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Virulence. 2013 May 15; 4(4): 307–314.
Published online 2013 March 27. doi:  10.4161/viru.24290
PMCID: PMC3710333

Cellular microbiology and molecular ecology of Legionella–amoeba interaction

Abstract

Legionella pneumophila is an aquatic organism that interacts with amoebae and ciliated protozoa as the natural hosts, and this interaction plays a central role in bacterial ecology and infectivity. Upon transmission to humans, L. pneumophila infect and replicate within alveolar macrophages causing pneumonia. Intracellular proliferation of L. pneumophila within the two evolutionarily distant hosts is facilitated by bacterial exploitation of evolutionarily conserved host processes that are targeted by bacterial protein effectors injected into the host cell by the Dot/Icm type VIB translocation system. Although cysteine is semi-essential for humans and essential for amoeba, it is a metabolically favorable source of carbon and energy generation by L. pneumophila. To counteract host limitation of cysteine, L. pneumophila utilizes the AnkB Dot/Icm-translocated F-box effector to promote host proteasomal degradation of polyubiquitinated proteins within amoebae and human cells. Evidence indicates ankB and other Dot/Icm-translocated effector genes have been acquired through inter-kingdom horizontal gene transfer.

Keywords: polyubiquitin, farnesylation, prenylation, effectors AnkB, Ankyrin B, proteasomes, cysteine, ppGpp, RelA, SpoT, Ankyrin, Dot/Icm, pneumophila, Legionnaire

Legionnaire Disease

The first recognized outbreak of Legionnaire disease occurred in 1976 during the 56th annual American Legion Convention in Philadelphia.1 There were 180 individuals who were diagnosed with severe pneumonia and of these individuals, 29 died. Less than one year after the outbreak the causative agent of the disease was isolated.2 The bacterium, which was designated Legionella pneumophila, is a gram-negative facultative intracellular bacterium that proliferates within alveolar macrophages, causing Legionnaire disease.3 There are more than 50 species of Legionellae, but L. pneumophila continues to be responsible for more than 80% of cases of Legionnaire disease in most of the world. The exception is Western Australia, where L. longbeachae is the most predominant species in causing disease, but its pathogenesis is distinct from L. pneumophila.4-6

Legionnaire disease and Pontiac fever, which is a flu-like illness caused by Legionella, have only emerged within the past few decades due to human alterations to the environment. These alterations include the use of air conditioning systems, whirlpools, and water cooling towers that generate aerosols as a vehicle to transmit Legionella from aquatic sources (Fig. 1).3 To date there has been no documented cases of L. pneumophila transmission between individuals, and transmission from the environment to the human host is considered to be the main mode of transmission of L. pneumophila. Once L. pneumophila infects the human host, its intracellular lifecycle has a striking similarity to that within the evolutionarily distant natural host, amoebae (Fig. 1).7

figure viru-4-307-g1
Figure 1. The environmental life cycle of L. pneumophila within protozoa. (1) Flagellated L. pneumophila infect protozoa in the aquatic environment. (2) The LCV evades the default endosomal–lysosomal degradation pathway and becomes rapidly ...

Legionella–Amoebae Interaction

L. pneumophila is ubiquitous in freshwater environments as well as many man-made water systems worldwide, often in close association with freshwater protozoa.8L. pneumophila replicate at temperatures of 25–42 °C with an optimal growth temperature of 35 °C. Consistent with what L. pneumophila would encounter in the environment, motility and adherence to host cells are optimal at temperatures below 37 °C.9 When the temperature in the aquatic environment is increased, the balance between bacteria and amoebae can shift, which results in rapid multiplication of Legionella.10 The increase in the number of L. pneumophila in the water as a result of proliferation within protozoa increases the chance of transmission and disease manifestation.3 Upon decrease of temperature or exposure to environmental stress such as chlorine, the amoebae differentiate into cyst,10 and intracellular L. pneumophila are capable of survival within the cyst11 (Fig. 1). Encysted amoeba is a highly resistant developmental stage that contributes to the resistance of intracellular L. pneumophila to different chemical and physical agents.12 Therefore, the relationship between L. pneumophila and amoebae plays an important role in ecology and pathogenicity of the bacterium.13

One mechanism by which the bacteria are released from amoebae is within excreted vesicles similar to exocytosis of food vacuoles.14,15 L. pneumophila can still be cultured after 6 mo of residence within excreted vesicles.16,17 The number of bacteria isolated from water sources of transmission to humans during Legionnaire disease outbreaks is usually low or undetectable.7,8 It is thought that the enhanced infectivity of L. pneumophila as a result of growth within amoebae could compensate for the low infectious dose in the water sources.17-20 The ability of L. pneumophila to parasitize human macrophages and to cause human disease is thought to be a consequence of its prior adaptation to intracellular growth within various protozoan hosts.7,8 This is most likely due to bacterial acquisition of eukaryotic genes during its co-evolution with amoebae and adaptation to the intracellular life within primitive eukaryotic hosts.7,21-24

L. pneumophila and amoebae have been isolated from the same source of infection during outbreaks of Legionnaire disease.8 The isolated amoebae have also been shown to support intracellular replication of L. pneumophila.25 It has been shown that L. pneumophila that cannot be cultured in vitro using classical methods can be resuscitated and proliferate if they are co-cultured with amoebae.8,26,27Dictyostelium discoideum has been adapted as a genetically amenable amoeba model system to decipher molecular and cellular bases of L. pneumophila–amoeba interaction.28 Therefore, amoebae are not only the environmental host for this human pathogen, but constitute a genetically amenable model system to study pathogenesis of L. pneumophila. These findings demonstrate that studies on the L. pneumophila–amoeba interaction will continue to contribute to our knowledge of the central role of the amoeba host in the pathogenesis of these bacteria.

