumophila IcmX protein. From these data, we conclude that the IcmX protein is an essential component of the dot/icm secretion apparatus, and that a conserved mechanism of host cell parasitism exists for members of the Legionellaceae family.

Sequence analysis of the mip gene of the soilborne pathogen Legionella longbeachae

To understand the basis of pathogenesis by Legionella longbeachae serogroup 1, the importance of the Mip protein in this species was examined. Amino-terminal analysis of the purified, cloned L. longbeachae serogroup 1 ATCC 33462 Mip protein confirmed that the cloned gene protein was expressed and processed in an Escherichia coli background. DNA sequence analysis of plasmid pIMVS27, containing the entire L. longbeachae serogroup 1 mip gene, revealed a high degree of homology to the mip gene of Legionella pneumophila serogroup 1, 76% homology at the DNA level and 87% identity at the amino acid level. Primer extension analysis determined that the start site of transcription was the same for both species, with some differences observed for the -10 and -35 promoter regions. Primers designed from the mip gene sequence obtained for L. longbeachae serogroup 1 ATCC 33462 were used to amplify the mip genes from L. longbeachae serogroup 2 ATCC 33484 and an Australian clinical isolate of L. longbeachae serogroup 1 A5H5. The mip gene from A5H5 was 100% identical to the type strain sequence. The serogroup 2 strain of L. longbeachae differed by 2 base pairs in third-codon positions. Allelic exchange mutagenesis was used to generate an isogenic mip mutant in ATCC 33462 and strain A5H5. The ATCC mip mutant was unable to infect a strain of Acanthamoebae sp. both in liquid and in a potting mix coculture system, while the A5H5 mip mutant behaved in a manner siilar to that of L. pneumophila serogroup 1, i.e., it displayed a reduced capacity to infect and multiply within Acanthamoebae. To determine if this mutation resulted in reduced virulence in the guinea pig animal model, the A5H5 mip mutant and its parent strain were assessed for their abilities to establish an infection after aerosol exposure. Unlike the virulent parent strain, the mutant strain did not kill any animals under two different dose regimes. The data indicate that the Mip protein plays an important role in the intracellular life cycle of L. longbeachae serogroup 1 species and is required for full virulence.

Identification and temperature regulation of Legionella pneumophila genes involved in type IV pilus biogenesis and type II protein secretion

Previously, we had isolated by transposon mutagenesis a Legionella pneumophila mutant that appeared defective for intracellular iron acquisition. While sequencing in the proximity of the mini-Tn10 insertion, we found a locus that had a predicted protein product with strong similarity to PilB from Pseudomonas aeruginosa. PilB is a component of the type II secretory pathway, which is required for the assembly of type IV pili. Consequently, the locus was cloned and sequenced. Within this 4-kb region were three genes that appeared to be organized in an operon and encoded homologs of P. aeruginosa PilB, PilC, and PilD, proteins essential for pilus production and type II protein secretion. Northern blot analysis identified a transcript large enough to include all three genes and showed a substantial increase in expression of this operon when L. pneumophila was grown at 30 degrees C as opposed to 37 degrees C. The latter observation was then correlated with an increase in piliation when bacteria were grown at the lower temperature. Southern hybridization analysis indicated that the pilB locus was conserved within L. pneumophila serogroups and other Legionella species. These data represent the first isolation of type II secretory genes from an intracellular parasite and indicate that the legionellae express temperature-regulated type IV pili.

Isolation of a gene encoding a novel spectinomycin phosphotransferase from Legionella pneumophila

A gene capable of conferring spectinomycin resistance was isolated from Legionella pneumophila, the agent of Legionnaires' disease. The gene (aph) encoded a 36-kDa protein which has similarity to aminoglycoside phosphotransferases. Biochemical analysis confirmed that aph encodes a phosphotransferase which modifies spectinomycin but not hygromycin, kanamycin, or streptomycin. The strain that was the source of aph demonstrated resistance to spectinomycin, and Southern hybridizations determined that aph also exists in other legionellae.

