Interspecies sequence differences in the Mip protein from the genus Legionella: implications for function and evolutionary relatedness

Legionella maioricensis sp. nov., a new species isolated from the hot water distribution systems of a hospital and a shopping center during routine sampling

Two Legionella-like strains isolated from hot water distribution systems in 2012 have been characterized phenotypically, biochemically and genomically in terms of DNA relatedness. Both strains, HCPI-6^T and EUR-108, exhibited biochemical phenotypic profiles typical of Legionella species. Cells were Gram-negative motile rods which grew on BCYEα agar but not on blood agar and displayed phenotypic characteristics typical of the family Legionellaceae, including a requirement for l-cysteine and testing catalase positive. Both strains were negative for oxidase, urease, nitrate reduction and hippurate negative, and non-fermentative. The major ubiquinone was Q12 (59.4 % HCPI-6^T) and the dominant fatty acids were C_16 : 1 ω7c (28.4 % HCPI-6^T, ≈16 % EUR-108), C_16 : 0 iso (≈22.5 % and ≈13 %) and C_15 : 0 anteiso (19.5 % and ≈23.5 %, respectively). The percent G+C content of genomic DNA was determined to be 39.3 mol %. The 16S rRNA gene, mip sequence and comparative genome sequence-based analyses (average nucleotide identity, ANI; digital DNA-DNA hybridization, dDDH; and phylogenomic treeing) demonstrated that the strains represent a new species of the genus Legionella. The analysis based on the 16S rRNA gene sequences showed that the sequence similarities for both strains ranged from 98.8-90.1 % to other members of the genus. The core genome-based phylogenomic tree (protein-concatemer tree based on concatenation of 418 proteins present in single copy) revealed that these two strains clearly form a separate cluster within the genus Legionella. ANI and dDDH values confirmed the distinctiveness of the strains. Based on the genomic, genotypic and phenotypic findings from a polyphasic study, the isolates are considered to represent a single novel species, for which the name Legionella maioricensis sp. nov. is proposed. The type strain is HCPI-6^T (=CCUG 75071^T=CECT 30569^T).

Legionella feeleii: Ubiquitous Pathogen in the Environment and Causative Agent of Pneumonia

L. feeleii is one of the most frequent Legionella species isolated from natural pools of the central region of Spain. This study aimed to evaluate its ecology and to identify this Legionella species as a respiratory pathogen. A PCR assay for detecting the L. feeleii mip gene was developed to identify it in clinical and environmental samples. Culture and PCR were performed in environmental samples from four drinking water treatment plants (DWTPs). Free L. feeleii was only detected in raw water samples (3.4%), while L. feeleii as an Acanthamoeba endosymbiont was found in 30.7% of raw water, 11.5% of decanter biofilm, and 32% of finished water samples. Therefore, Acanthamoeba spp. plays an essential role in the multiplication, persistence, and spread of Legionella species in the environment. The first case of Legionnaires’ disease caused by L. feeleii in Spain is described in this study. The case was diagnosed in an older woman through PCR and sequencing from urine and sputum samples. A respiratory infection could be linked with health care procedures, and the patient presented several risk factors (age, insulin-dependent diabetes, and heart disease). The detection of non-L. pneumophila, such as L. feeleii, is a factor that must be considered when establishing or reviewing measures for the control and prevention of legionellosis.

Evaluation of MALDI-TOF Mass Spectrometry in Diagnostic and Environmental Surveillance of Legionella Species: A Comparison With Culture and Mip-Gene Sequencing Technique

