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Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Pseudomonas aeruginosa
  5. Pseudomonas aeruginosa– plant infection models
  6. Enterococcus faecalis
  7. Staphylococcus aureus
  8. Plant infection models and biosecurity select agents
  9. Conclusions
  10. Acknowledgements
  11. References

Several bacteria that are pathogenic to animals also infect plants. Mechanistic studies have proven that some human/animal pathogenic bacteria employ a similar subset of virulence determinants to elicit disease in animals, invertebrates and plants. Therefore, the results of plant infection studies are relevant to animal pathogenesis. This discovery has resulted in the development of convenient, cost-effective, and reliable plant infection models to study the molecular basis of infection by animal pathogens. Plant infection models provide a number of advantages in the study of animal pathogenesis. Using a plant model, mutations in animal pathogenic bacteria can easily be screened for putative virulence factors, a process which if done using existing animal infection models would be time-consuming and tedious. High-throughput screening of plants also provides the potential for unravelling the mechanisms by which plants resist animal pathogenic bacteria, and provides a means to discover novel therapeutic agents such as antibiotics and anti-infective compounds. In this review, we describe the developing technique of using plants as a model system to study Pseudomonas aeruginosa, Enterococcus faecalis and Staphylococcus aureus pathogenesis, and discuss ways to use this new technology against disease warfare and other types of bioterrorism.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Pseudomonas aeruginosa
  5. Pseudomonas aeruginosa– plant infection models
  6. Enterococcus faecalis
  7. Staphylococcus aureus
  8. Plant infection models and biosecurity select agents
  9. Conclusions
  10. Acknowledgements
  11. References

Infectious diseases transmitted by bacteria are a major cause of mortality worldwide. Contagious diseases like tuberculosis, leprosy and cholera continue to be a great threat to public health in many developing countries. Opportunistic pathogenic bacteria like Pseudomonas aeruginosa and Staphylocccus aureus, which are capable of infecting individuals who are affected by AIDS, extensive burn injury, cystic fibrosis, or are otherwise immuno-compromised, have become a major concern in developed countries. Treating infections caused by such nosocomial pathogens is complicated by the emergence of drug-resistant strains and specialized physiological adaptations like biofilm formation (Costerton et al., 1999). For more than half a century, a handful of antibiotics that interfere with specific metabolic events in the bacterium have been used to treat bacterial infections. However, the selective pressure mounted by this approach, the ability of bacteria to transfer plasmid-mediated genetic information and the widespread occurrence of efflux pumps contributing to intrinsic and acquired resistance (Li and Nikaido, 2004) has resulted in untreatable antibiotic resistant strains. Thus, in this post-antibiotic era we are left with a reduced capacity to fight infectious diseases, creating an urgent need to develop novel strategies for controlling bacterial infections (Hentzer and Givskov, 2003). Furthermore, the potential threat from bio-terrorism agents makes the search for novel therapeutics for infectious diseases a critically compelling area of research.

Bacteria elicit disease by deploying an array of virulence determinants that assist in their attack against host cells, leading to invasive and toxinogenic infections (Ran et al., 2003). Identifying these virulence factors and determining their mechanisms of regulation and action is pivotal in designing new methods to control bacterial diseases. Traditionally, virulence factors of animal pathogenic bacteria have been discovered using costly and time-consuming biochemical and genetic approaches in animal models. More recently, methods such as in vivo expression technology (IVET) (Mahan et al., 1993; Wang et al., 1996), signature-tagged mutagenesis (STM) (Potvin et al., 2003) and transposon site hybridization (TraSH) (Sassetti et al., 2001), have provided more efficient, high-throughput screening abilities for the detection of bacterial virulence factors. However, the mutants developed using these techniques still need to be tested on a large number of animals and therefore are cost- and labour-intensive. In contrast, the identification of plant pathogenic virulence determinants is accomplished by fast and inexpensive screening to determine the pathogenicity of random mutants on individual plants (Willis et al., 1990; Rahme et al., 1991). This approach is advantageous as the gene product(s) involved in pathogenicity can readily be determined. Applying this approach to human pathogens would provide a fast and efficient model to identify and study virulence factors, and may lead to the discovery of new drug targets.

