Plant models for animal pathogenesis

A Marine Bacterium with Animal-Pathogen-Like Type III Secretion Elicits the Nonhost Hypersensitive Response in a Land Plant

Active plant immune response involving programmed cell death called the hypersensitive response (HR) is elicited by microbial effectors delivered through the type III secretion system (T3SS). The marine bacterium Hahella chejuensis contains two T3SSs that are similar to those of animal pathogens, but it was able to elicit HR-like cell death in the land plant Nicotiana benthamiana. The cell death was comparable with the transcriptional patterns of H. chejuensis T3SS-1 genes, was mediated by SGT1, a general regulator of plant resistance, and was suppressed by AvrPto1, a type III-secreted effector of a plant pathogen that inhibits HR. Thus, type III-secreted effectors of a marine bacterium are capable of inducing the nonhost HR in a land plant it has never encountered before. This suggests that plants may have evolved to cope with a potential threat posed by alien pathogen effectors. Our work documents an exceptional case of nonhost HR and provides an expanded perspective for studying plant nonhost resistance.

Duckweed (Lemna minor) and Alfalfa (Medicago sativa) as Bacterial Infection Model Systems

Alternative animal host models of bacterial infection have been developed which reproduce some of the disease conditions observed in higher animals. Analogously, plants are useful for modeling bacterial pathogenesis, in some cases revealing broadly conserved infection mechanisms. Similar to animals, plants have been shown to possess innate immune systems that respond to invading viruses, bacteria, and fungi. Plant infection models often yield results faster, are more convenient, and less expensive than many animal infection models. Here, we describe the use of two different plant-based infection models for the discovery of virulence genes and factors involved in bacterial pathogenesis.

EXPERIMENTAL EVIDENCES ON A CROP PLANT ROLE IN EPIDEMIOLOGY OF SAPRONOTIC (SOIL-BORNE) BACTERIAL INFECTIONS

Specific epidemiology of sapronotic (soil-borne) bacteria is characterized from the ecological point of view. The characteristic feature of soil-borne pathogens is an ability to exist autonomously in the environment. This analytical review is focused on crops as alternative hosts for a number of soil-borne pathogenic bacteria (Yersinia, Salmonella, Listeria, Escherichia etc). Published experimental results evidence capabilities of human and animal pathogens to colonize plant tissues. Novel approaches are discussed to minimize risks of infection spreading with crops. These approaches include an analysis of wild plant natural resistance to pathogenic bacteria and a construction of transgenic plant crops expressing antimicrobial peptides. Multiple studies are cited that established wild plants used in traditional medicine as a source for obtaining molecules effective against resistant pathogens. The review includes recent author results on activity of wild plant extracts against Listeria and toxin-producing Escherichia.

The cysteine dioxygenase homologue from Pseudomonas aeruginosa is a 3-mercaptopropionate dioxygenase

Thiol dioxygenation is the initial oxidation step that commits a thiol to important catabolic or biosynthetic pathways. The reaction is catalyzed by a family of specific non-heme mononuclear iron proteins each of which is reported to react efficiently with only one substrate. This family of enzymes includes cysteine dioxygenase, cysteamine dioxygenase, mercaptosuccinate dioxygenase, and 3-mercaptopropionate dioxygenase. Using sequence alignment to infer cysteine dioxygenase activity, a cysteine dioxygenase homologue from Pseudomonas aeruginosa (p3MDO) has been identified. Mass spectrometry of P. aeruginosa under standard growth conditions showed that p3MDO is expressed in low levels, suggesting that this metabolic pathway is available to the organism. Purified recombinant p3MDO is able to oxidize both cysteine and 3-mercaptopropionic acid in vitro, with a marked preference for 3-mercaptopropionic acid. We therefore describe this enzyme as a 3-mercaptopropionate dioxygenase. Mössbauer spectroscopy suggests that substrate binding to the ferrous iron is through the thiol but indicates that each substrate could adopt different coordination geometries. Crystallographic comparison with mammalian cysteine dioxygenase shows that the overall active site geometry is conserved but suggests that the different substrate specificity can be related to replacement of an arginine by a glutamine in the active site.