Legionella spp. are widespread bacteria in aquatic environments with a growing impact on human health. Between the 61 species, Legionella pneumophila is the most prevalent in human diseases; on the contrary, Legionella non-pneumophila species are less detected in clinical diagnosis or during environmental surveillance due to their slow growth in culture and the absence of specific and rapid diagnostic/analytical tools. Reliable and rapid isolate identification is essential to estimate the source of infection, to undertake containment measures, and to determine clinical treatment. Matrix-assisted laser desorption ionization–time-of-flight mass spectrometry (MALDI–TOF MS), since its introduction into the routine diagnostics of laboratories, represents a widely accepted method for the identification of different bacteria species, described in a few studies on the Legionella clinical and environmental surveillance. The focus of this study was the improvement of MALDI–TOF MS on Legionella non-pneumophila species collected during Legionella nosocomial and community surveillance. Comparative analysis with cultural and mip-gene sequencing results was performed. Moreover, a phylogenetic analysis was carried out to estimate the correlations amongst isolates. MALDI–TOF MS achieved correct species-level identification for 45.0% of the isolates belonging to the Legionella anisa, Legionella rubrilucens, Legionella feeleii, and Legionella jordanis species, displaying a high concordance with the mip-gene sequencing results. In contrast, less reliable identification was found for the remaining 55.0% of the isolates, corresponding to the samples belonging to species not yet included in the database. The phylogenetic analysis showed relevant differences inside the species, regruped in three main clades; among the Legionella anisa clade, a subclade with a divergence of 3.3% from the main clade was observed. Moreover, one isolate, identified as Legionella quinlivanii, displayed a divergence of 3.8% from the corresponding reference strain. However, these findings require supplementary investigation. The results encourage the implementation of MALDI–TOF MS in routine diagnostics and environmental Legionella surveillance, as it displays a reliable and faster identification at the species level, as well as the potential to identify species that are not yet included in the database. Moreover, phylogenetic analysis is a relevant approach to correlate the isolates and to track their spread, especially in unconventional reservoirs, where Legionella prevention is still underestimated.

Optimization of viability qPCR for selective detection of membrane-intact Legionella pneumophila

Although a number of viability qPCR assays have been reported to selectively detect signals from membrane-intact Legionella pneumophila, the efficient suppression of amplification of DNA from dead membrane-compromised bacteria remains an ongoing challenge. This research aimed at establishing a new oligonucleotide combination that allows for a better exclusion of dead Legionella pneumophila on basis of the mip gene. Propidium monoazide (PMA) was chosen as viability dye. An oligonucleotide combination for the amplification of a 633 bp sequence was established with 100% specificity for different Legionella pneumophila strains compared with 17 other Legionella species tested. Apart from increasing amplicon length, the study aimed at optimizing dye incubation time and temperature. An incubation temperature of 45 °C for 10 min was found optimal. Dye treatment of heat-killed bacteria in the presence of EDTA improved signal suppression, whereas deoxycholate also affected signals from live intact bacteria. Suppression of signals from heat-treated bacteria was found to be approx. twice as efficient compared to a commercial kit, although the detection sensitivity is superior when targeting short amplicons. With a limit of detection of 10 genome copies per PCR well and a 6-log signal reduction of bacteria killed at 80 °C, the assay appears useful for applications where pathogen numbers are not limiting and where the priority is on the distinction between intact and damaged Legionella pneumophila for the evaluation of hygienic risk and of disinfection efficiency.

Convective Mixing in Distal Pipes Exacerbates Legionella pneumophila Growth in Hot Water Plumbing

Legionella pneumophila is known to proliferate in hot water plumbing systems, but little is known about the specific physicochemical factors that contribute to its regrowth. Here, L. pneumophila trends were examined in controlled, replicated pilot-scale hot water systems with continuous recirculation lines subject to two water heater settings (40 °C and 58 °C) and three distal tap water use frequencies (high, medium, and low) with two pipe configurations (oriented upward to promote convective mixing with the recirculating line and downward to prevent it). Water heater temperature setting determined where L. pneumophila regrowth occurred in each system, with an increase of up to 4.4 log gene copies/mL in the 40 °C system tank and recirculating line relative to influent water compared to only 2.5 log gene copies/mL regrowth in the 58 °C system. Distal pipes without convective mixing cooled to room temperature (23-24 °C) during periods of no water use, but pipes with convective mixing equilibrated to 30.5 °C in the 40 °C system and 38.8 °C in the 58 °C system. Corresponding with known temperature effects on L. pneumophila growth and enhanced delivery of nutrients, distal pipes with convective mixing had on average 0.2 log more gene copies/mL in the 40 °C system and 0.8 log more gene copies/mL in the 58 °C system. Importantly, this work demonstrated the potential for thermal control strategies to be undermined by distal taps in general, and convective mixing in particular.