Infection of Acanthamoeba castellanii by Chlamydia pneumoniae

Chlamydia pneumoniae is an intracellular respiratory pathogen, which, similar to Legionella, might have developed mechanisms to escape the intracellular bactericidal activity of both human host cells and amoeba. We therefore investigated the intracellular growth and survival of C. pneumoniae in Acanthamoeba castellanii by using cell culture, immunofluorescence microscopy, and electron microscopy. A castellanii was incubated with purified elementary bodies of C. pneumoniae TW 183 at a concentration of 10(6) inclusion-forming units (IFU)/ml to give a ratio of approximately 1 IFU of C. pneumoniae per amoeba. Quantitative determination of chlamydial growth within A. castellanii revealed viable and infective C. pneumoniae in the range of 10(4) to 10(5) IFU/ml between days 7 and 14 postinfection. Immunofluorescence analysis and transmission electron microscopy with subsequent immunogold staining confirmed evidence of infection of the amoebae by C. Pneumoniae and additionally revealed that C. pneumoniae entered the typical growth cycle. Our results show that amoebae allow the survival of C. pneumoniae, suggesting that amoebae may serve as an additional reservoir for Chlamydia or Chlamydia-related organisms.

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

Survival and distribution of legionellae in the environment are assumed to be associated with their multiplication in amoebae, whereas the ability to multiply in macrophages is usually regarded to correspond to pathogenicity. Since most investigations focused on Legionella pneumophila serogroup 1, we examined the intracellular multiplication of different Legionella species in Mono Mac 6 cells, which express phenotypic and functional features of mature monocytes, and in Acanthamoeba castellanii, an environmental host of Legionella spp. According to the bacterial doubling time in Mono Mac 6 cells and in A. castellanii, seven clusters of legionellae could be defined which could be split further with regard to finer differences. L. longbeachae serogroup 1, L. jordanis, and L. anisa were not able to multiply in either A. castellanii or Mono Mac 6 cells and are members of the first cluster. L. dumoffi did not multiply in Mono Mac 6 cells but showed a delayed multiplication in A. castellanii 72 h after infection and is the only member of the second cluster. L. steigerwaltii, L. gormanii, L. pneumophila serogroup 6 ATCC 33215, L. bozemanii, and L. micdadei showed a stable bacterial count in Mono Mac 6 cells after infection but a decreasing count in amoebae. They can be regarded as members of the third cluster. As the only member of the fourth cluster, L. oakridgensis was able to multiply slight in Mono Mac 6 cells but was killed within amoebae. A strain of L. pneumophila serogroup 1 Philadelphia obtained after 30 passages on SMH agar and a strain of L. pneumophila serogroup 1 Philadelphia obtained after intraperitoneal growth in guinea pigs are members of the fifth cluster, which showed multiplication in Mono Mac 6 cells but a decrease of bacterial counts in A. castellanii. The sixth cluster is characterized by intracellular multiplication in both host cell systems and consists of several strains of L. pneumophila serogroup 1 Philadelphia, a strain of L. pneumophila serogroup 2, and a fresh clinical isolate of L. pneumophila serogroup 6. Members of the seventh cluster are a strain of agar-adapted L. pneumophila serogroup 1 Bellingham and a strain of L. pneumophila serogroup 1 Bellingham which was passaged fewer than three times on BCYE alpha agar after inoculation and intraperitoneal growth in guinea pigs. In comparison to members of the sixth cluster, both strains showed a slightly enhanced multiplication in Mono Mac 6 cells but a reduced multiplication in amoebae. From our investigations, we could demonstrate a correlation between prevalence of a given Legionella species and their intracellular multiplication in Mono Mac 6 cells. Multiplication of members of the genus Legionella in A. castellanii seems to be dependent on mechanisms different from those in monocytes.