Common duckweed (Lemna minor) is a versatile high-throughput infection model for the Burkholderia cepacia complex and other pathogenic bacteria

Members of the Burkholderia cepacia complex (Bcc) have emerged in recent decades as problematic pulmonary pathogens of cystic fibrosis (CF) patients, with severe infections progressing to acute necrotizing pneumonia and sepsis. This study presents evidence that Lemna minor (Common duckweed) is useful as a plant model for the Bcc infectious process, and has potential as a model system for bacterial pathogenesis in general. To investigate the relationship between Bcc virulence in duckweed and Galleria mellonella (Greater wax moth) larvae, a previously established Bcc infection model, a duckweed survival assay was developed and used to determine LD₅₀ values. A strong correlation (R² = 0.81) was found between the strains’ virulence ranks in the two infection models, suggesting conserved pathways in these vastly different hosts. To broaden the application of the duckweed model, enteropathogenic Escherichia coli (EPEC) and five isogenic mutants with previously established LD₅₀ values in the larval model were tested against duckweed, and a strong correlation (R² = 0.93) was found between their raw LD₅₀ values. Potential virulence factors in B. cenocepacia K56-2 were identified using a high-throughput screen against single duckweed plants. In addition to the previously characterized antifungal compound (AFC) cluster genes, several uncharacterized genes were discovered including a novel lysR regulator, a histidine biosynthesis gene hisG, and a gene located near the gene encoding the recently characterized virulence factor SuhB_Bc. Finally, to demonstrate the utility of this model in therapeutic applications, duckweed was rescued from Bcc infection by treating with bacteriophage at 6-h intervals. It was observed that phage application became ineffective at a timepoint that coincided with a sharp increase in bacterial invasion of plant tissue. These results indicate that common duckweed can serve as an effective infection model for the investigation of bacterial virulence factors and therapeutic strategies to combat them.

Wide screening of phage-displayed libraries identifies immune targets in planta

Microbe-Associated Molecular Patterns and virulence effectors are recognized by plants as a first step to mount a defence response against potential pathogens. This recognition involves a large family of extracellular membrane receptors and other immune proteins located in different sub-cellular compartments. We have used phage-display technology to express and select for Arabidopsis proteins able to bind bacterial pathogens. To rapidly identify microbe-bound phage, we developed a monitoring method based on microarrays. This combined strategy allowed for a genome-wide screening of plant proteins involved in pathogen perception. Two phage libraries for high-throughput selection were constructed from cDNA of plants infected with Pseudomonas aeruginosa PA14, or from combined samples of the virulent isolate DC3000 of Pseudomonas syringae pv. tomato and its avirulent variant avrRpt2. These three pathosystems represent different degrees in the specificity of plant-microbe interactions. Libraries cover up to 2×10⁷ different plant transcripts that can be displayed as functional proteins on the surface of T7 bacteriophage. A number of these were selected in a bio-panning assay for binding to Pseudomonas cells. Among the selected clones we isolated the ethylene response factor ATERF-1, which was able to bind the three bacterial strains in competition assays. ATERF-1 was rapidly exported from the nucleus upon infiltration of either alive or heat-killed Pseudomonas. Moreover, aterf-1 mutants exhibited enhanced susceptibility to infection. These findings suggest that ATERF-1 contains a microbe-recognition domain with a role in plant defence. To identify other putative pathogen-binding proteins on a genome-wide scale, the copy number of selected-vs.-total clones was compared by hybridizing phage cDNAs with Arabidopsis microarrays. Microarray analysis revealed a set of 472 candidates with significant fold change. Within this set defence-related genes, including well-known targets of bacterial effectors, are over-represented. Other genes non-previously related to defence can be associated through this study with general or strain-specific recognition of Pseudomonas.