Multiplex real-time PCR assay for Legionella species

Legionella pneumophila serogroup 1 (sg1) accounts for the majority of infections in humans, but other Legionella species are also associated with human disease. In this study, a new SYBR Green I-based multiplex real-time PCR assay in a single reaction was developed to allow the rapid detection and differentiation of Legionella species by targeting specific gene sequences. Candidate target genes were selected, and primer sets were designed by referring to comparative genomic hybridization data of Legionella species. The Legionella species-specific groES primer set successfully detected all 30 Legionella strains tested. The xcpX and rfbA primers specifically detected L. pneumophila sg1-15 and L. pneumophila sg1, respectively. In addition, this assay was validated by testing clinical samples and isolates. In conclusion, this novel multiplex real-time PCR assay might be a useful diagnostic tool for the rapid detection and differentiation of Legionella species in both clinical and epidemiological studies.

A new oligonucleotide microarray for detection of pathogenic and non-pathogenic Legionella spp

Legionella pneumophila has been recognized as the major cause of legionellosis since the discovery of the deadly disease. Legionella spp. other than L. pneumophila were later found to be responsible to many non-pneumophila infections. The non-L. pneumophila infections are likely under-detected because of a lack of effective diagnosis. In this report, we have sequenced the 16S-23S rRNA gene internal transcribed spacer (ITS) of 10 Legionella species and subspecies, including L. anisa, L. bozemanii, L. dumoffii, L. fairfieldensis, L. gormanii, L. jordanis, L. maceachernii, L. micdadei, L. pneumophila subspp. fraseri and L. pneumophila subspp. pasculleii, and developed a rapid oligonucleotide microarray detection technique accordingly to identify 12 most common Legionella spp., which consist of 11 pathogenic species of L. anisa, L. bozemanii, L. dumoffii, L. gormanii, L. jordanis, L. longbeachae, L. maceachernii, L. micdadei, and L. pneumophila (including subspp. pneumophila, subspp. fraseri, and subspp. pasculleii) and one non-pathogenic species, L. fairfieldensis. Twenty-nine probes that reproducibly detected multiple Legionella species with high specificity were included in the array. A total of 52 strains, including 30 target pathogens and 22 non-target bacteria, were used to verify the oligonucleotide microarray assay. The sensitivity of the detection was at 1.0 ng with genomic DNA or 13 CFU/100 mL with Legionella cultures. The microarray detected seven samples of air conditioner-condensed water with 100% accuracy, validating the technique as a promising method for applications in basic microbiology, clinical diagnosis, food safety, and epidemiological surveillance. The phylogenetic study based on the ITS has also revealed that the non-pathogenic L. fairfieldensis is the closest to L. pneumophila than the nine other pathogenic Legionella spp.

Sequence-based identification of legionella

Legionella strains are considered biologically inert with respect to traditional identification schemes. Various phenotypic alternatives have been attempted but all have lacked resolution as additional species have been added to what is proving to be a large genus. Only sequence-based schemes have the required resolution to confidently speciate or recognize potentially novel strains. The mip gene target is the most comprehensive currently available, with the added advantage of a Web-based analysis tool. Other gene targets are available for most if not all species, the best of which target 16S rRNA, rpoB, rnpB, or proA genes. One or several of these should be used to confirm important strains or clarify apparent novelness. The increased resolution of these sequence-based schemes has recognized many new species, and many more remain to be characterized. I provide a mip analysis of 44 such strains along with the recognized species, and a SplitsTree network analysis of recognized species and 20 novel strains for which sequence for the five targets is available.