Observations that a number of Gram-negative bacteria like P. aeruginosa, Burkolderia cepacia (formerly Pseudomonas) and Erwinia sp. can infect both plants and animals (Elrod and Braun, 1942; Burkholder, 1950; Starr and Chaterjee, 1972; Fick, 1993), and that type III secretion systems, which participate in the export of virulence factors, were conserved between plant and animal pathogens (Van Gijsegem et al., 1993; Hardt and Galan, 1997; Galan and Collmer, 1999; Staskawicz et al., 2001), led to the development of the first plant model to study human pathogenesis. This model was developed using the plant Arabidopsis thaliana in conjunction with P. aeruginosa, and provided the first evidence that virulence factors are conserved in both plant and animal pathogenicity (Rahme et al., 1995). More recent studies have demonstrated that in addition to Gram-negative human pathogens, several Gram-positive human pathogens can also infect plants (Jha et al., 2005; B. Prithiviraj, H.P. Bais, A.K. Jha and J.M. Vivanco, submitted). These findings increase the usefulness of plant/human pathogen models by broadening their potential application and increasing the number of infectious diseases for which we can seek to develop novel treatments. The plant/human pathogen model is also of evolutionary interest as it bridges the divide between plant and animal pathogenesis. In this review, we will discuss plant models that have been developed to study P. aeruginosa, Enterococcus faecalis and Staphylococcus aureus infections and outline the insights gained from using these models to study human pathogenesis.

Pseudomonas aeruginosa

  1. Top of page
  2. Summary
  3. Introduction
  4. Pseudomonas aeruginosa
  5. Pseudomonas aeruginosa– plant infection models
  6. Enterococcus faecalis
  7. Staphylococcus aureus
  8. Plant infection models and biosecurity select agents
  9. Conclusions
  10. Acknowledgements
  11. References

Pseudomonas aeruginosa (Family: Pseudomonadaceae) is an aerobic, Gram-negative, rod-shaped bacterium that inhabits soil, water and biotic surfaces. Although it is a saprophyte and rarely infects healthy tissue, it is the embodiment of an opportunistic pathogen in humans and animals. P. aeruginosa is primarily a nosocomial pathogen that is most commonly found in cystic fibrosis patients, burn victims and other immuno-compromised patients, as well as soft contact lens wearers and those who use prostheses (Fick, 1993). Eradication of P. aeruginosa infections has remained a clinical challenge, as the bacteria are notoriously resistant to antibiotic treatment. One of the contributing factors to its antibiotic resistance is that as a soil-dwelling bacteria, P. aeruginosa is frequently in contact with bacilli, actinomycetes and molds, coevolving resistance to their naturally produced antibiotics (Rahme et al., 1995). The main mechanism responsible for acquired and intrinsic drug resistance is synergy between an impermeable outer membrane and active efflux from the cell (Nikaido, 2003; Li and Nikaido, 2004). In addition, P. aeruginosa maintains transferable antibiotic resistant plasmids (Stewart, 2002) and often colonizes surfaces in its more impervious biofilm form (Singh et al., 2000). Biofilm  bacteria  are  notoriously  antibiotic  resistant  and the molecular basis for this observation is slowly being unravelled  (Costerton  et al.,  1999;  Drenkard  and Ausubel, 2002; Singh et al., 2002; Stewart, 2002; Mah et al., 2003). In human pathogenic infections P. aeruginosa has proven to be recalcitrant, as it is resistant to high doses of antibiotics such as ciprofloxacin (Poole, 2000). To date, although chronic P. aeruginosa infections in the lungs of cystic fibrosis (CF) patients can be temporarily controlled by antibiotic treatment, in the long run they are incurable.