Exploring the applications of invertebrate host-pathogen models for in vivo biofilm infections

In the natural environment, microorganisms exist together in self-produced polymeric matrix biofilms. Often, several species, which can belong to both bacterial and fungal kingdoms, coexist and interact in ways which are not completely understood. Biofilm infections have become prevalent largely in medical settings because of the increasing use of indwelling medical devices such as catheters or prosthetics. These infections are resistant to common antimicrobial therapies because of the inherent nature of their structure. In terms of infectious biofilms, it is important to understand the microbe-microbe interactions and how the host immune system reacts in order to discover therapeutic targets. Currently, single infection immune response studies are thriving with the use of invertebrate models. This review highlights the advances in single microbial-host immune response as well as the promising aspects of polymicrobial biofilm study in five invertebrate models: Lemna minor (duckweed), Arabidopsis thaliana (thale cress), Dictyostelium discoideum (slime mold), Drosophila melanogaster (common fruit fly), and Caenorhabditis elegans (roundworm).

Virulence of an exotoxin A-deficient strain of Pseudomonas aeruginosa toward the silkworm, Bombyx mori

We studied the contribution of exotoxin A to the virulence of Pseudomonas aeruginosa against the silkworm, Bombyx mori. First, an exotoxin A-deficient mutant strain (PAO1toxA) was created, and its virulence compared with that of the parental PAO1 strain. In a short-term mortality assay, the mutant harboring pBBR1MCS2 did not kill B. mori until 120 h after inoculation and complementation of the corresponding gene in trans restored the strain's virulence. Next, to ascertain whether or not it lost all virulence, PAO1toxA (pBBR1MCS2, pGFP) was used in a long-term mortality assay. B. mori inoculated with the mutant strain did not die until early in the 5th instar (240 h after inoculation). However, 50% of the inoculated B. mori died late in the 5th instar or in the early pupal stage (408 h after inoculation). All had died by the pupal stage (600 h after inoculation). The mutant strain was isolated from dead larvae and cocoons. The bacterial population of PAO1toxA in hemolymph reached 4.77 × 10(7) cfu/ml. These results indicated that exotoxin A acts as a virulence factor in B. mori and that other virulence factor(s) are involved during the late stages of infection.

Conservation of Salmonella infection mechanisms in plants and animals

Salmonella virulence in animals depends on effectors injected by Type III Secretion Systems (T3SSs). In this report we demonstrate that Salmonella mutants that are unable to deliver effectors are also compromised in infection of Arabidopsis thaliana plants. Transcriptome analysis revealed that in contrast to wild type bacteria, T3SS mutants of Salmonella are compromised in suppressing highly conserved Arabidopsis genes that play a prominent role during Salmonella infection of animals. We also found that Salmonella originating from infected plants are equally virulent for human cells and mice. These results indicate a high degree of conservation in the defense and infection mechanism of animal and plant hosts during Salmonella infection.

Function of the bacteriophytochrome BphP in the RpoS/Las quorum-sensing network of Pseudomonas aeruginosa

The bacterial phytochrome of Pseudomonas aeruginosa (PaBphP) is an in vitro-active red/far-red light sensor histidine kinase of a two-component regulatory system. Despite solid biochemical data, its function in this heterotrophic, opportunistic pathogen is still unknown. Previous studies established that the genes encoding the two necessary phytochrome components BphO, a chromophore-producing haem oxygenase, and BphP, the apo-phytochrome, are co-transcribed in a bicistronic operon. Transcription has been shown to be induced in the stationary phase and to be dependent on the alternative sigma factor RpoS. Here we show an additional regulation of bphP expression through the quorum-sensing (QS) regulator LasR. This regulation is also reflected in a combination of expression profile experiments and proteome analyses of wild-type and phytochrome-deficient strains. While PaBphP has a pleiotropic effect on global gene expression, 66 % of the downregulated genes in the phytochrome mutant display a link to the Las QS system. Most of these genes seem to be indirectly regulated by LasR through BphP and the unknown response regulator BphR. A model of phytochrome function within the Las QS network is presented.