Identification of Legionella pneumophila serogroups and other Legionella species by mip gene sequencing

The virulence factor known as the macrophage infectivity potentiator (mip) is responsible for the intracellular survival of Legionella species. In this study, we investigated the potential of the mip gene sequence to differentiate isolates of different species of Legionella and different serogroups of Legionella pneumophila. We used 35 clinical L. pneumophila isolates and one clinical isolate each of Legionella micdadei, Legionella longbeachae, and Legionella dumoffii (collected from hospitals all over Japan between 1980 and 2007). We used 19 environmental Legionella anisa isolates (collected in the Okinawa, Nara, Osaka, and Hyogo prefectures between 1987 and 2007) and two Legionella type strains. We extracted bacterial genomic DNA and amplified out the mip gene by PCR. PCR products were purified by agarose gel electrophoresis and the mip gene was then sequenced. The L. pneumophila isolates could be divided into two groups: one group was very similar to the type strain and was composed of serogroup (SG) 1 isolates only; the second group had more sequence variations and was composed of SG1 isolates as well as SG2, SG3, SG5, and SG10 isolates. Phylogenetic analysis displayed one cluster for L. anisa isolates, while other Legionella species were present at discrete levels. Our findings show that mip gene sequencing is an effective technique for differentiating L. pneumophila strains from other Legionella species.

A Legionella effector acquired from protozoa is involved in sphingolipids metabolism and is targeted to the host cell mitochondria

Legionella pneumophila infects alveolar macrophages and protozoa through establishment of an intracellular replication niche. This process is mediated by bacterial effectors translocated into the host cell via the Icm/Dot type IV secretion system. Most of the effectors identified so far are unique to L. pneumophila; however, some of the effectors are homologous to eukaryotic proteins. We performed a distribution analysis of many known L. pneumophila effectors and found that several of them, mostly eukaryotic homologous proteins, are present in different Legionella species. In-depth analysis of LegS2, a L. pneumophila homologue of the highly conserved eukaryotic enzyme sphingosine-1-phosphate lyase (SPL), revealed that it was most likely acquired from a protozoan organism early during Legionella evolution. The LegS2 protein was found to translocate into host cells using a C-terminal translocation domain absent in its eukaryotic homologues. LegS2 was found to complement the sphingosine-sensitive phenotype of a Saccharomyces serevisia SPL-null mutant and this complementation depended on evolutionary conserved residues in the LegS2 catalytic domain. Interestingly, unlike the eukaryotic SPL that localizes to the endoplasmic reticulum, LegS2 was found to be targeted mainly to host cell mitochondria. Collectively, our results demonstrate the remarkable adaptations of a eukaryotic protein to the L. pneumophila pathogenesis system.

Tracking airborne Legionella and Legionella pneumophila at a biological treatment plant

Biological treatment plants are frequently used to degrade organic substances in wastewater from wood refinement processes. Aeration ponds in such plants provide an optimal growth environment for many microorganisms, including Legionella species. To investigate whether legionellae could be dispersed as aerosols from the ponds and transported by the wind, the wetted-wall cyclone SASS 2000(PLUS) and the impactors MAS-100 and STA-204 were used to collect air samples directly above, upwind, and downwind of aeration ponds during a 4-month period. Computational fluid dynamics was used a priori to estimate the aerosol paths and to determine suitable air-sampling locations. Several Legionella species, including Legionella pneumophila, were identified in air samples at the biological treatment plant using microbiological and molecular methods. L. pneumophila was identified up to distances of 200 m downwind from the ponds, but, in general, not upwind nor outside the predicted aerosol paths. The highest concentration level of viable legionellae was identified directly above the aeration ponds (3300 CFU/m3). This level decreased as the distance from the aeration ponds increased. Molecular typing indicated that a single clone of L. pneumophila was dispersed from the ponds during the period of the study. Thus, our study demonstrated that aerosols generated at aeration ponds of biological treatment facilities may contain L. pneumophila, which then can be transported by the wind to the surroundings. The methods used in this study may be generically applied to trace biological aerosols that may pose a challenge to environmental occupational health.