Pseudomonas aeruginosa– plant infection models

  1. Top of page
  2. Summary
  3. Introduction
  4. Pseudomonas aeruginosa
  5. Pseudomonas aeruginosa– plant infection models
  6. Enterococcus faecalis
  7. Staphylococcus aureus
  8. Plant infection models and biosecurity select agents
  9. Conclusions
  10. Acknowledgements
  11. References

Aside from being an effective and deadly opportunistic human pathogen, P. aeruginosa has been known to infect a number of plants (Elrod and Braun, 1942; Burkholder, 1950). However, it was Dr Rahme and coworkers, using the clinical isolate of P. aeruginosa strain UCBPP-PA14 and the model plant A. thaliana (Thalle cress), as well as Lettuca sativa (lettuce), who first showed that this bacterium employs a similar subset of virulence factors to elicit disease in animals and plants (Rahme et al., 1995; 1997). They took advantage of the fact that in addition to producing visual disease symptoms on plant leaf surfaces, similar to plant pathogenic bacteria, P. aeruginosa was capable of multiplying rapidly in the apoplast, correlating with disease severity. The virulence factors toxA and phospholipase (plcS) are essential for animal pathogenesis. Inoculation with toxA and plcS mutants resulted in reduced disease severity in plant and mouse models, correlating with low bacterial load in infected tissues. Conversely, Rahme et al. (1995) showed that mutations in gacA, a virulence factor involved in pathogenesis of the plant pathogens Pseudomonas syringae, P. viridiflava and P. marginalis, also caused reduced virulence in a mouse model. These findings established the validity of using a plant model to study pathogenesis of animal pathogens and led to the speculation that a plant model could aid in identification of novel virulence factors through screening of random P. aeruginosa mutants. To test this hypothesis, 2500 random transposon mutants of P. aeruginosa were inoculated on lettuce leaves and the disease severity was scored (Rahme et al., 1997). Nine mutants showed reduced virulence in the plant model. Astonishingly, eight of the nine mutants that displayed attenuated virulence in the plant assay were also less pathogenic in animal infection assays. Molecular analysis of the mutants revealed that two of the mutated genes produced the known virulence factors gacA and dsbA (Peeke and Taylor, 1992; Shevchik et al., 1995; Watarai et al., 1995). However, seven of the mutations were in genes that showed no homology to genes known to be implicated in pathogenesis. Several of these genes appear to encode global virulence factors. For instance, insertion in the gene mvfR (LysR-like transcriptional regulator) showed 50% reduced haemolytic activity and no pyocyanin production, while dsbA mutants exhibited 60–70% reduced elastase activity and a mutation in gacA had significantly reduced pyocyanin production. Therefore, these loci might be involved in or coordinate the expression of other virulence factors responsible for infection of animals. As the screening of such a large number of mutants is not practical using vertebrates, the plant model provided an efficient, high-throughput screening tool for the identification of these virulence factors.

Plant models have also assisted in the characterization of several other suspected virulence factors in P. aeruginosa. The degP gene product in E. coli is reportedly responsible for the degradation of damaged proteins (Strauch and Beckwith, 1988), growth at high temperatures (Strauch et al., 1989) and protection from heat stress (Speiss et al., 1999). Homologues of degP have also been shown to play a role in pathogenesis in both animal (Johnson et al., 1991) and plant pathogens (Stevens, 1998). The role of mucD, a PA14 homologue of degP, was elucidated using multihost pathogenesis models, including Arabidopsis as a host plant (Yorgey et al., 2001). P. aeruginosa mucD mutants form colonies with a mucoid phenotype, which has been attributed to the overproduction of alginate (Boucher et al., 1996), a virulence factor associated with chronic infection in cystic fibrosis patients (Govan and Deretic, 1996). Interestingly, a mucD mutant, producing more alginate, showed decreased pathogenicity in Arabidopsis, Caenorhabditis elegans and mouse models (Yorgey et al., 2001). To clarify the role of alginate in PA14 pathogenicity, algD, a gene involved in alginate biosynthesis, was also mutated. The PA14 algD mutant did not show reduced virulence in any of the models tested. This suggests that alginate does not play a significant role as a virulence factor in Arabidopsis, C. elegans and mouse pathogenicity models, and may only be an important virulence factor in chronic infections requiring biofilm formation, such as infections found in the lungs of cystic fibrosis patients (Yorgey et al., 2001).

A similar multihost study was used to characterize the rpoN gene, which encodes an alternate sigma factor in P. aeruginosa. This sigma factor is implicated in virulence factor regulation in both plant and animal pathogens (Totten et al., 1990; Goldberg and Dahnke, 1992), and was therefore thought to be required for multihost pathogenesis. However, an rpoN insertion mutation elicited disease symptoms similar to wild type in Arabidopsis by 7 days post infection, and only showed significantly less killing in the C. elegans model, surprisingly suggesting that rpoN does not regulate the expression of any genes that are universally required for virulence (Hendrickson et al., 2001).