Amoebae Aid in the Persistence of Legionella pneumophila in the Environment

Amoebae not only enhance the pathogenicity of L. pneumophila, but they also enable the bacteria to persist in the environment. Fourteen species of amoebae, with Hartmannellae and Acanthamoeba being the most prominent, and two species of ciliated protozoa have been shown to support intracellular replication of L. pneumophila.8 L. pneumophila infects the trophozoite form of amoebae and the presence of the bacteria within amoebae serves to protect the bacteria from harsh environments.27 L. pneumophila does not proliferate within encysted amoeba but when conditions become unfavorable, protozoa can differentiate from their trophozoite form into a cyst form that protects the organisms and ensures their survival (Fig. 1). L. pneumophila released from free-living amoebae also show an increased resistance to harsh conditions compared with those grown in vitro.29-32 When compared with bacteria grown in vitro, bacteria grown in amoebae have changes in biochemistry, physiology, and virulence potential.12 These changes include an enhanced resistance to chemicals and antibiotics, an altered fatty acid profile and protein profile, shorter size, motility, an increased ability to infect amoebae and mammalian cells, an increase in environmental fitness, and an increase in uptake.8,19,20,26,27 In addition, the bacteria found within vesicles excreted from amoebae are highly resistant to biocides while the vesicle is resistant to freezing and sonication.17 After prolonged starvation of L. pneumophila or treatment with chlorine that renders L. pneumophila non-culturable in aquatic environments, the bacteria can’t be cultured on rich media but they can be resuscitated by infection of amoebae, clearly indicating the remarkable protection of L. pneumophila through its intracellular niche within amoebae.26,27

Numerous methods have been employed to attempt to eradicate L. pneumophila from aquatic environments, with little success. These attempts, which include chemical biocides, overheating water, and UV irradiation, have been successful for short periods after which the bacteria can be again detected. It has been suggested that in order to eradicate L. pneumophila from aquatic environments continuous treatments effective against both the bacteria and the protozoan host should be employed.8,12,27,33

Legionella-Like Amoebal Pathogens

There are Legionella-like species that cannot be grown on bacteriologic media but must be co-cultured with protozoa and are referred to as Legionella-like amoebal pathogens (LLAP).14 The LLAPs are closely related to Legionella phylogenetically and acquired their name because of their ability to infect and multiply within amoebae.14 It is thought the LLAPs play a role in community-acquired pneumonia and usually act as a co-pathogen but not as the sole pathogen.34 Little is known about LLAPs and future studies are needed to gain a better understanding of the significance of these organisms in pulmonary infections.

Entry and Intracellular Trafficking of L. pneumophila

L. pneumophila infection of human alveolar macrophages is an accidental infection and is thought to be a diversion from its natural life cycle within amoeba. In the aquatic environment, L. pneumophila resides in protozoa or in biofilms.35,36 Amoebae play a central role in the life cycle of L. pneumophila, and this was first described in 1980.37 Upon initial interaction between L. pneumophila and amoebae, L. pneumophila is often engulfed by coiling phagocytosis but other forms of internalization also occur through a Gal/Gal-NAC specific receptor.38,39 After internalization of L. pneumophila into the trophozoite of amoeba, proliferation occurs within the Legionella-containing vacuole (LCV) followed by bacterial release from the cell to seek a new host (Fig. 1).40-42 Once amoebae or mammalian cells engulf L. pneumophila, the bacteria evade the default trafficking pathways into the lysosomal network (Fig. 1). The LCV recruits host cell organelles, like mitochondria, ribosomes, and small vesicles to its surface.43 This accumulation begins during uptake into the cell and is completed within a few minutes.35 The ER-to-Golgi vesicle traffic is intercepted by the LCV and the LCV membrane becomes derived from the ER (Fig. 1).13,43 The LCV becomes rapidly decorated with polyubiquitinated proteins within amoebae and human cells.44-46 Following maturation of the ER-remodeled LCV and its decoration with polyubiquitinated proteins, rapid replication of L. pneumophila commences (Fig. 1). During late stages of intracellular proliferation, L. pneumophila escape from the LCV to the cytosol where the bacteria finish the last 1–2 rounds of proliferation along with phenotypic modulations in response to nutrient depletion in the host (Fig. 1).8,40,47

The strategy used by L. pneumophila to avoid lysosomes and to modulate cellular processes is dependent on the Dot/Icm type IVB secretion system.43,48,49 This secretion system injects ~300 effector proteins into the host cell, which accounts for ~10% of the coding capacity of the genome of L. pneumophila.7,50,51 Although a large number of effectors are injected into the host cell, but with only few exceptions, deletion of individual effectors does not result in reduced intracellular proliferation, suggesting potential functional redundancy.7,43,51,52 Strikingly, many L. pneumophila effector proteins harbor eukaryotic protein domains, which include ankyrin repeats, leucine-rich repeats, Sel-1, U-box, F-box, and a C-terminal CaaX prenylation motif.23,53-58 Expression of a large number of effectors is induced during the exponential phase of intracellular growth within macrophages and Acanthamoeba,47,59-61 but the function of most of these proteins has yet to be determined.

Growth phase-dependent regulation of bacterial virulence

The intracellular lifecycle of L. pneumophila consists of a replicative phase within the LCV, and a transmissive phase, exhibited upon escape into the cytosol.40,47,62,63 This biphasic lifestyle is characterized by dramatic changes in the transcriptome, that result in phenotypic modulations.59-61 During the replicative phase, the bacterium is undergoing exponential (E) growth; it is non-motile and represses transmissive traits, such as lysosomal evasion (Fig. 1).64 The stringent-like response is triggered upon transition of L. pneumophila into post-exponential (PE) growth, when the bacteria become cytotoxic, motile, sodium-sensitive, osmotic-resistant, and capable of lysosomal evasion.63,64 These phenotypic modulations are necessary to escape from the wasted host and invade a new host to start a second cycle of intracellular proliferation.41,42,47,59,60,63-65

The transition between replicative and transmissive phenotypes is highly orchestrated, and is governed by many factors that are influenced by intracellular nutrient levels.7,24,63,66 Upon amino acid depletion, uncharged bacterial tRNAs activate RelA to synthesize the bacterial alarmone 3′,5′-bispyrophosphate (ppGpp), a master regulator of numerous genes of L. pneumophila, which triggers phenotypic modulations upon transition into the PE phase.63 SpoT, a bifunctional synthetase/hydrolase that responds to a variety of stimuli, such as fatty acid starvation, also synthesizes ppGpp leading to increased levels of the alarmone (Fig. 1).67 RpoS and several global response two-component regulators, such as LetA/S and PmrA/B, function as downstream cascades of regulatory networks that govern phenotypic modulations at the PE phase.61,66,68-70 Small non-coding RNAs, such as RsmY and RsmZ, are induced at the PE phase by the regulatory cascade of networks triggered by elevated ppGpp levels.66,68,71 The RNA polymerase interacting protein, DskA, also responds to increased levels of ppGpp and other stress signals to coordinate phenotypic modulations of L. pneumophila at the PE phase and its transmission to a new host.67

In addition to triggering flagellation and various virulence-related traits, elevated ppGpp levels result in upregulation of the type IV-secretion components and many of its exported effectors.59-61 One of the Dot-Icm-translocated effectors important in the intracellular infection of amoebae and human cells is the eukaryotic-like AnkB,33,44,45 which is temporally and differentially regulated at the PE phase.33,35,61,72 Therefore, complex cascades of regulatory networks govern phenotypic transition at the PE phase and most or all of these networks are under the direct or indirect control of ppGpp.