Requirement of a mip-like gene for virulence in the phytopathogenic bacterium Xanthomonas campestris pv. campestris

Macrophage infectivity potentiators (Mips) are FKBP domain-containing proteins reported as virulence factors in several human pathogens, such as members of genera Legionella, Salmonella and Chlamydia. The putative peptidylprolyl cis-trans isomerase (PPIase) encoded by XC2699 of the plant bacterial pathogen Xanthomonas campestris pv. campestris 8004 exhibits a 49% similarity at the amino-acid level to the Mip protein of Legionella pneumophila. This mip-like gene, XC2699, was overexpressed in Escherichia coli and the purified (His)6-tagged Mip-like protein encoded by XC2699 exhibited a PPIase activity specifically inhibited by FK-506. A mutation in the mip-like gene XC2699 led to significant reductions in virulence and replication capacity in the host plant Chinese radish (Raphanus sativus L. var. radiculus Pers.). Furthermore, the production of exopolysaccharide and the activity of extracellular proteases, virulence factors of X. campestris pv. campestris, were significantly decreased in the mip-like mutant. These results reveal that the mip-like gene is involved in the pathogenesis of X. campestris pv. campestris through an effect on the production of these virulence factors.

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.

Analysis of the genetic diversity of Legionella by sequencing the 23S-5S ribosomal intergenic spacer region: from phylogeny to direct identification of isolates at the species level from clinical specimens

This study focuses on the interest of the hypervariable 23S-5S ribosomal intergenic spacer region (ISR) of the genus Legionella to analyze the phylogenic diversity of Legionella at the species and subspecies levels and to identify isolates directly from clinical specimens. The method, using a real-time PCR assay with a single primer pair followed by sequencing, was able to identify correctly 49 reference strains of Legionella belonging to 37 different species, including those implicated in human infections, and to clearly differentiate the three subspecies of L. pneumophila. Based on sequence similarities, the 23S-5S ISR sequences were much more variable than the rpoB and mip sequences (P<0.0001 by the Wilcoxon signed rank test). The 23S-5S ISR method was able to cluster Legionella species in accordance with phenotypic traits, such as autofluorescence or fatty acid membrane composition. Using maximum parsimony methods, the rpoB and 23S-5S ISR data sets were shown to be incongruent (P<0.001). In contrast, the 23S-5S ISR and the mip data sets were found to be congruent (P=0.313), suggesting the interest of combining these two regions to demonstrate phylogenetic links between Legionella species. This molecular assay was shown able to both detect Legionella DNA directly in respiratory specimens from patients exhibiting a Legionella infection and provide accurate identification of the bacterium at the species level in the tested specimens. These properties open a wide range of applications to the 23S-5S ISR sequencing method, from taxonomic analyses to clinical and epidemiological investigations.

Phylogenetic characterization of Legionella-like endosymbiotic X-bacteria in Amoeba proteus: a proposal for 'Candidatus Legionella jeonii' sp. nov

The X-bacteria which initiated organismic association with the D strain of Amoeba proteus in 1966 as parasites have changed to obligate endosymbionts on which the host depends for survival. Owing to the difficulty in cultivating the bacteria in vitro, the identity of X-bacteria has not been determined. As the life cycle of X-bacteria is similar to that of Legionella spp. in soil amoebae, we applied the polymerase chain reaction method with specific primers aimed at Legionella spp. for the detection and cloning of 16S rRNA gene. The identity and intracellular localization of the endosymbiont were confirmed by the application of a specific fluorescently labelled 16S rRNA-targeted probe. In addition we cloned RNA polymerase beta-subunit gene (rpoB) of X-bacteria by genomic library tagging. A phylogenetic analysis of the 16S rRNA gene placed the bacterium within a unique monophyletic group containing all other members of the genus Legionella. Phylogeny from rpoB and mip genes further confirmed the taxonomic context of X-bacteria to be a Legionella sp. In all three phylogenic analyses, X-bacterium was placed apart from Legionella-like amoebal pathogens present in soil amoebae. Thus, we propose the name 'Candidatus Legionella jeonii' sp. nov. for the endosymbiotic X-bacteria in Amoeba proteus.