Another interesting application of the plant model is the characterization of virulence factors found in the non-typeable, less virulent strains of P. aeruginosa often found in chronic lung infections of cystic fibrosis patients. One of the major constraints in studying the pathogenesis of these isolates is that they usually display a dramatically attenuated virulence in classic animal infection models. For example, the P. aeruginosa cystic fibrosis strain FRD1 which is characteristically mucoid was tested for pathogenicity by transtracheal installation of an agar bead containing 104 bacteria in rats (Cash et al., 1979; Woods et al., 1991). FRD1 was unable to establish infection, and the bacterial count reduced over a period of time with only 4.1 × 101 cfu detected in the lung homogenates 14 days post inoculation. However, the development of a wounded alfalfa (Medicago sativa) seedling model was sufficiently sensitive to show disease symptoms using low infectious doses of strain FRD1 (Silo-Suh et al., 2002). Different virulence factor mutations were tested using this model to determine which of these factors play a role in chronic rather than acute P. aeruginosa infections. RhlR and algT were found to be important for virulence using the alfalfa seedling model, although mutations of other common virulence factors such as RpoS, PvdS and LasR did not decrease the virulence of FRD1 on alfalfa. Interestingly, the FRD1 algD mutant, also defective in alginate biosynthesis, did not show reduced virulence in the alfalfa seedling model, agreeing with the previously discussed conclusions of Yorgey et al. (2001) that suggest that alginate is not an important virulence factor in plant, C. elegans and mouse models. Furthermore, an algT mutant in a PAO1 background did not show attenuated virulence, suggesting that algT controls an unidentified virulence determinant that is important in an attenuated strain like FRD1, but masked in highly virulent strains of the pathogen like PAO1 (Silo-Suh et al., 2002).

The mode of P. aeruginosa infections has been extensively studied in leaf tissue (Plotnikova et al., 2000) and wounded stems (Silo-Suh et al., 2002). However, these models cannot be used to study a characteristic feature of infection, biofilm formation. Recent studies have led to the development of a root infection model (Fig. 1A-C) where the biofilm formation on the root surface can be readily monitored microscopically using appropriate staining procedures (Walker et al., 2004). This system has two obvious advantages: first, it provides a model for identifying compounds that limit biofilm formation and provides a platform for observing biofilm-deficient mutants. Second, screening plant roots for their susceptibility to bacterial biofilms could lead to discovery of plant-derived factors that inhibit biofilm formation and could aid in treating P. aeruginosa infections. Using this system, Walker et al. (2004) tested PAO1, the quorum-sensing mutants lasI and rhlI and the lasIrhlI double mutant for pathogenicity against Ocimum basilicum (basil). They found that the plant responded to bacterial root infection by increasing the secretion of the antimicrobial compound rosmarinic acid (RA) from its roots. This compound was able to kill planktonic cells, but had little effect on P. aeruginosa biofilms. Further, only the lasI mutant, but not the rhamnolipid-deficient rhlI or lasIrhlI mutants, retained virulence against O. basilicum, strengthening the hypothesis that it is the biofilm formation that confers resistance to the pathogen against the antimicrobial effect of rosmarinic acid. Interestingly, Mathesius et al. (2003) reported that acylated homoserine lactones (AHLs) induced changes in the secretion of plant secondary metabolites that could affect a plant's interaction with bacteria. Medicago truncatula was found to secrete AHL signal mimics that have the potential to interfere with quorum-sensing signalling in bacteria (Teplitski et al., 2000; Bauer and Robinson, 2002). While research on the relationship between bacterial quorum-sensing and plant metabolites is still a largely unexplored area, it suggests that plants may be a valuable source of novel compounds which inhibit or interfere with bacterial quorum-sensing, and which could be developed as novel anti-infective therapeutic agents.

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Figure 1. Pseudomonas aeruginosa PA01 infects Arabidposis thaliana roots. A. P. aeruginosa PA01 forms confluent biofilms on the surface of A. thaliana roots (arrows). B. Colonization of Arabidopsis roots by P. aeruginosa PA01. C. P. aeruginosa cell embedded in a polysaccharide matrix.