In addition to phenotypic modulations at the PE phase, L. pneumophila undergoes a differentiation cycle that is dimorphic, cycling between a replicating form and a planktonic spore-like cyst form, designated as mature intracellular form (MIF).73-75 The MIF is near dormant metabolically, resistant to detergents and antibiotics, and is more invasive.73 The MIFs are detectable in HeLa cells but do not form in macrophages, which is likely due to early apoptotic lysis within 1–3 d of the infection, while the MIFs are formed later in HeLa cells.73 The MIFs of L. pneumophila germinate following entry into a susceptible eukaryotic host cell or in rich media in vitro. It is possible that the MIFs contribute to the ecology of L. pneumophila during starvation in the water system when nutrients are depleted and the amoebal hosts are encysted and not susceptible to infection.

Exploitation of conserved eukaryotic processes by the eukaryotic-like AnkB effector of L. pneumophila

L. pneumophila harbors a plethora of eukaryotic-like effectors that interfere with host processes by mimicking eukaryotic functions.21,23,53,54 Many translocated effectors of L. pneumophila are functionally and structurally similar to eukaryotic proteins and interact with and disrupt various eukaryotic processes such as signaling, protein synthesis, apoptosis, posttranslational modification, vesicular trafficking, ubiquitination, and proteasomal degradation.43 Among the ~300 effectors of L. pneumophila, AnkB is the only effector known to be indispensable for the intracellular infection of both human cells and amoebae, and the biological function of this effector has been deciphered. It is not surprising that recent studies on the AnkB effector and its exploitation of multiple highly conserved eukaryotic processes may just be the tip of the iceberg of our continued unraveling of L. pneumophila-host interaction and its evolution from invading amoebae to invading human cells and causing pneumonia.

The AnkB effector harbors multiple eukaryotic domains that enable this protein to hijack a number of evolutionarily conserved eukaryotic processes, and is essential for intracellular proliferation of L. pneumophila in amoebae and human cells and for virulence in the mouse model.33,44,45,76 The AnkB effector harbors two Ankyrin domains (ANK), 33-residue repeats involved in protein-protein interactions, and is the most common domain in eukaryotic proteins.56 AnkB also contains a C-terminal eukaryotic CaaX motif (C, cysteine; a, aliphatic amino acid; X, any amino acid) that allows the protein to be lipidated through farnesylation by the host farnesyltransferase (FTase), which anchors AnkB into the LCV membrane (Fig. 2).58,77-79 Farnesylation is a type of prenylation that covalently links a 15-carbon lipid moiety to a conserved cysteine residue within the C-terminus “CaaX” motif, which confers hydrophobicity enabling the lipidated protein to be anchored into the lipid bi-layer of eukaryotic membranes.58 However, there is variation in the C-terminus CaaX motif of AnkB, as some isolates, such as the Paris strain of L. pneumophila, have a truncated C-terminus without a CaaX motif.45 The biological relevance of this variation is still to be determined.

figure viru-4-307-g2
Figure 2. Nutritional and metabolic adaptation of L. pneumophila to the intracellular life within amoebae and human cells is facilitated by the AnkB effector and its exploitation of multiple highly conserved eukaryotic processes. The AnkB effector ...

Ubiquitination of proteins is a highly conserved eukaryotic post-translation modification that is mediated by three enzymes (E1–E3); E1 is the activating enzyme which transfers a 76-amino acid ubiquitin polypeptide to the conjugating enzyme (E2) while the E3 ubiquitin ligase links ubiquitin to the target protein.7,76 Polyubiquitin is formed by linking ubiquitin monomers through one of the 7 lysine (K) residues of ubiquitin. Polyubiquitination through K48 linkages targets the modified protein for proteolytic degradation by the proteasomes.76 The AnkB effector is a bona fide F-box protein that binds the E3 eukaryotic ubiquitin ligase and functions as a platform for the assembly of K48-linked polyubiquitinated proteins on the LCV (Fig. 2),44,45 which occurs within a few minutes of bacterial entry.45,80 Proteasomal degradation of K48-linked polyubiquitinated proteins results in increased cellular levels of amino acids (Fig. 2),7,80 which are essential for intracellular proliferation of L. pneumophila that is dependent on amino acids as the major source of carbon and energy to feed the tri-carboxylic acid (TCA) cycle.81

The ankB mutant of L. pneumophila is severely defective in intracellular proliferation in amoebae and human macrophages due to the defect in assembly of K48-linked polyubiquitinated proteins decorating the LCV (Fig. 1).7,45,80 Due to lack of proteasomal degradation of K48-linked polyubiquitin during infection by the ankB mutant, cellular levels of amino acid do not increase. This triggers a bacterial starvation response, mediated by the induced expression of RelA and SpoT, and results in elevated ppGpp levels.7,80 Intracellular growth can be restored to the ankB mutant within amoebae and human cells by supplementing excess amino acids.7,24,80 Thus, higher levels of cellular amino acids are needed for intracellular replication of L. pneumophila. Remarkably, supplementation of infected cells with certain single amino acids, such as cysteine, reverses the growth defect of the ankB mutant in amoebae and human cells. Interestingly, in human cells cysteine is semi-essential and is the least abundant amino acid, but in amoebae cysteine is essential.24,80 Similar to cysteine, supplementation of infected cells with pyruvate or citrate to feed the TCA cycle, rescues the ankB mutant for intracellular proliferation.7,24,80 Interestingly, in vitro growth of L. pneumophila in rich medium requires supplementation with 3.3 mM cysteine.80 Therefore, AnkB is a remarkable example of an effector involved in exploitation of multiple host processes that are highly conserved in unicellular eukaryotes and mammals.7 By promoting proteasomal degradation in amoebae and human cells though the AnkB F-box effector, L. pneumophila generates a gratuitous supply of cellular amino acids (Fig. 2), particularly the limiting ones such as Cys, which is a metabolically favorable source of carbon and energy for L. pneumophila to power intracellular growth within amoebae and human cells.24