Pseudomonas aeruginosa PAO1 virulence factors and poplar tree response in the rhizosphere

Whole-transcriptome analysis was used here for the first time in the rhizosphere to discern the genes involved in the pathogenic response of Pseudomonas aeruginosa PAO1 as well as to discern the response of the poplar tree. Differential gene expression shows that 185 genes of the bacterium and 753 genes of the poplar tree were induced in the rhizosphere. Using the P. aeruginosa transcriptome analysis, isogenic knockout mutants, and two novel plant assays (poplar and barley), seven novel PAO1 virulence genes were identified (PA1385, PA2146, PA2462, PA2463, PA2663, PA4150 and PA4295). The uncharacterized putative haemolysin repressor, PA2463, upon inactivation, resulted in greater poplar virulence and elevated haemolysis while this mutant remained competitive in the rhizosphere. In addition, disruption of the haemolysin gene itself (PA2462) reduced the haemolytic activity of P. aeruginosa, caused less cytotoxicity and reduced barley virulence, as expected. Inactivating PA1385, a putative glycosyl transferase, reduced both poplar and barley virulence. Furthermore, disrupting PA2663, a putative membrane protein, reduced biofilm formation by 20-fold. Inactivation of PA3476 (rhlI) increased virulence with barley as well as haemolytic activity and cytotoxicity, so quorum sensing is important in plant pathogenesis. Hence, this strategy is capable of elucidating virulence genes for an important pathogen.

Duckweed (Lemna minor) as a model plant system for the study of human microbial pathogenesis

Background

Plant infection models provide certain advantages over animal models in the study of pathogenesis. However, current plant models face some limitations, e.g., plant and pathogen cannot co-culture in a contained environment. Development of such a plant model is needed to better illustrate host-pathogen interactions.

Methodology/Principal Findings

We describe a novel model plant system for the study of human pathogenic bacterial infection on a large scale. This system was initiated by co-cultivation of axenic duckweed (Lemna minor) plants with pathogenic bacteria in 24-well polystyrene cell culture plate. Pathogenesis of bacteria to duckweed was demonstrated with Pseudomonas aeruginosa and Staphylococcus aureus as two model pathogens. P. aeruginosa PAO1 caused severe detriment to duckweed as judged from inhibition to frond multiplication and chlorophyll formation. Using a GFP-marked PAO1 strain, we demonstrated that bacteria colonized on both fronds and roots and formed biofilms. Virulence of PAO1 to duckweed was attenuated in its quorum sensing (QS) mutants and in recombinant strains overexpressing the QS quenching enzymes. RN4220, a virulent strain of S. aureus, caused severe toxicity to duckweed while an avirulent strain showed little effect. Using this system for antimicrobial chemical selection, green tea polyphenols exhibited inhibitory activity against S. aureus virulence. This system was further confirmed to be effective as a pathogenesis model using a number of pathogenic bacterial species.

Conclusions/Significance

Our results demonstrate that duckweed can be used as a fast, inexpensive and reproducible model plant system for the study of host-pathogen interactions, could serve as an alternative choice for the study of some virulence factors, and could also potentially be used in large-scale screening for the discovery of antimicrobial chemicals.

Identification of tomato plant as a novel host model for Burkholderia pseudomallei

BACKGROUND

Burkholderia pseudomallei is the causative agent for melioidosis, a disease with significant mortality and morbidity in endemic regions. Its versatility as a pathogen is reflected in its relatively huge 7.24 Mb genome and the presence of many virulence factors including three Type Three Secretion Systems known as T3SS1, T3SS2 and T3SS3. Besides being a human pathogen, it is able to infect and cause disease in many different animals and alternative hosts such as C. elegans.

RESULTS

Its host range is further extended to include plants as we demonstrated the ability of B. pseudomallei and the closely related species B. thailandensis to infect susceptible tomato but not rice plants. Bacteria were found to multiply intercellularly and were found in the xylem vessels of the vascular bundle. Disease is substantially attenuated upon infection with bacterial mutants deficient in T3SS1 or T3SS2 and slightly attenuated upon infection with the T3SS3 mutant. This shows the importance of both T3SS1 and T3SS2 in bacterial pathogenesis in susceptible plants.