Real-time PCR assay targets the 23S-5S spacer for direct detection and differentiation of Legionella spp. and Legionella pneumophila

A real-time PCR for the ABI Prism 7000 system targeting the 23S-5S spacer of Legionella spp. was developed. Simultaneous detection and differentiation of Legionella spp. and Legionella pneumophila within 90 min and without post-PCR melting-curve analysis was achieved using two TaqMan probes. In sputum samples from 23 controls and 17 patients with legionellosis, defined by positive culture, urinary antigen testing, or seroconversion, 94% sensitivity and 100% specificity were observed.

Biochemical and functional analyses of the Mip protein: influence of the N-terminal half and of peptidylprolyl isomerase activity on the virulence of Legionella pneumophila

The virulence factor Mip (macrophage infectivity potentiator) contributes to the intracellular survival of Legionella pneumophila, the causative agent of Legionnaires' disease. The protein consists of two domains that are connected via a very long alpha-helix (A. Riboldi-Tunnicliffe et al., Nat. Struct. Biol. 8:779-783, 2001). The fold of the C-terminal domain (residues 100 to 213) is closely related to human FK506-binding protein (FKBP12), and like FKBP12, Mip exhibits peptidylprolyl cis/trans isomerase (PPIase) activity. The alpha-helical N-terminal domain is responsible for the formation of very stable Mip homodimers. In order to determine the importance of the homodimeric state of Mip for its biochemical activities and for infectivity of Legionella, a truncated, monomeric Mip variant [Mip((77-213))] was overexpressed in Escherichia coli and characterized biochemically. In vitro isomerase activity assays revealed that the altered protein exhibits full isomerase activity towards peptide substrates. However, the deletion resulted in a dramatic loss in the efficiency of refolding of reduced and carboxy-methylated RNase T(1). By cis complementation of the Mip-negative mutant strain L. pneumophila JR32-2, we constructed the strain L. pneumophila JR32-2.4, which expresses an N-terminally truncated variant of Mip. Infection studies with these strains revealed that the N-terminal part and the dimerization of Mip but not its PPIase activity are necessary for full virulence in Acanthamoeba castellanii. Infection of guinea pigs showed that strains with dimerization-deficient Mip (JR32-2.4) or a very low PPIase activity (JR32-2.2) were significantly attenuated in the animal model. These results suggest a different role of the PPIase activity and the N-terminally mediated dimeric state of Mip in monocellular systems and during the infection of guinea pigs.

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.

Extrapulmonary Legionella micdadei infection in a previously healthy child

A previously healthy 9-year-old girl presented with a 1-week history of left neck mass and low grade fever. Surgical drainage specimens from the mass were bacterial culture-negative. Legionella micdadei infection was diagnosed by a two step PCR-nucleic acid sequencing method and supported by organism-specific serology. A backyard hot tub was suspected as the source of infection.

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.

Application of RNA polymerase beta-subunit gene (rpoB) sequences for the molecular differentiation of Legionella species

The nucleotide sequences of the partial rpoB gene were determined from 38 Legionella species, including 15 serogroups of Legionella pneumophila. These sequences were then used to infer the phylogenetic relationships among the Legionella species in order to establish a molecular differentiation method appropriate for them. The sequences (300 bp) and the phylogenetic tree of rpoB were compared to those from analyses using 16S rRNA gene and mip sequences. The trees inferred from these three gene sequences revealed significant differences. This sequence incongruence between the rpoB tree and the other trees might have originated from the high frequency of synonymous base substitutions and/or from horizontal gene transfer among the Legionella species. The nucleotide variation of rpoB enabled more evident differentiation among the Legionella species than was achievable by the 16S rRNA gene and even by mip in some cases. Two subspecies of L. pneumophila (L. pneumophila subsp. pneumophila and subsp. fraseri) were clearly distinguished by rpoB but not by 16S rRNA gene and mip analysis. One hundred and five strains isolated from patient tissues and environments in Korea and Japan could be identified by comparison of rpoB sequence similarity and phylogenetic trees. These results suggest that the partial sequences of rpoB determined in this study might be applicable to the molecular differentiation of Legionella species.