Nutritional adaptation of L. pneumophila to amoebae

Amino acids are the main sources of carbon and energy to feed the TCA cycle of L. pneumophila, which has a defect in the glycolytic pathway, but carbohydrate metabolism plays a minor role in contribution to central metabolism.81 Cysteine and Serine are particularly important to feed central metabolism of L. pneumophila. Both amino acids are converted into pyruvate that feed the TCA cycle, which is the primary pathway of carbon and energy production in L. pneumophila.24,80,81 L. pneumophila and its primary host (Acanthamoebae or Dictyostelium discoideum) are both auxotrophic for cysteine while L. pneumophila is also auxotrophic for arginine, isoleucine, leucine, methionine, valine, and threonine.7,24 Interestingly, toxicity of proteasomal inhibition in human cells results from the lack of protein synthesis due to low levels of limiting amino acids, particularly cysteine.82 Therefore, through high nutritional dependence of L. pneumophila on host limiting amino acids, such as Cys, and synchronization of amino acid auxotrophy with its host, L. pneumophila synchronizes its nutritional needs for growth with the availability of nutrients for the host.7,24 This remarkable nutritional adaptation could be what allows the bacteria to be protected during amoebal encystation upon nutrient depletion, leading to cessation of intracellular bacterial growth (Fig. 1). Encysted amoebae are thought to be protected against invasion by L. pneumophila.75 Therefore, potential growth and release of L. pneumophila from amoeba in an aquatic environment in which amoebae have encysted in response to nutrient depletion is unlikely to happen, since it would result in free-living bacteria without a susceptible host, which may result is eventual loss of bacterial viability. However, potential differentiation into the MIF under starvation conditions may facilitate continued presence of L. pneumophila in the aquatic environment under these conditions.73,74

Acquisition of eukaryotic genes through inter-kingdom horizontal gene transfer

The long-term co-evolution of L. pneumophila with various protists and metazoa has influenced the genomic structure of this organism through inter-kingdom horizontal gene transfer (HGT).7,23,83,84 This long-term co-evolution is likely what gave rise to the acquisition of eukaryotic host genes encoding proteins with eukaryotic-like functions and structures. Amoeba may act as a gene melting pot, allowing diverse microorganisms to evolve by gene acquisition and loss, and then either adapt to the intra-amoebal lifestyle or evolve into new pathogens. Interestingly, mammalian F-box proteins do not have the ANK domain, while F-box proteins from amoebae do.7,24,56,76 Therefore, it is more likely that ankB had been acquired through inter-kingdom HGT from a primitive eukaryotic host.7,24,76 L. pneumophila is a naturally competent organism that takes up DNA and can exchange DNA between bacteria through conjugation.9,48,49 Long-term convergent evolution and modification of the genes acquired through HGT, splicing of introns, acquisition of prokaryotic promoters and regulators, and translocation motifs is likely what allowed eukaryotic-like proteins to become translocated effectors with functional activities in the host cell.83 It is to be expected that many of the eukaryotic-like proteins in L. pneumophila are still undergoing convergent evolution through modifications that might enable them to become translocated and functionally active effectors.7

Long-term co-evolution with its protozoan hosts has likely contributed to the ability of L. pneumophila to cause disease in humans, perpetuated by changes in human lifestyle. Understanding its association with amoebae will give us a better understanding of how L. pneumophila causes human disease though exploitation of evolutionary conserved eukaryotic processes.7,24,80 Since L. pneumophila also exploits mammalian-specific processes such as the inflammasomes and pro- and anti-apoptosis,85-92 it is likely that additional virulence properties have been acquired by L. pneumophila to enhance its capacity to infect humans. Since many other pathogens are detected within amoebae, this primitive eukaryotic host may represent a reservoir for many human pathogens.93