An iron- and fur-repressed Legionella pneumophila gene that promotes intracellular infection and encodes a protein with similarity to the Escherichia coli aerobactin synthetases

Legionella pneumophila, a parasite of alveolar macrophages, requires iron for intra- and extracellular growth. Although its mechanisms for iron assimilation are poorly understood, this bacterium produces Fur, a protein that can repress gene transcription in response to iron concentration. Because iron- and Fur-regulated genes are important for infection in other bacteria, the identification of similar genes in L. pneumophila was undertaken. A wild-type strain of L. pneumophila was randomly mutated with a mini-Tn10' lacZ transposon, and the resulting gene fusions were tested for iron regulation by assessing beta-galactosidase production in the presence and absence of iron chelators. Of the initial six mutants with iron-repressed lacZ fusions, two strains, NU229 and NU232, possessed fusions that were stably iron regulated. To assay for Fur regulation, the levels of beta-galactosidase were measured in strains no longer producing Fur. As in a number of pathogenic bacteria, L. pneumophila fur could not be insertionally inactivated, but spontaneous Fur- derivatives were generated by selecting for manganese resistance. Strain NU229 contained a Fur-repressed fusion based on derepression of lacZ expression in its manganese-resistant derivative. Extracellular growth of NU229 in bacteriological media was similar to that of wild-type strain 130b. To assess the role of an iron- and Fur-regulated (frgA) gene in intracellular infection, the ability of NU229 to grow within U937 cell monolayers was tested. Quantitative infection assays demonstrated that intracellular growth of NU229 was impaired as much as 80-fold. Reconstruction of the mutant by allelic exchange proved that the infectivity defect in NU229 was due to the inactivation of frgA and not to a second-site mutation. Subsequently, complementation of the interrupted gene by an intact plasmid-encoded gene demonstrated that the infectivity defect was due to the loss of frgA and not to a polar effect. Nucleotide sequence analysis revealed that the 63-kDa FrgA protein has homology with the aerobactin synthetases IucA and IucC of Escherichia coli, raising the possibility that L. pneumophila encodes a siderophore which is required for optimal intracellular replication. Southern hybridization analysis determined that frgA is specific to L. pneumophila.

Distribution of mip-related sequences in 39 species (48 serogroups) of Legionellaceae

The macrophage infectivity potentiator gene (mip) from Legionella pneumophila is a major virulence factor of the species. Thus, mip-detection by amplification has been proposed to assess the presence of L. pneumophila in clinical and environmental samples. The distribution of mip-related sequences within the Legionellaceae was studied by DNA amplification using mip-specific primers followed by Southern blot hybridization with an internal probe. Thirty-nine species (48 serogroups) of Legionellaceae were screened in this attempt. Using this approach, sequences related to mip were observed in 89% of the tested species including the most recently described L. fairfieldensis, L. lansingensis and L. shakespearei. In several cases, cloning and sequencing of the amplified products confirmed the high levels of similarity between the sequence found in non-pneumophila species with that of the L. pneumophila mip gene. This confirms previous reports that mip related genes are widespread among Legionellaceae and therefore specific detection of the species L. pneumophila cannot be based on mip-targeted amplification.

Effector mechanisms responsible for gamma interferon-mediated host resistance to Legionella pneumophila lung infection: the role of endogenous nitric oxide differs in susceptible and resistant murine hosts