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.

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.

Legionella pneumophila: an aquatic microbe goes astray

Legionella pneumophila is naturally found in fresh water were the bacteria parasitize within protozoa. It also survives planctonically in water or biofilms. Upon aerosol formation via man-made water systems, L. pneumophila can enter the human lung and cause a severe form of pneumonia, called Legionnaires' disease. The pathogenesis of Legionnaires' disease is largely due to the ability of L. pneumophila to invade and grow within macrophages. An important characteristic of the intracellular survival strategy is the replication within the host vacuole that does not fuse with endosomes or lysosomes. In recent times a great number of bacterial virulence factors which affect growth of L. pneumophila in both macrophages and protozoa have been identified. The ongoing Legionella genome project and the use of genetically tractable surrogate hosts are expected to significantly contribute to the understanding of bacterium-host interactions and the regulation of virulence traits during the infection cycle. Since person-to-person transmission of legionellosis has never been observed, the measures for disease prevention have concentrated on eliminating the pathogen from water supplies. In this respect detection and analysis of Legionella in complex environmental consortia become increasingly important. With the availability of new molecular tools this area of applied research has gained new momentum.

Sequence diversification of the FK506-binding proteins in several different genomes

Sequences of FK506-binding proteins (FKBPs) from four genomes of the following organisms were compared: the prokaryote Escherichia coli, the lower eukaryote Saccharomyces cerevisiae, the plant Arabidopsis thaliana, the nematode Caenorhabditis elegans and a composite of 14 unique FKBPs from two mammalian organisms Homo sapiens (man) and Mus musculus (domestic mouse). A singular FK506-like binding domain (FKBD) has about 12 kDa and occurs in the form of archetypal FKBP-12 and as a part of different proteins ranging in size from 13 to 135 kDa. Some organisms may contain a variable number of proteins which consist from two to four consecutively fused FKBDs. In the 12-kDa subgroup of archetypal FKBPs sequence identity (ID) varies from 100 to 83% (mammalian FKBPs-12), 75-50% in mammalian vs. invertebrate FKBPs-12, and fall to about 30% for pairwise sequence comparisons of mammalian and bacterial FKBPs-12 which suggests that their sequences are divergent. Multiple sequence alignment of FKBPs from the four genomes and a set of unique mammalian FKBPs does not contain any explicit consensus sequence but certain sequence positions have conserved physico-chemical characteristics. Variations of hydrophobicity and bulkiness in the multiple sequence alignment are nonsymmetrical because the physico-chemical properties of the aligned sequences changed during evolution. These variations at the sequence positions which are crucial for binding the immunosuppressive macrolide FK506 and peptidyl-prolyl cis/trans isomerase (PPIase) activity are small.

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.

From protozoa to mammalian cells: a new paradigm in the life cycle of intracellular bacterial pathogens

It is becoming apparent that several intracellular bacterial pathogens of humans can also survive within protozoa. This interaction with protozoa may protect these pathogens from harsh conditions in the extracellular environment and enhance their infectivity in mammals. This relationship has been clearly established in the case of the interaction between Legionella pneumophila and its protozoan hosts. In addition, the adaptation of bacterial pathogens to the intracellular life within the primitive eukaryotic protozoa may have provided them with the means to infect the more evolved mammalian cells. This is evident from the existence of several similarities, at both the phenotypic and the molecular levels, between the infection of mammalian and protozoan cells by L. pneumophila. Thus, protozoa appear to play a central role in the transition of bacteria from the environment to mammals. In essence, protozoa may be viewed as a 'biological gym', within which intracellular bacterial pathogens train for their encounters with the more evolved mammalian cells. Thus, intracellular bacterial pathogens have benefited from the structural and biochemical conservation of cellular processes in eukaryotes. The interaction of intracellular bacterial pathogens and protozoa highlights this conservation and may constitute a simplified model for the study of these pathogens and the evolution of cellular processes in eukaryotes. Furthermore, in addition to being environmental reservoirs for known intracellular pathogens of humans and animals, protozoa may be sources of emerging pathogenic bacteria. It is thus critical to re-examine the relationship between bacteria and protozoa to further our understanding of current human bacterial pathogenesis and, possibly, to predict the appearance of emerging pathogens.