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Footnotes

References

1. Fraser DW, Tsai TR, Orenstein W, Parkin WE, Beecham HJ, Sharrar RG, et al. Legionnaires’ disease: description of an epidemic of pneumonia. N Engl J Med. 1977;297:1189–97. doi: 10.1056/NEJM197712012972201. [PubMed] [Cross Ref]
2. McDade JE, Shepard CC, Fraser DW, Tsai TR, Redus MA, Dowdle WR. Legionnaires’ disease: isolation of a bacterium and demonstration of its role in other respiratory disease. N Engl J Med. 1977;297:1197–203. doi: 10.1056/NEJM197712012972202. [PubMed] [Cross Ref]
3. Fields BS, Benson RF, Besser RE. Legionella and Legionnaires’ disease: 25 years of investigation. Clin Microbiol Rev. 2002;15:506–26. doi: 10.1128/CMR.15.3.506-526.2002. [PMC free article] [PubMed] [Cross Ref]
4. Asare R, Santic M, Gobin I, Doric M, Suttles J, Graham JE, et al. Genetic susceptibility and caspase activation in mouse and human macrophages are distinct for Legionella longbeachae and L. pneumophila. Infect Immun. 2007;75:1933–45. doi: 10.1128/IAI.00025-07. [PMC free article] [PubMed] [Cross Ref]
5. Asare R, Abu Kwaik Y. Early trafficking and intracellular replication of Legionella longbeachaea within an ER-derived late endosome-like phagosome. Cell Microbiol. 2007;9:1571–87. doi: 10.1111/j.1462-5822.2007.00894.x. [PubMed] [Cross Ref]
6. Cazalet C, Gomez-Valero L, Rusniok C, Lomma M, Dervins-Ravault D, Newton HJ, et al. Analysis of the Legionella longbeachae genome and transcriptome uncovers unique strategies to cause Legionnaires’ disease. PLoS Genet. 2010;6:e1000851. doi: 10.1371/journal.pgen.1000851. [PMC free article] [PubMed] [Cross Ref]
7. Al-Quadan T, Price CT, Abu Kwaik Y. Exploitation of evolutionarily conserved amoeba and mammalian processes by Legionella. Trends Microbiol. 2012;20:299–306. doi: 10.1016/j.tim.2012.03.005. [PMC free article] [PubMed] [Cross Ref]
8. Molmeret M, Horn M, Wagner M, Santic M, Abu Kwaik Y. Amoebae as training grounds for intracellular bacterial pathogens. Appl Environ Microbiol. 2005;71:20–8. doi: 10.1128/AEM.71.1.20-28.2005. [PMC free article] [PubMed] [Cross Ref]
9. Stone BJ, Kwaik YA. Natural competence for DNA transformation by Legionella pneumophila and its association with expression of type IV pili. J Bacteriol. 1999;181:1395–402. [PMC free article] [PubMed]
10. Ohno A, Kato N, Sakamoto R, Kimura S, Yamaguchi K. Temperature-dependent parasitic relationship between Legionella pneumophila and a free-living amoeba (Acanthamoeba castellanii) Appl Environ Microbiol. 2008;74:4585–8. doi: 10.1128/AEM.00083-08. [PMC free article] [PubMed] [Cross Ref]
11. Kilvington S, Price J. Survival of Legionella pneumophila within cysts of Acanthamoeba polyphaga following chlorine exposure. J Appl Bacteriol. 1990;68:519–25. doi: 10.1111/j.1365-2672.1990.tb02904.x. [PubMed] [Cross Ref]
12. Abu Kwaik Y, Gao L-Y, Stone BJ, Venkataraman C, Harb OS. Invasion of protozoa by Legionella pneumophila and its role in bacterial ecology and pathogenesis. Appl Environ Microbiol. 1998;64:3127–33. [PMC free article] [PubMed]
13. Segal G, Shuman HA. Legionella pneumophila utilizes the same genes to multiply within Acanthamoeba castellanii and human macrophages. Infect Immun. 1999;67:2117–24. [PMC free article] [PubMed]
14. Adeleke A, Pruckler J, Benson R, Rowbotham T, Halablab M, Fields BS. Legionella-like amebal pathogens--phylogenetic status and possible role in respiratory disease. Emerg Infect Dis. 1996;2:225–30. doi: 10.3201/eid0203.960311. [PMC free article] [PubMed] [Cross Ref]
15. Chen J, de Felipe KS, Clarke M, Lu H, Anderson OR, Segal G, et al. Legionella effectors that promote nonlytic release from protozoa. Science. 2004;303:1358–61. doi: 10.1126/science.1094226. [PubMed] [Cross Ref]
16. Bouyer S, Imbert C, Rodier MH, Héchard Y. Long-term survival of Legionella pneumophila associated with Acanthamoeba castellanii vesicles. Environ Microbiol. 2007;9:1341–4. doi: 10.1111/j.1462-2920.2006.01229.x. [PubMed] [Cross Ref]
17. Berk SG, Ting RS, Turner GW, Ashburn RJ. Production of respirable vesicles containing live Legionella pneumophila cells by two Acanthamoeba spp. Appl Environ Microbiol. 1998;64:279–86. [PMC free article] [PubMed]
18. O’Brien SJ, Bhopal RS. Legionnaires’ disease: the infective dose paradox. Lancet. 1993;342:5–6. doi: 10.1016/0140-6736(93)91877-O. [PubMed] [Cross Ref]
19. Cirillo JD, Falkow S, Tompkins LS. Growth of Legionella pneumophila in Acanthamoeba castellanii enhances invasion. Infect Immun. 1994;62:3254–61. [PMC free article] [PubMed]
20. Cirillo JD, Cirillo SL, Yan L, Bermudez LE, Falkow S, Tompkins LS. Intracellular growth in Acanthamoeba castellanii affects monocyte entry mechanisms and enhances virulence of Legionella pneumophila. Infect Immun. 1999;67:4427–34. [PMC free article] [PubMed]
21. Lurie-Weinberger MN, Gomez-Valero L, Merault N, Glöckner G, Buchrieser C, Gophna U. The origins of eukaryotic-like proteins in Legionella pneumophila. Int J Med Microbiol. 2010;300:470–81. doi: 10.1016/j.ijmm.2010.04.016. [PubMed] [Cross Ref]
22. Buchrieser C. Legionella: from protozoa to humans. Front Microbiol. 2011;2:182. doi: 10.3389/fmicb.2011.00182. [PMC free article] [PubMed] [Cross Ref]
23. Gomez-Valero L, Rusniok C, Cazalet C, Buchrieser C. Comparative and functional genomics of legionella identified eukaryotic like proteins as key players in host-pathogen interactions. Front Microbiol. 2011;2:208. doi: 10.3389/fmicb.2011.00208. [PMC free article] [PubMed] [Cross Ref]
24. Price C, Abu Kwaik Y. Amoebae and Mammals Deliver Protein-Rich Atkins Diet Meals to Legionella. Microbe. 2012;7:506–13.
25. Fields BS, Nerad TA, Sawyer TK, King CH, Barbaree JM, Martin WT, et al. Characterization of an axenic strain of Hartmannella vermiformis obtained from an investigation of nosocomial legionellosis. J Protozool. 1990;37:581–3. [PubMed]