To facilitate identification of the effector mechanism(s) responsible for gamma interferon (IFN-gamma)-mediated host resistance to Legionella pneumophila, a murine model of legionellosis in BALB/c mice with a targeted disruption in the IFN-gamma gene (gamma knockout [GKO] mice) was developed. Immunocompetent BALB/c mice and GKO mice were inoculated intratracheally with virulent L. pneumophila (10(6) bacteria per mouse), and bacterial clearance and the pulmonary inflammatory response were assessed. L. pneumophila did not replicate in, and was rapidly cleared from, the lungs of immunocompetent BALB/c mice, demonstrating that immunocompetent BALB/c mice are resistant to replicative L. pneumophila pulmonary infections. In contrast, similarly infected GKO mice developed persistent, replicative intrapulmonary L. pneumophila infections with extrapulmonary dissemination of the bacteria to the spleen. Histopathologic and flow cytometric analysis of L. pneumophila-infected lung tissue demonstrated that while immunocompetent BALB/c mice develop multifocal pneumonitis which resolves, similarly infected GKO mice develop diffuse pneumonitis with persistent neutrophil recruitment into the lung. Intratracheal administration of exogenous IFN-gamma to L. pneumophila-infected GKO mice facilitated intrapulmonary clearance of the bacteria, confirming the pivotal role of IFN-gamma in innate host defenses to L. pneumophila lung infection in this murine host. The potential role of endogenous reactive nitrogen intermediates, including nitric oxide (NO), in IFN-gamma-mediated resistance to L. pneumophila pulmonary infections in immunocompetent BALB/c mice was subsequently assessed. Macrophage inducible nitric oxide synthetase (an enzyme responsible for the production of NO) was induced in alveolar cells from L. pneumophila-infected immunocompetent BALB/c mice (with maximal expression at 48 h postinfection) but was not induced in similarly infected GKO mice. However, administration of the NO synthetase inhibitor N-monomethyl-L-arginine did not significantly inhibit clearance of L. pneumophila from the lung of immunocompetent BALB/c mice (compared with that in similarly infected mice not administered N-monomethyl-L-arginine). In contrast, we have previously demonstrated that IFN-gamma-induced host resistance to replicative L. pneumophila lung infections in a susceptible murine host (A/J mice) is mediated, in part, by endogenous NO. Taken together, these studies identify a differing role of endogenous NO in IFN-gamma-mediated resistance to L. pneumophila pulmonary infection in susceptible and resistant murine hosts.

Isolation and amino acid sequence of a new 22-kDa FKBP-like peptidyl-prolyl cis/trans-isomerase of Escherichia coli. Similarity to Mip-like proteins of pathogenic bacteria

We identified a periplasmic peptidyl-prolyl cis/trans-isomerase (PPIase) of the (FK506-binding protein (FKBP) type in Escherichia coli (FK506 represents a natural peptidomacrolide containing an acylated pipecolic acid residue). After purification to homogeneity, its complete amino acid sequence was determined by a combination of Edman degradation and electrospray mass spectrometry of the authentic protein and peptides generated by proteolysis. The molecular mass calculated from the amino acid sequence of the protein was 22,085.53 Da, which corresponded perfectly with the value of 22,084 +/- 1.47 Da as determined by mass spectrometry. The corresponding gene was cloned and analyzed, and Southern blot experiments revealed the existence of similar genes in various Gram-negative bacteria. The amino acid sequence of the novel FKBP22 shows similarity to Mip (macrophage infectivity potentiator)-like proteins produced by a number of pathogenic bacteria. However, FKBP22 is inhibited more strongly by FK506 than are other Mip-homologues, as indicated by the Ki value of 25 nM. The subsite specificity regarding the P1 position of the substrate resembles that for Mip-FKBP25 from Legionella pneumophila. The mature FKBP22 enzyme of 205 amino acids exists as a dimer in solution.

A Legionella pneumophila gene that promotes hemin binding

The ability to bind and utilize hemin is a trait common to many human pathogens. Nevertheless, the relationship between Legionella pneumophila, the agent of Legionnaires' disease, and hemin has received little attention. Thus, we explored the capacity of a virulent, serogroup 1 strain of L. pneumophila to bind hemin and use it as an iron source. Hemin, but not protoporphyrin IX, restored bacterial growth in iron-limiting media, indicating that it can serve as an iron source for L. pneumophila. In support of this idea, we observed that wildtype legionellae were able to bind 50 to 60% of added hemin, a binding capacity that was comparable to those of other pathogens. To begin to identify proteins involved in hemin acquisition, we identified a Legionella locus that conferred hemin binding upon Escherichia coli. Subcloning and nucleotide sequence analysis determined that a single open reading frame, which was designated hbp for hemin-binding promotion, was responsible for this binding activity. The hbp gene was predicted to encode a secreted, 15.5-kDa protein. To ascertain the importance of this gene in L. pneumophila biology, we used allelic exchange to construct an hbp mutant. Importantly, the mutant displayed a 42% reduction in hemin binding, confirming that hbp potentiates hemin acquisition by L. pneumophila. However, the strain was unaltered in its ability to grow within macrophage-like cells and freshwater amoebae, indicating that hbp is not required for intracellular infection. Despite this, Southern hybridization analysis and database searches demonstrated that hbp is nearly exclusive to the L. pneumophila species.