CONCLUSIONS

The potential of B. pseudomallei as a plant pathogen raises new possibilities of exploiting plant as an alternative host for novel anti-infectives or virulence factor discovery. It also raises issues of biosecurity due to its classification as a potential bioterrorism agent.

Potassium and sodium transporters of Pseudomonas aeruginosa regulate virulence to barley

We investigated the role of three uncharacterized cation transporters of Pseudomonas aeruginosa PAO1 as virulence factors for barley: PA1207, PA5021, and PA2647. PAO1 displayed reduced barley virulence with inactivated PA1207, PA5021, and PA2647 as well as with one known Na(+)/H(+) antiporter, PA1820. Using the Escherichia coli LB2003 mutant lacking three K(+) uptake systems, the expression of the PA5021 gene repressed LB2003 growth with low K(+), but the strain acquired tolerance to high K(+). In contrast, the expression of the PA1207 gene enhanced growth of LB2003 with low K(+) but repressed its growth with high K(+); therefore, the PA5021 protein exports K(+), while the PA1207 protein imports K(+). The PA5021 mutant of P. aeruginosa also showed impaired growth at 400 mM KCl and at 400 mM NaCl; therefore, the PA5021 protein may also export Na(+). The loss of the PA5021 protein also decreased production of the virulence factor pyocyanin; corroborating this result, pyocyanin production decreased in wild-type PAO1 under high salinity. Whole-genome transcriptome analysis showed that PAO1 induced more genes in barley upon infection compared to the PA5021 mutant. Additionally, PAO1 infection induced water stress-related genes in barley, which suggests that barley may undergo water deficit upon infection by this pathogen.

Inactivation of pyocyanin synthesis genes has no effect on the virulence of Pseudomonas aeruginosa PAO1 toward the silkworm, Bombyx mori

The contribution of pyocyanin to the virulence of Pseudomonas aeruginosa against the silkworm Bombyx mori was studied. First, purified pyocyanin was injected into the hemocoel of B. mori. Acute toxicity was observed only when a high dose of pyocyanin was injected. The lethal dose 50% value of pyocyanin was found to be 9.52 microg per larva. Next, mutant strains of phzM and phzS, which encode putative phenazine-specific methytransferase and flavin-containing monooxygenase, respectively, were created, and their virulence was compared with that of the PAO1 parent strain. Although the ability to produce pyocyanin was completely lost in the phz-mutant strains, they maintained the same level of virulence as the PAO1 parent strain. In addition, the complementation of the corresponding gene in trans in the mutant strains did not have any effect on the virulence of those mutant strains. These results indicated that pyocyanin does not act as a virulence factor in B. mori after invasion, which was different from the results obtained in other Lepidopteran host models.

Bacteria-derived peptidoglycans constitute pathogen-associated molecular patterns triggering innate immunity in Arabidopsis

Pathogen-associated molecular pattern (PAMP)-triggered immunity constitutes the primary plant immune response that has evolved to recognize invariant structures of microbial surfaces. Here we show that Gram-positive bacteria-derived peptidoglycan (PGN) constitutes a novel PAMP of immune responses in Arabidopsis thaliana. Treatment with PGN from Staphylococcus aureus results in the activation of plant responses, such as medium alkalinization, elevation of cytoplasmic calcium concentrations, nitric oxide, and camalexin production and the post-translational induction of MAPK activities. Microarray analysis performed with RNA prepared from PGN-treated Arabidopsis leaves revealed enhanced transcript levels for 236 genes, many of which are also altered upon administration of flagellin. Comparison of cellular responses after treatment with bacteria-derived PGN and structurally related fungal chitin indicated that both PAMPs are perceived via different perception systems. PGN-mediated immune stimulation in Arabidopsis is based upon recognition of the PGN sugar backbone, while muramyl dipeptide, which is inactive in this plant, triggers immunity-associated responses in animals. PGN adds to the list of PAMPs that induce innate immune programs in both plants and animals. However, we propose that PGN perception systems arose independently in both lineages and are the result of convergent evolution.