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Enterococcus faecalis

  1. Top of page
  2. Summary
  3. Introduction
  4. Pseudomonas aeruginosa
  5. Pseudomonas aeruginosa– plant infection models
  6. Enterococcus faecalis
  7. Staphylococcus aureus
  8. Plant infection models and biosecurity select agents
  9. Conclusions
  10. Acknowledgements
  11. References

Enterococcus faecalis (Family: Micrococcaceae), a Gram-positive bacterium normally growing as a commensal organism in the gut, is a leading cause of nosocomial infections (Moellering, 1991; 1992). Although the ability of E. faecalis to cause serious disease is well recognized, not much is known about enterococcal virulence factors that contribute to its pathogenesis. Recent advances in this field have resulted in elucidation of some of the virulence factors from E. faecalis, including cytolysin (Cyl), a factor called aggregation substance (AS), a zinc metalloprotease (gelatinase), and fsr (an E. faecalis regulator), a putative quorum-sensing system thought to be involved in gelatinase and/or serine protease regulation (Singh et al., 1998; Qin et al., 2000; 2001). The limited knowledge of enterococcal virulence factors is due to the cumbersome and expensive nature of mammalian models for enterococcal infections. Recently, a C. elegans model, which can potentially aid in the search for new virulence factors, has been developed to study E. faecalis pathogenesis (Garsin et al., 2001; Sifri et al., 2002). Using plant infection models would further aid in our quest to understand E. faecalis pathogenesis.

It has recently been demonstrated that E. faecalis can colonize and inflict disease on the model plant A. thaliana (Jha et al., 2005). When inoculated to leaves or roots, three clinical isolates of E. faecalis, FA-2-2, V583 and OG1RF, exhibited potent pathogenicity in A. thaliana involving a sequential array of events, including attachment to the root surface, congregation of bacteria in stomata, colonization in intercellular spaces (Fig. 2), and formation of communities on the root surfaces (Jha et al., 2005). However, the strains differ in their virulence. Interestingly, some of the E. faecalis virulence factors required for mammalian pathogenesis are also involved in plant pathogenicity, further indicating the validity of using plant models to study E. faecalis pathogenesis.

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Figure 2. Enterococcus feacalis OG1RF multiplies in the intercellular spaces of Arabidopsis thaliana (Col-0) leaves.

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As previously outlined, quorum sensing is an important mechanism used by many prokaryotes to adapt to different environments encountered during pathogenesis (Haas et al., 2002). In E. faecalis, the fsr system positively regulates the expression of the pathogenesis factors gelatinase and serine protease in a cell-density-dependent manner (Jarraud et al., 2002). Qin et al. (2000; 2001) have characterized three genes in the fsr regulatory locus: fsrA, fsrB and fsrC using a non-polar deletion mutant in fsrB. They showed that fsrB is required for the regulatory function of the fsr system (Qin et al., 2000; 2001). In line with the existing knowledge of the positive role of fsr regulatory systems in mammalian pathogenesis, Jha et al. (2005) found that fsrB plays an important role in regulating bacterial colonization of A. thaliana root surfaces. Accordingly, a deletion mutant, ΔfsrB, failed to colonize A. thaliana roots and exhibited attenuated pathogenicity on Arabidopsis plants. Along with the fsr system, serine protease (sprE) is an additional virulence factor thought to play a role in systemic disease in mammalian hosts (Qin et al., 2001). The serine protease gene sprE, which lies immediately downstream of and is co-transcribed with gelE, encodes a secreted 26 kDa serine protease that shares homology with the S. aureus V8 protease (Qin et al., 2000; 2001). Insertion disruption of sprE also attenuates virulence in the mouse peritonitis and C. elegans model systems (Sifri et al., 2002). As observed in mammalian and invertebrate models (Garsin et al., 2001), the serine protease deletion mutant ΔsprE showed reduced pathogenicity in the A. thaliana root pathogenicity model (Jha et al., 2005). The prevalence of antibiotic resistance in E. faecalis has reached disturbing proportions and there is a need to develop alternate antimicrobial agents to cure infections. Such an endeavour requires a better understanding of the virulence factors of E. faecalis. Plant infection models can aid in high-throughput screening of E. faecalis mutants for putative virulence factors. Furthermore, the screening of different plant species for differential infection by E. faecalis may provide leads to novel antimicrobial agents.