Characterization of a Legionella micdadei mip mutant

The pathogenesis of Legionella micdadei is dependent upon its ability to infect alveolar phagocytes. To better understand the basis of intracellular infection by this organism, we examined the importance of its Mip surface protein. In Legionella pneumophila, Mip promotes infection of both human macrophages and freshwater protozoa. Southern hybridization and immunoblot analyses demonstrated that mip sequences were present and expressed within a panel of virulent L. micdadei strains. Using allelic exchange mutagenesis, we then constructed an L. micdadei strain that completely and specifically lacked Mip. Although unimpaired in its ability to grow in bacteriologic media, this Mip mutant was defective in its capacity to infect U937 cells, a human macrophage-like cell line. Most significantly, the Mip- organism displayed a 24-fold reduction in survivability immediately after its entry into the phagocyte. Similarly, the mutant was less able to parasitize Hartmannella amoebae. Taken together, these data argue that Mip specifically potentiates intracellular growth by L. micdadei.

Cross-reacting lipopolysaccharide antigens in Legionella pneumophila serogroups 1 to 14

Immunological cross-reactions among Legionella species were investigated with sonicated, proteinase K-digested cell lysates. The antigens separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were either analyzed for lipopolysaccharides (LPSs) by silver staining or transferred to nitrocellulose membranes for serological characterization with rabbit antibodies directed against Legionella pneumophila serogroups 1 and 5. When antiserum prepared against serogroup 5 was used to probe the LPSs from L. pneumophila serogroups 1 to 14, the antibodies recognized a common epitope harbored by all L. pneumophila serogroups but not by other Legionella species or by the gram-negative bacteria tested as controls. Hence, the serogroup 5 antiserum correctly identified all serogroups of L. pneumophila tested in the LPS immunoblot assay. Moreover, the silver-stained profiles of the isolated LPSs revealed characteristic patterns allowing the identification of the individual serogroups of L. pneumophila.

Escherichia coli and other species of the Enterobacteriaceae encode a protein similar to the family of Mip-like FK506-binding proteins

A newly identified gene in Escherichia coli, fkpA, encodes a protein with extensive similarity to the macrophage infectivity potentiator (Mip) proteins of Legionella pneumophila and Chlamydia trachomatis. The FkpA protein may be a new member of the family of FK506-binding proteins (FKBPs) because its carboxyl domain includes a sequence that matches the consensus FK506-binding motif in 40 of 48 positions, including those amino acids at the active site that form hydrogen bonds with the drug FK506. The amino acid sequence of the 29 kDa FkpA protein is 30-35% identical to the Mip proteins of L. pneumophila, L. micdadei, and C. trachomatis. Of the 270 amino acids of FkpA, 113 (42%) are identical to the sequence of one or another of these Mip proteins. Overexpression of FkpA or deletion of fkpA from the E. coli chromosome had no detrimental effect on bacterial growth, indicating that fkpA is not an essential gene. Hybridization of fkpA-specific DNA probes to genomic blots revealed that similar genes exist in several representatives of the Enterobacteriaceae. Thus, mip-like genes are not found exclusively in bacteria having a predominately intracellular life style, but instead appear to be a new FKBP subfamily that is a common constituent of many bacteria.