Molecular mechanisms of pathogenicity: how do pathogenic microorganisms develop cross-kingdom host jumps?

It is common knowledge that pathogenic viruses can change hosts, with avian influenza, the HIV, and the causal agent of variant Creutzfeldt-Jacob encephalitis as well-known examples. Less well known, however, is that host jumps also occur with more complex pathogenic microorganisms such as bacteria and fungi. In extreme cases, these host jumps even cross kingdom of life barriers. A number of requirements need to be met to enable a microorganism to cross such kingdom barriers. Potential cross-kingdom pathogenic microorganisms must be able to come into close and frequent contact with potential hosts, and must be able to overcome or evade host defences. Reproduction on, in, or near the new host will ensure the transmission or release of successful genotypes. An unexpectedly high number of cross-kingdom host shifts of bacterial and fungal pathogens are described in the literature. Interestingly, the molecular mechanisms underlying these shifts show commonalities. The evolution of pathogenicity towards novel hosts may be based on traits that were originally developed to ensure survival in the microorganism's original habitat, including former hosts.

Disease induction by human microbial pathogens in plant-model systems: potential, problems and prospects

Relatively simple eukaryotic model organisms such as the genetic model weed plant Arabidopsis thaliana possess an innate immune system that shares important similarities with its mammalian counterpart. In fact, some human pathogens infect Arabidopsis and cause overt disease with human symptomology. In such cases, decisive elements of the plant's immune system are likely to be targeted by the same microbial factors that are necessary for causing disease in humans. These similarities can be exploited to identify elementary microbial pathogenicity factors and their corresponding targets in a green host. This circumvents important cost aspects that often frustrate studies in humans or animal models and, in addition, results in facile ethical clearance.

Down regulation of virulence factors of Pseudomonas aeruginosa by salicylic acid attenuates its virulence on Arabidopsis thaliana and Caenorhabditis elegans

Salicylic acid (SA) is a phenolic metabolite produced by plants and is known to play an important role in several physiological processes, such as the induction of plant defense responses against pathogen attack. Here, using the Arabidopsis thaliana-Pseudomonas aeruginosa pathosystem, we provide evidence that SA acts directly on the pathogen, down regulating fitness and virulence factor production of the bacteria. Pseudomonas aeruginosa PA14 showed reduced attachment and biofilm formation on the roots of the Arabidopsis mutants lox2 and cpr5-2, which produce elevated amounts of SA, as well as on wild-type Arabidopsis plants primed with exogenous SA, a treatment known to enhance endogenous SA concentration. Salicylic acid at a concentration that did not inhibit PA14 growth was sufficient to significantly affect the ability of the bacteria to attach and form biofilm communities on abiotic surfaces. Furthermore, SA down regulated three known virulence factors of PA14: pyocyanin, protease, and elastase. Interestingly, P. aeruginosa produced more pyocyanin when infiltrated into leaves of the Arabidopsis transgenic line NahG, which accumulates less SA than wild-type plants. This finding suggests that endogenous SA plays a role in down regulating the synthesis and secretion of pyocyanin in vivo. To further test if SA directly affects the virulence of P. aeruginosa, we used the Caenorhabditis elegans-P. aeruginosa infection model. The addition of SA to P. aeruginosa lawns significantly diminished the bacterium's ability to kill the worms, without affecting the accumulation of bacteria inside the nematodes' guts, suggesting that SA negatively affects factors that influence the virulence of P. aeruginosa. We employed microarray technology to identify SA target genes. These analyses showed that SA treatment affected expression of 331 genes. It selectively repressed transcription of exoproteins and other virulence factors, while it had no effect on expression of housekeeping genes. Our results indicate that in addition to its role as a signal molecule in plant defense responses, SA works as an anti-infective compound by affecting the physiology of P. aeruginosa and ultimately attenuating its virulence.