Staphylococcus aureus

  1. Top of page
  2. Summary
  3. Introduction
  4. Pseudomonas aeruginosa
  5. Pseudomonas aeruginosa– plant infection models
  6. Enterococcus faecalis
  7. Staphylococcus aureus
  8. Plant infection models and biosecurity select agents
  9. Conclusions
  10. Acknowledgements
  11. References

Staphylococcus aureus (Family: Micrococcaceae) is a Gram-positive bacterium that is also capable of infecting humans and animals. In humans it causes a wide array of diseases from superficial skin infections to life-threatening systemic diseases like septicemia, pneumonia, bone infection, meningitis and endocarditis (Bradley, 2002). Several virulence factors make S. aureus a versatile pathogen, including cell-associated products, secreted exotoxins and regulatory proteins (Archer, 1998). The secreted exotoxins include haemolysins, nucleases, proteases, lipases, hyaluonidases and collagenases (Dinges et al., 2000). In humans, these toxins have direct cytotoxic effects, impairing the immune system. The production of these toxins is controlled by regulatory genes like sarA and agr (Cheung and Zhang, 2002). Staphylococci also form biofilms, resulting in serious infections in which several other factors orchestrate to cause full virulence (Kupferwasser et al., 2003).

The whole genome of S. aureus has been sequenced (Iandolo et al., 2002). To make better use of this information it is imperative to screen mutants of this bacterium for their potential to cause infection. Therefore, it is necessary to develop simple model systems to study S. aureus pathogenesis. Recently, the model nematode, C. elegans, has been used to study the pathogenesis of S. aureus (Sifri et al., 2003). A plant model has also been developed to test S. aureus pathogenicity. Leaf or root inoculation of A. thaliana with wild-type S. aureus elicited symptoms reminiscent of typical bacterial plant pathogens including colonization of the leaf and trichomes (Fig. 3A and B), formation of water soaked lesions, chlorosis and necrosis (B. Prithiviraj, H.P. Bais, A.K. Jha and J.M. Vivanco, submitted). However, the severity of the symptoms depended on the bacterial strains tested. Such differences in the pathogenicity of plant, animal and human pathogens are known and are thought to be due to the differences in virulence as result of differential gene expression (Choi et al., 2002). Similar to the studies with mutants of P. aeruginosa, which suggested conservation of some of the virulence factors and regulatory factors in plant and animal pathogenesis (Rahme et al., 1997), the function of virulence factors and two global transcription regulators was found to be essential for S. aureus pathogenesis in both animals and A. thaliana. Similar attenuation of virulence was observed in a C. elegans infection model (Sifri et al., 2003). Three mutants attenuated in global regulatory systems that affect the synthesis of virulence factors, RN6911 (agr), ALC 488 (sarA) and ALC 842 (agr/sar), showed reduced virulence on animal models (Cheung and Ying, 1994; Booth et al., 1997; Chan and Foster, 1998; Blevins et al., 1999; 2002; Kielian et al., 2001) and were also defective in pathogenicity on Arabidopsis thaliana. Similarly, a mutant DU1019 (hla) defective in alpha-toxin and biofilm formation (DU1019) (Beenken et al., 2003) was less pathogenic in A. thaliana. Interestingly, S. aureus colonization was also influenced by host factors. Some genotypes of A. thaliana accumulate high concentrations of the phenolic compound salicylic acid (a precursor of aspirin), while others produce less (Shah, 2003). The differences in salicylic acid (SA) levels in plants have been correlated with disease resistance by a process in which SA is able to activate gene cascades involved in plant defence response against pathogens (Shah, 2003). In our studies, S. aureus colonization was inhibited on roots of plants that hyper-accumulated salicylic acid (Fig. 3B and C). In such roots, the bacteria were not able to form biofilms, and the lack of biofilms was correlated to a lack of virulence. Similar effects of salicylic acid on S. aureus was observed in animal pathogenesis models in which SA-treated bacteria showed less virulence in a C. elegans model. Accordingly, treatment of an invasive experimental S. aureus infection (endocarditis) with intravenous aspirin resulted in a significant reduction in bacterial densities within target tissues and kidneys; and SA mediated these effects (Kupferwasser et al., 1999; 2003). The molecular targets of SA in decreasing S. aureus virulence have not been identified. These results highlight the value of using plant systems not only for the screening of virulence factors needed by bacterial pathogens to cause infection but most importantly for the discovery of novel therapeutics.