Enzyme-linked immunoassay for detection of PCR-amplified DNA of legionellae in bronchoalveolar fluid

A nonradioactive method is described that detects 10 to 100 legionellae in 1 ml of bronchoalveolar lavage fluid. DNA is purified by a proteinase K-phenol protocol or with a commercial DNA preparation kit and amplified by PCR with amplimers specific for the 16S rRNA gene of Legionella pneumophila. The upstream primer is 5' biotinylated. The amplification product is immobilized on streptavidin-coated microtiter plates. Because of the high binding capacity, no removal of nonincorporated biotin from the PCR product is required. After alkaline denaturation, the single-stranded PCR product is hybridized with a 5' digoxigenin-labeled probing oligomer. The amplification product is then detected by using peroxidase-labeled anti-digoxigenin antibodies in a luminescence or colorimetric reaction. The assay detects as few as 10 legionellae in 1-ml bronchoalveolar lavage fluid specimens. It is specific for medically relevant Legionella species, including Legionella pneumophila, L. bozemanii, and L. longbeachae. Of over 250 clinical specimens examined, 8 were positive for legionellae by both culture and the PCR assay. Six further specimens were culture negative but PCR positive for legionellae; of these, five specimens were from patients receiving high-dose erythromycin therapy for suspected or previously diagnosed legionella pneumonia. None of the remaining 240 specimens that were culture negative for legionellae yielded a positive PCR test, although a total of over 30 different bacterial species were cultured from these specimens. The PCR assay therefore appears to exhibit high sensitivity and specificity and thus could prove suitable for use in the routine microbiological diagnostic laboratory.

Monoclonal antibodies to Legionella Mip proteins recognize genus- and species-specific epitopes

Monoclonal antibodies (MAbs) against the virulence-associated Mip protein of Legionella spp. were raised by immunizing BALB/c mice with (i) Legionella pneumophila, (ii) Legionella micdadei, and (iii) purified recombinant native Mip protein cloned from L. pneumophila Philadelphia 1. Following screening of seeded wells by immunoblot analysis with homologous antigens, eight Mip-specific MAbs were found. These MAbs were chosen to investigate the antigenic diversity of Mip proteins in the genus Legionella. Mip was detected in 82 Legionella strains representing all 34 species tested. One of these MAbs, obtained from immunization with L. micdadei, recognized an epitope common to all Legionella species tested by immunoblot analysis. Another MAb was discovered to be specific for the Mip protein of L. pneumophila. The remaining six MAbs recognized 18 to 79% of Legionella species included in this study. By making use of the MAbs introduced in this study, it could be shown that, based on Mip protein epitope expression, Legionella species can be divided into at least six antigenetically distinct groups. As demonstrated by 43 L. pneumophila strains representing all serogroups, no antigenic diversity of Mip proteins was found for this species. In addition, 18 non-Legionella species, including Chlamydia trachomatis, Neisseria meningitidis, Pseudomonas aeruginosa, and Saccharomyces cerevisiae, all of which are known to carry genes homologous to the Legionella mip genes, were reacted against all eight MAbs. No cross-reactivity was detectable in any of those strains.

Detection of mip-like sequences and Mip-related proteins within the family Rickettsiaceae

The Mip surface protein, a prokaryotic analog of the FK506-binding proteins, enhances the ability of Legionella pneumophila to infect macrophages and protozoa. Using mip-specific probes and low-stringency Southern hybridizations, we have detected DNA sequences homologous to mip within Coxiella burnetii and Rochalimaea quintana. Using specific anti-Mip antisera and immunoblot analysis, we also detected Mip-related proteins within these bacteria as well as within Rickettsia and Ehrlichia species. These data suggest that Mip-related proteins have broad significance for host-parasite interactions. However, they also indicate that care must be exercised when using mip probes or anti-Mip antibodies for the detection of Legionella organisms in water or clinical samples.