26. Steinert M, Emödy L, Amann R, Hacker J. Resuscitation of viable but nonculturable Legionella pneumophila Philadelphia JR32 by Acanthamoeba castellanii. Appl Environ Microbiol. 1997;63:2047–53. [PMC free article] [PubMed]
27. García MT, Jones S, Pelaz C, Millar RD, Abu Kwaik Y. Acanthamoeba polyphaga resuscitates viable non-culturable Legionella pneumophila after disinfection. Environ Microbiol. 2007;9:1267–77. doi: 10.1111/j.1462-2920.2007.01245.x. [PubMed] [Cross Ref]
28. Hägele S, Köhler R, Merkert H, Schleicher M, Hacker J, Steinert M. Dictyostelium discoideum: a new host model system for intracellular pathogens of the genus Legionella. Cell Microbiol. 2000;2:165–71. doi: 10.1046/j.1462-5822.2000.00044.x. [PubMed] [Cross Ref]
29. Abu Kwaik Y, Gao L-Y, Harb OS, Stone BJ. Transcriptional regulation of the macrophage-induced gene (gspA) of Legionella pneumophila and phenotypic characterization of a null mutant. Mol Microbiol. 1997;24:629–42. doi: 10.1046/j.1365-2958.1997.3661739.x. [PubMed] [Cross Ref]
30. Barker J, Brown MRW, Collier PJ, Farrell I, Gilbert P. Relationship between Legionella pneumophila and Acanthamoeba polyphaga: physiological status and susceptibility to chemical inactivation. Appl Environ Microbiol. 1992;58:2420–5. [PMC free article] [PubMed]
31. Barker J, Lambert PA, Brown MRW. Influence of intra-amoebic and other growth conditions on the surface properties of Legionella pneumophila. Infect Immun. 1993;61:3503–10. [PMC free article] [PubMed]
32. Barker J, Scaife H, Brown MRW. Intraphagocytic growth induces an antibiotic-resistant phenotype of Legionella pneumophila. Antimicrob Agents Chemother. 1995;39:2684–8. doi: 10.1128/AAC.39.12.2684. [PMC free article] [PubMed] [Cross Ref]
33. Al-Khodor S, Price CT, Habyarimana F, Kalia A, Abu Kwaik YA. A Dot/Icm-translocated ankyrin protein of Legionella pneumophila is required for intracellular proliferation within human macrophages and protozoa. Mol Microbiol. 2008;70:908–23. [PMC free article] [PubMed]
34. Marrie TJ, Raoult D, La Scola B, Birtles RJ, de Carolis E, Canadian Community-Acquired Pneumonia Study Group Legionella-like and other amoebal pathogens as agents of community-acquired pneumonia. Emerg Infect Dis. 2001;7:1026–9. doi: 10.3201/eid0706.010619. [PMC free article] [PubMed] [Cross Ref]
35. Bitar DM, Molmeret M, Abu Kwaik Y. Molecular and cell biology of Legionella pneumophila. Int J Med Microbiol. 2004;293:519–27. doi: 10.1078/1438-4221-00286. [PubMed] [Cross Ref]
36. Molmeret M, Bitar DM, Han L, Kwaik YA. Cell biology of the intracellular infection by Legionella pneumophila. Microbes Infect. 2004;6:129–39. doi: 10.1016/j.micinf.2003.11.004. [PubMed] [Cross Ref]
37. Rowbotham TJ. Preliminary report on the pathogenicity of Legionella pneumophila for freshwater and soil amoebae. J Clin Pathol. 1980;33:1179–83. doi: 10.1136/jcp.33.12.1179. [PMC free article] [PubMed] [Cross Ref]
38. Venkataraman C, Haack BJ, Bondada S, Abu Kwaik Y. Identification of a Gal/GalNAc lectin in the protozoan Hartmannella vermiformis as a potential receptor for attachment and invasion by the Legionnaires’ disease bacterium. J Exp Med. 1997;186:537–47. doi: 10.1084/jem.186.4.537. [PMC free article] [PubMed] [Cross Ref]
39. Venkataraman C, Gao L-Y, Bondada S, Kwaik YA. Identification of putative cytoskeletal protein homologues in the protozoan host Hartmannella vermiformis as substrates for induced tyrosine phosphatase activity upon attachment to the Legionnaires’ disease bacterium, Legionella pneumophila. J Exp Med. 1998;188:505–14. doi: 10.1084/jem.188.3.505. [PMC free article] [PubMed] [Cross Ref]
40. Molmeret M, Bitar DM, Han L, Kwaik YA. Disruption of the phagosomal membrane and egress of Legionella pneumophila into the cytoplasm during the last stages of intracellular infection of macrophages and Acanthamoeba polyphaga. Infect Immun. 2004;72:4040–51. doi: 10.1128/IAI.72.7.4040-4051.2004. [PMC free article] [PubMed] [Cross Ref]
41. Alli OAT, Gao L-Y, Pedersen LL, Zink S, Radulic M, Doric M, et al. Temporal pore formation-mediated egress from macrophages and alveolar epithelial cells by Legionella pneumophila. Infect Immun. 2000;68:6431–40. doi: 10.1128/IAI.68.11.6431-6440.2000. [PMC free article] [PubMed] [Cross Ref]
42. Gao L-Y, Kwaik YA. The mechanism of killing and exiting the protozoan host Acanthamoeba polyphaga by Legionella pneumophila. Environ Microbiol. 2000;2:79–90. doi: 10.1046/j.1462-2920.2000.00076.x. [PubMed] [Cross Ref]
43. Isberg RR, O’Connor TJ, Heidtman M. The Legionella pneumophila replication vacuole: making a cosy niche inside host cells. Nat Rev Microbiol. 2009;7:13–24. doi: 10.1038/nrmicro1967. [PMC free article] [PubMed] [Cross Ref]
44. Price CT, Al-Khodor S, Al-Quadan T, Santic M, Habyarimana F, Kalia A, et al. Molecular mimicry by an F-box effector of Legionella pneumophila hijacks a conserved polyubiquitination machinery within macrophages and protozoa. PLoS Pathog. 2009;5:e1000704. doi: 10.1371/journal.ppat.1000704. [PMC free article] [PubMed] [Cross Ref]
45. Lomma M, Dervins-Ravault D, Rolando M, Nora T, Newton HJ, Sansom FM, et al. The Legionella pneumophila F-box protein Lpp2082 (AnkB) modulates ubiquitination of the host protein parvin B and promotes intracellular replication. Cell Microbiol. 2010;12:1272–91. doi: 10.1111/j.1462-5822.2010.01467.x. [PubMed] [Cross Ref]
46. Dorer MS, Kirton D, Bader JS, Isberg RR. RNA interference analysis of Legionella in Drosophila cells: exploitation of early secretory apparatus dynamics. PLoS Pathog. 2006;2:e34. doi: 10.1371/journal.ppat.0020034. [PMC free article] [PubMed] [Cross Ref]
47. Molmeret M, Jones S, Santic M, Habyarimana F, Esteban MT, Kwaik YA. Temporal and spatial trigger of post-exponential virulence-associated regulatory cascades by Legionella pneumophila after bacterial escape into the host cell cytosol. Environ Microbiol. 2010;12:704–15. doi: 10.1111/j.1462-2920.2009.02114.x. [PMC free article] [PubMed] [Cross Ref]