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Figure 3. Staphylococcus aureus NCTC 8325 infects Arabidopsis thaliana. A. Extensive colonization of the leaf by S. aureus; note the polysaccharide matrix. B. Colonization of the leaf trichomes. C. Biofilm formation on the roots of wild-type A. thaliana (Col-0) plants. D. S. aureus did not form good biofilm on the roots of transgenic A. thaliana (lox2) which accumulates high levels of salicylic acid.

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Plant infection models and biosecurity select agents

  1. Top of page
  2. Summary
  3. Introduction
  4. Pseudomonas aeruginosa
  5. Pseudomonas aeruginosa– plant infection models
  6. Enterococcus faecalis
  7. Staphylococcus aureus
  8. Plant infection models and biosecurity select agents
  9. Conclusions
  10. Acknowledgements
  11. References

The increased risk of bioterrorism has become a concern in recent years. In response to increased bioterrorism awareness, the Centers for Disease Control and Prevention as part of the Department of Health and Human Services (HHS) established the select agents program (http://www.cdc.gov/od/sap) to regulate the possession of biological agents and toxins that have the potential to pose a severe threat to public health and safety. Realizing the shared potential threat that a number of the biological agents pose to both humans and animals, a list detailing the potential overlap of high consequence HHS/USDA livestock agents was generated, which to date only includes animal pathogens. However, in light of the fact that many animal pathogens can also infect plants and cause disease symptoms similar to those in animals, some of the select agents on the overlap list, especially saprophytic bacteria like Burkholderia (formerly Pseudomonas) pseudomallei the causative agent of melioidosis (White, 2003), should be evaluated in plant models. The outcomes of such studies may not only be new disease models for these emerging pathogens and potential plant-derived therapeutics, but also may necessitate a evaluation of the notion that overlap agents may pose threats beyond the animal kingdom. Furthermore, a developing and complementary research area suggests that plant systems could be used as environmental indicators of the presence of such select agents, providing an early warning system of intentional releases of these agents.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. Pseudomonas aeruginosa
  5. Pseudomonas aeruginosa– plant infection models
  6. Enterococcus faecalis
  7. Staphylococcus aureus
  8. Plant infection models and biosecurity select agents
  9. Conclusions
  10. Acknowledgements
  11. References

Using plants as model systems has proven to be an effective way to study the molecular basis of pathogenesis of several animal pathogenic bacteria. The data obtained thus far suggest considerable overlap of virulence between plant and animal pathogens. An additional advantage of using plant models over invertebrate models is the occurrence of anti-infective secondary metabolites in plants that are induced by pathogen infections. Isolation of these compounds may prove useful as novel agents in the treatment of bacterial infections (Fig. 4). Availability of the complete genome sequence of a number of pathogenic bacteria and the model plant A. thaliana and the ease of handling plant infection models provide an opportunity to study the molecular basis of host–pathogen interaction. Furthermore, the potential to combine several host models for bacterial infection to screen for the interapplicability of organism-secreted anti-infective compounds presents a powerful new tool for developing novel therapeutics.

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Figure 4. Schematic representation of the advantages of developing plant models for animal pathogenic bacteria.

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Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Pseudomonas aeruginosa
  5. Pseudomonas aeruginosa– plant infection models
  6. Enterococcus faecalis
  7. Staphylococcus aureus
  8. Plant infection models and biosecurity select agents
  9. Conclusions
  10. Acknowledgements
  11. References

The research presented in this paper was supported by the Colorado State University Agricultural Experiment Station (J.M.V.). J.M.V. is an NSF-CAREER Faculty Fellow (MCB-0093014). H.P.S. is supported by grants from NIH.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Pseudomonas aeruginosa
  5. Pseudomonas aeruginosa– plant infection models
  6. Enterococcus faecalis
  7. Staphylococcus aureus
  8. Plant infection models and biosecurity select agents
  9. Conclusions
  10. Acknowledgements
  11. References
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