Genetic approaches to study Legionella pneumophila pathogenicity

Legionella pneumophila is an intracellular pathogen replicating in human macrophages during the course of infection of the lungs. Infection by legionellae often leads to severe pneumonia, termed Legionnaires' disease. Genetic approaches to identify the factors responsible for L. pneumophila pathogenicity started with the construction of genomic libraries in Escherichia coli. Various L. pneumophila-specific genes were cloned in E. coli K-12 by identification using functional assays, antibody screening and hybridization ('reverse genetics'). By disrupting the genes via allelic exchange, mutants have been created to assess the influence of the factors on pathogenicity. Among the cloned genes, only for the gene product of the mip gene, encoding a 24-kDa surface-associated protein (macrophage infectivity potentiator) unequivocal evidence for its contribution to pathogenicity could be provided. Two hemolytic factors that have been cloned do not seem to play a role in L. pneumophila pathogenicity. Genetic systems for transposon mutagenesis of the L. pneumophila genome (Tn5, Tn903dIIlacZ, MudphoA), including Tn phoA shuttle mutagenesis, have been established and specifically adapted to identify mutants which displayed an impaired capability to multiply inside macrophages and with a reduced in vivo virulence. Furthermore, by complementation of avirulent mutants, genetic loci could be identified which restored the virulence.

Evaluation of commercial amplification kit for detection of Legionella pneumophila in clinical specimens

A commercial kit (EnviroAmp) designed to detect the DNA of Legionella species in environmental water samples using PCR and reverse dot hybridization was applied to clinical specimens. Results correlated well with culture for bronchoalveolar lavages. In addition, this test was easy to perform and showed good sensitivity.

Cloning and sequencing of the Legionella pneumophila fur gene

Iron is required for the intracellular and extracellular growth of Legionella pneumophila (Lp). In addition, variations in iron levels may serve as a signal for changes in gene expression. In a number of bacterial pathogens, the regulation of gene expression by iron is usually mediated by the Fur (ferric uptake regulation) repressor protein. Through complementation of an Escherichia coli fur mutation and nucleotide sequence analysis, we have cloned and characterized the Lp fur gene. Lp fur encoded a 15.0-kDa protein whose repressive activity was, as expected, highest in bacteria grown in iron-rich media. Computer analysis determined that Lp Fur had an amino-acid identity of over 54% and a similarity of over 72% to the Fur of E. coli, Yersinia pestis, Vibrio species and Pseudomonas aeruginosa. The promoter region of Lp fur contained sequences homologous to the Fur-binding site, suggesting that fur is autoregulated in Lp. Finally, Southern blot hybridizations demonstrated that fur is conserved among Lp strains and Legionella species.

Rapid detection of Legionella species in bronchoalveolar lavage fluids with the EnviroAmp Legionella PCR amplification and detection kit

A molecular assay based on a rapid DNA extraction protocol and the EnviroAmp Legionella Kits was used to detect Legionella species in bronchoalveolar fluid specimens. All Legionella strains isolated from tap water in hospitals could be detected distinctly. Both sensitivity and specificity were tested. In a prospective study, bronchoalveolar lavage fluids obtained from patients with atypical pneumonia were investigated. Three positive samples were detected with the molecular techniques and were subsequently confirmed by culture. Application of the system described may lead to safe and early diagnosis of Legionnaires' disease in patients with atypical pneumonia.

Detection of Legionella species in sewage and ocean water by polymerase chain reaction, direct fluorescent-antibody, and plate culture methods

Legionella spp. are ubiquitous in most environmental water sources; however, sewage treatment plants have not been examined as potential environmental reservoirs for these bacteria. This study used polymerase chain reaction, direct fluorescent-antibody staining, and culture methods to examine raw and treated sewage, ocean-receiving waters, and nearshore coastal environments for the presence of Legionella pneumophila and other Legionella spp. The study concluded that Legionella spp. are present in all phases of sewage treatment and that population numbers do not significantly decline through the treatment process. Ocean-receiving waters located 5 miles offshore, where the treated sewage is discharged, were found to contain Legionella spp., but ocean water between the discharge site and coastal bathing beaches was negative. This suggests that the Legionella spp. from the ocean discharge site were not reaching the nearshore beach waters. A flood control channel and river that entered the ocean were found to contain Legionella spp., and a nearby beach swimming area was also found to be positive, suggesting that land runoff from the flood control channel and river were the source of the Legionella spp. in the beach water samples that tested positive.