Legionella pneumophila pathogesesis: a fateful journey from amoebae to macrophages

Legionella pneumophila first commanded attention in 1976, when investigators from the Centers for Disease Control and Prevention identified it as the culprit in a massive outbreak of pneumonia that struck individuals attending an American Legion convention (). It is now clear that this gram-negative bacterium flourishes naturally in fresh water as a parasite of amoebae, but it can also replicate within alveolar macrophages. L. pneumophila pathogenesis is discussed using the following model as a framework. When ingested by phagocytes, stationary-phase L. pneumophila bacteria establish phagosomes which are completely isolated from the endosomal pathway but are surrounded by endoplasmic reticulum. Within this protected vacuole, L. pneumophila converts to a replicative form that is acid tolerant but no longer expresses several virulence traits, including factors that block membrane fusion. As a consequence, the pathogen vacuoles merge with lysosomes, which provide a nutrient-rich replication niche. Once the amino acid supply is depleted, progeny accumulate the second messenger guanosine 3',5'-bispyrophosphate (ppGpp), which coordinates entry into the stationary phase with expression of traits that promote transmission to a new phagocyte. A number of factors contribute to L. pneumophila virulence, including type II and type IV secretion systems, a pore-forming toxin, type IV pili, flagella, and numerous other factors currently under investigation. Because of its resemblance to certain aspects of Mycobacterium, Toxoplasma, Leishmania, and Coxiella pathogenesis, a detailed description of the mechanism used by L. pneumophila to manipulate and exploit phagocyte membrane traffic may suggest novel strategies for treating a variety of infectious diseases. Knowledge of L. pneumophila ecology may also inform efforts to combat the emergence of new opportunistic macrophage pathogens.

A PCR test for the identification and discrimination of Legionella longbeachae serogroups 1 and 2

A PCR test has been developed for the specific identification of Legionella longbeachae. The test targeted sequence unique to both L. longbeachae serogroups 1 and 2 within the mip gene and permitted both species and serogroup identification. The test was trialed on a range of closely related species and 20 clinical isolates originating from Australia, the USA and Israel. Results were consistent with previous identification analyses. From 20 water samples known to contain Legionella spp. one sample yielded isolates which consistently tested positive by L. longbeachae serogroup 1 PCR. DNA sequencing of the PCR product, 5S rRNA gene sequence and hybridisation analysis with a specific oligonucleotide probe definitively identified one isolate as L. longbeachae serogroup 1. PCR testing was demonstrated as a superior method of identification to traditional seroagglutination reactions, which were ambiguous and could explain the previous failure to identify the presence of this microorganism in water.

Sequence-based classification scheme for the genus Legionella targeting the mip gene

The identification and speciation of strains of Legionella is often difficult, and even the more successful chromatographic classification techniques have struggled to discriminate newly described species. A sequence-based genotypic classification scheme is reported, targeting approximately 700 nucleotide bases of the mip gene and utilizing gene amplification and direct amplicon sequencing. With the exception of Legionella geestiana, for which an amplicon was not produced, the scheme clearly and unambiguously discriminated among the remaining 39 Legionella species and correctly grouped 26 additional serogroup and reference strains within those species. Additionally, the genotypic classification of approximately 150 wild strains from several continents was consistent with their phenotypic classification, with the exception of a few strains where serological cross-reactivity was complex, potentially confusing the latter classification. Strains thought to represent currently uncharacterized species were also found to be genotypically unique. The scheme is technically simple for a laboratory with even basic molecular capabilities and equipment, if access to a sequencing laboratory is available.

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.