48. Vogel JP, Andrews HL, Wong SK, Isberg RR. Conjugative transfer by the virulence system of Legionella pneumophila. Science. 1998;279:873–6. doi: 10.1126/science.279.5352.873. [PubMed] [Cross Ref]
49. Segal G, Purcell M, Shuman HA. Host cell killing and bacterial conjugation require overlapping sets of genes within a 22-kb region of the Legionella pneumophila genome. Proc Natl Acad Sci U S A. 1998;95:1669–74. doi: 10.1073/pnas.95.4.1669. [PubMed] [Cross Ref]
50. Zhu W, Banga S, Tan Y, Zheng C, Stephenson R, Gately J, et al. Comprehensive identification of protein substrates of the Dot/Icm type IV transporter of Legionella pneumophila. PLoS One. 2011;6:e17638. doi: 10.1371/journal.pone.0017638. [PMC free article] [PubMed] [Cross Ref]
51. Luo ZQ. Targeting One of its Own: Expanding Roles of Substrates of the Legionella Pneumophila Dot/Icm Type IV Secretion System. Front Microbiol. 2011;2:31. doi: 10.3389/fmicb.2011.00031. [PMC free article] [PubMed] [Cross Ref]
52. Luo ZQ. Striking a balance: modulation of host cell death pathways by legionella pneumophila. Front Microbiol. 2011;2:36. doi: 10.3389/fmicb.2011.00036. [PMC free article] [PubMed] [Cross Ref]
53. Cazalet C, Rusniok C, Brüggemann H, Zidane N, Magnier A, Ma L, et al. Evidence in the Legionella pneumophila genome for exploitation of host cell functions and high genome plasticity. Nat Genet. 2004;36:1165–73. doi: 10.1038/ng1447. [PubMed] [Cross Ref]
54. Chien M, Morozova I, Shi S, Sheng H, Chen J, Gomez SM, et al. The genomic sequence of the accidental pathogen Legionella pneumophila. Science. 2004;305:1966–8. doi: 10.1126/science.1099776. [PubMed] [Cross Ref]
55. Habyarimana F, Al-Khodor S, Kalia A, Graham JE, Price CT, Garcia MT, et al. Role for the Ankyrin eukaryotic-like genes of Legionella pneumophila in parasitism of protozoan hosts and human macrophages. Environ Microbiol. 2008;10:1460–74. doi: 10.1111/j.1462-2920.2007.01560.x. [PubMed] [Cross Ref]
56. Al-Khodor S, Price CT, Kalia A, Abu Kwaik Y. Functional diversity of ankyrin repeats in microbial proteins. Trends Microbiol. 2010;18:132–9. doi: 10.1016/j.tim.2009.11.004. [PMC free article] [PubMed] [Cross Ref]
57. Price CTD, Jones SC, Amundson KE, Kwaik YA. Host-mediated post-translational prenylation of novel dot/icm-translocated effectors of legionella pneumophila. Front Microbiol. 2010;1:131. doi: 10.3389/fmicb.2010.00131. [PMC free article] [PubMed] [Cross Ref]
58. Al-Quadan T, Price CT, London N, Schueler-Furman O, AbuKwaik Y. Anchoring of bacterial effectors to host membranes through host-mediated lipidation by prenylation: a common paradigm. Trends Microbiol. 2011;19:573–9. doi: 10.1016/j.tim.2011.08.003. [PubMed] [Cross Ref]
59. Faucher SP, Mueller CA, Shuman HA. Legionella Pneumophila Transcriptome during Intracellular Multiplication in Human Macrophages. Front Microbiol. 2011;2:60. doi: 10.3389/fmicb.2011.00060. [PMC free article] [PubMed] [Cross Ref]
60. Brüggemann H, Hagman A, Jules M, Sismeiro O, Dillies MA, Gouyette C, et al. Virulence strategies for infecting phagocytes deduced from the in vivo transcriptional program of Legionella pneumophila. Cell Microbiol. 2006;8:1228–40. doi: 10.1111/j.1462-5822.2006.00703.x. [PubMed] [Cross Ref]
61. Al-Khodor S, Kalachikov S, Morozova I, Price CT, Abu Kwaik Y. The PmrA/PmrB two-component system of Legionella pneumophila is a global regulator required for intracellular replication within macrophages and protozoa. Infect Immun. 2009;77:374–86. doi: 10.1128/IAI.01081-08. [PMC free article] [PubMed] [Cross Ref]
62. Amer AO, Swanson MS. A phagosome of one’s own: a microbial guide to life in the macrophage. Curr Opin Microbiol. 2002;5:56–61. doi: 10.1016/S1369-5274(02)00286-2. [PubMed] [Cross Ref]
63. Molofsky AB, Swanson MS. Differentiate to thrive: lessons from the Legionella pneumophila life cycle. Mol Microbiol. 2004;53:29–40. doi: 10.1111/j.1365-2958.2004.04129.x. [PubMed] [Cross Ref]
64. Joshi AD, Swanson MS. Comparative analysis of Legionella pneumophila and Legionella micdadei virulence traits. Infect Immun. 1999;67:4134–42. [PMC free article] [PubMed]
65. Swanson MS, Hammer BK. Legionella pneumophila pathogesesis: a fateful journey from amoebae to macrophages. Annu Rev Microbiol. 2000;54:567–613. doi: 10.1146/annurev.micro.54.1.567. [PubMed] [Cross Ref]
66. Rasis M, Segal G. The LetA-RsmYZ-CsrA regulatory cascade, together with RpoS and PmrA, post-transcriptionally regulates stationary phase activation of Legionella pneumophila Icm/Dot effectors. Mol Microbiol. 2009;72:995–1010. doi: 10.1111/j.1365-2958.2009.06705.x. [PubMed] [Cross Ref]
67. Dalebroux ZD, Yagi BF, Sahr T, Buchrieser C, Swanson MS. Distinct roles of ppGpp and DksA in Legionella pneumophila differentiation. Mol Microbiol. 2010;76:200–19. doi: 10.1111/j.1365-2958.2010.07094.x. [PMC free article] [PubMed] [Cross Ref]
68. Molofsky AB, Swanson MS. Legionella pneumophila CsrA is a pivotal repressor of transmission traits and activator of replication. Mol Microbiol. 2003;50:445–61. doi: 10.1046/j.1365-2958.2003.03706.x. [PubMed] [Cross Ref]
69. Bachman MA, Swanson MS. RpoS co-operates with other factors to induce Legionella pneumophila virulence in the stationary phase. Mol Microbiol. 2001;40:1201–14. doi: 10.1046/j.1365-2958.2001.02465.x. [PubMed] [Cross Ref]
70. Abu-Zant A, Asare R, Graham JE, Abu Kwaik Y. Role for RpoS but not RelA of Legionella pneumophila in modulation of phagosome biogenesis and adaptation to the phagosomal microenvironment. Infect Immun. 2006;74:3021–6. doi: 10.1128/IAI.74.5.3021-3026.2006. [PMC free article] [PubMed] [Cross Ref]
71. Sahr T, Brüggemann H, Jules M, Lomma M, Albert-Weissenberger C, Cazalet C, et al. Two small ncRNAs jointly govern virulence and transmission in Legionella pneumophila. Mol Microbiol. 2009;72:741–62. doi: 10.1111/j.1365-2958.2009.06677.x. [PMC free article] [PubMed] [Cross Ref]