Immunophilins: structure-function relationship and possible role in microbial pathogenicity

Immunophilins are housekeeping proteins present in a wide variety of organisms. Members of two protein superfamilies, cyclophilins (Cyps) and FK506-binding proteins (FKBPs) belong to this class of immunophilins. Despite the fact that the amino acid sequences of Cyp and FKBPs do not exhibit noticeable homology to each other, proteins of both classes are able to ligate immunosuppressive peptide derivatives. Cyps form complexes with the cyclic undercapeptide cyclosporin A and FKBPs are able to bind FK506 as well as rapamycin, both of which have a pipecolyl bond within their structure. In a ligand-bound form, immunophilins interfere with signal transduction in T cells. In addition, immunophilins have peptidyl prolyl cis-trans isomerase (PPlase) activity and are able to accelerate the rate of conformational events in proline-containing polypeptides. Microorganisms produce proteins that exhibit extensive sequence homologies to cyclophilins and FKBPs of higher organisms and which have considerable PPlase catalytic activity. While cyclophilins seem to be present in most if not all microbial species investigated, FKBPs are produced by yeasts as well as by a number of pathogenic bacteria, such as Legionella pneumophila, Chlamydia trachomatis and Neisseria meningitidis. The Mip protein of L. pneumophila is a virulence factor that plays an essential role in the ability of the bacteria to survive and multiply in phagocytic cells. Some results are summarized on the structure and putative functions of immunophilins and place special emphasis on the contribution of these polypeptides to the virulence of pathogenic microorganisms.

Peptidylproline cis-trans-isomerases: immunophilins

Two sequence-unrelated families of proteins possess peptidylproline cis-trans-isomerase activities (PPIase). PPIases are highly sequence conserved and multifunctional proteins which are present in many types of cells with a considerably divergent phylogenetic distribution. On the cellular level, PPIases occur in every compartment, both as free species and anchored to membranes. Diverse posttranslational modifications such as glycosylation, N-terminal modifications and phosphorylation constitute the additional functional features of PPIases. Folding, assembly and trafficking of proteins in the cellular milieu are regulated by PPIases. These enzymes accelerate the rate of in-vitro protein folding and they have the ability to bind proteins and act as chaperones. Some PPIases are coregulatory subunits of molecular complexes including heat-shock proteins, glucocorticoid receptors and ion channels. Secreted forms of PPIases are inflammatory and chemotactic agents for monocytes, eosinophils and basophils. The potent and clinically useful immunosuppressants CsA, FK506 or rapamycin bind with high affinities to PPIases (immunophilins). The binding criterion allows us to sort the PPIases for the following two superfamilies of proteins: the cyclophilins (CsA-binding proteins) and the FKBP (FK506/rapamycin-binding proteins). Although none of PPIases appeared to be essential for the viability of haploid yeast cells some of the immunophilin/immunosuppressant complexes are toxic both for yeast and mammalian cells. At least seven unlinked genes of cyclophilins and four unlinked genes of FKBP exist in human genomic DNA. Selected immunophilins regulate two different signalling pathways in lymphoid cells, namely the secretion of growth factors by stimulated T-cells and interleukin-2-induced T-cell proliferation. Moreover, selected FKBP mediate the cytotoxic effects of rapamycin in non-lymphoid cells. Accounts of the discovery of PPIases (immunophilins) and their functions are given in this review. A larger spectrum of proteins is analysed in relation to various signal-transduction pathways in lymphoid cells which involve immunophilins or their complexes with the immunosuppressants CsA, FK506 or rapamycin.