72. Al-Khodor S, Al-Quadan T, Abu Kwaik Y. Temporal and differential regulation of expression of the eukaryotic-like ankyrin effectors of Legionella pneumophila. Environ Microbiol Rep. 2010;2:677–84. doi: 10.1111/j.1758-2229.2010.00159.x. [PubMed] [Cross Ref]
73. Garduño RA, Garduño E, Hiltz M, Hoffman PS. Intracellular growth of Legionella pneumophila gives rise to a differentiated form dissimilar to stationary-phase forms. Infect Immun. 2002;70:6273–83. doi: 10.1128/IAI.70.11.6273-6283.2002. [PMC free article] [PubMed] [Cross Ref]
74. Faulkner G, Garduño RA. Ultrastructural analysis of differentiation in Legionella pneumophila. J Bacteriol. 2002;184:7025–41. doi: 10.1128/JB.184.24.7025-7041.2002. [PMC free article] [PubMed] [Cross Ref]
75. Greub G, Raoult D. Morphology of Legionella pneumophila according to their location within Hartmanella vermiformis. Res Microbiol. 2003;154:619–21. doi: 10.1016/j.resmic.2003.08.003. [PubMed] [Cross Ref]
76. Price CT, Kwaik YA. Exploitation of Host Polyubiquitination Machinery through Molecular Mimicry by Eukaryotic-Like Bacterial F-Box Effectors. Front Microbiol. 2010;1:122. doi: 10.3389/fmicb.2010.00122. [PMC free article] [PubMed] [Cross Ref]
77. Al-Quadan T, Kwaik YA. Molecular Characterization of Exploitation of the Polyubiquitination and Farnesylation Machineries of Dictyostelium Discoideum by the AnkB F-Box Effector of Legionella Pneumophila. Front Microbiol. 2011;2:23. doi: 10.3389/fmicb.2011.00023. [PMC free article] [PubMed] [Cross Ref]
78. Price CT, Al-Quadan T, Santic M, Jones SC, Abu Kwaik Y. Exploitation of conserved eukaryotic host cell farnesylation machinery by an F-box effector of Legionella pneumophila. J Exp Med. 2010;207:1713–26. doi: 10.1084/jem.20100771. [PMC free article] [PubMed] [Cross Ref]
79. Price CT, Al-Khodor S, Al-Quadan T, Abu Kwaik Y. Indispensable role for the eukaryotic-like ankyrin domains of the ankyrin B effector of Legionella pneumophila within macrophages and amoebae. Infect Immun. 2010;78:2079–88. doi: 10.1128/IAI.01450-09. [PMC free article] [PubMed] [Cross Ref]
80. Price CT, Al-Quadan T, Santic M, Rosenshine I, Abu Kwaik Y. Host proteasomal degradation generates amino acids essential for intracellular bacterial growth. Science. 2011;334:1553–7. doi: 10.1126/science.1212868. [PubMed] [Cross Ref]
81. Eylert E, Herrmann V, Jules M, Gillmaier N, Lautner M, Buchrieser C, et al. Isotopologue profiling of Legionella pneumophila: role of serine and glucose as carbon substrates. J Biol Chem. 2010;285:22232–43. doi: 10.1074/jbc.M110.128678. [PubMed] [Cross Ref]
82. Suraweera A, Münch C, Hanssum A, Bertolotti A. Failure of amino acid homeostasis causes cell death following proteasome inhibition. Mol Cell. 2012;48:242–53. doi: 10.1016/j.molcel.2012.08.003. [PMC free article] [PubMed] [Cross Ref]
83. de Felipe KS, Pampou S, Jovanovic OS, Pericone CD, Ye SF, Kalachikov S, et al. Evidence for acquisition of Legionella type IV secretion substrates via interdomain horizontal gene transfer. J Bacteriol. 2005;187:7716–26. doi: 10.1128/JB.187.22.7716-7726.2005. [PMC free article] [PubMed] [Cross Ref]
84. Franco IS, Shuman HA, Charpentier X. The perplexing functions and surprising origins of Legionella pneumophila type IV secretion effectors. Cell Microbiol. 2009;11:1435–43. doi: 10.1111/j.1462-5822.2009.01351.x. [PubMed] [Cross Ref]
85. Amer AO. Modulation of caspases and their non-apoptotic functions by Legionella pneumophila. Cell Microbiol. 2010;12:140–7. doi: 10.1111/j.1462-5822.2009.01401.x. [PubMed] [Cross Ref]
86. Molmeret M, Zink SD, Han L, Abu-Zant A, Asari R, Bitar DM, et al. Activation of caspase-3 by the Dot/Icm virulence system is essential for arrested biogenesis of the Legionella-containing phagosome. Cell Microbiol. 2004;6:33–48. doi: 10.1046/j.1462-5822.2003.00335.x. [PubMed] [Cross Ref]
87. Abu-Zant A, Santic M, Molmeret M, Jones S, Helbig J, Abu Kwaik Y. Incomplete activation of macrophage apoptosis during intracellular replication of Legionella pneumophila. Infect Immun. 2005;73:5339–49. doi: 10.1128/IAI.73.9.5339-5349.2005. [PMC free article] [PubMed] [Cross Ref]
88. Abu-Zant A, Jones S, Asare R, Suttles J, Price C, Graham J, et al. Anti-apoptotic signalling by the Dot/Icm secretion system of L. pneumophila. Cell Microbiol. 2007;9:246–64. doi: 10.1111/j.1462-5822.2006.00785.x. [PubMed] [Cross Ref]
89. Gao L-Y, Abu Kwaik Y. Activation of caspase-3 in Legionella pneumophila-induced apoptosis. Infect Immun. 1999;67:4886–94. [PMC free article] [PubMed]
90. Abdelaziz DH, Gavrilin MA, Akhter A, Caution K, Kotrange S, Khweek AA, et al. Apoptosis-associated speck-like protein (ASC) controls Legionella pneumophila infection in human monocytes. J Biol Chem. 2011;286:3203–8. doi: 10.1074/jbc.M110.197681. [PMC free article] [PubMed] [Cross Ref]
91. Abdelaziz DH, Gavrilin MA, Akhter A, Caution K, Kotrange S, Khweek AA, et al. Asc-dependent and independent mechanisms contribute to restriction of legionella pneumophila infection in murine macrophages. Front Microbiol. 2011;2:18. doi: 10.3389/fmicb.2011.00018. [PMC free article] [PubMed] [Cross Ref]
92. Akhter A, Caution K, Abu Khweek A, Tazi M, Abdulrahman BA, Abdelaziz DH, et al. Caspase-11 promotes the fusion of phagosomes harboring pathogenic bacteria with lysosomes by modulating actin polymerization. Immunity. 2012;37:35–47. doi: 10.1016/j.immuni.2012.05.001. [PMC free article] [PubMed] [Cross Ref]
93. Pagnier I, Raoult D, La Scola B. Isolation and identification of amoeba-resisting bacteria from water in human environment by using an Acanthamoeba polyphaga co-culture procedure. Environ Microbiol. 2008;10:1135–44. doi: 10.1111/j.1462-2920.2007.01530.x. [PubMed] [Cross Ref]
94. Sauer JD, Bachman MA, Swanson MS. The phagosomal transporter A couples threonine acquisition to differentiation and replication of Legionella pneumophila in macrophages. Proc Natl Acad Sci U S A. 2005;102:9924–9. doi: 10.1073/pnas.0502767102. [PubMed] [Cross Ref]

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