Virulence of plant pathogenic bacteria attenuated by degradation of fatty acid cell-to-cell signaling factors

Diffusible signal factor (DSF) is a fatty acid signal molecule involved in regulation of virulence in several Xanthomonas species as well as Xylella fastidiosa. In this study, we identified a variety of bacteria that could disrupt DSF-mediated induction of virulence factors in Xanthomonas campestris pv. campestris. While many bacteria had the ability to degrade DSF, several bacterial strains belonging to genera Bacillus, Paenibacillus, Microbacterium, Staphylococcus, and Pseudomonas were identified that were capable of particularly rapid degradation of DSF. The molecular determinants for rapid degradation of DSF in Pseudomonas spp. strain G were elucidated. Random transposon mutants of strain G lacking the ability to degrade DSF were isolated. Cloning and characterization of disrupted genes in these strains revealed that carAB, required for the synthesis of carbamoylphosphate, a precursor for pyrimidine and arginine biosynthesis is required for rapid degradation of DSF in strain G. Complementation of carAB mutants restored both pyrimidine prototrophy and DSF degradation ability of the strain G mutant. An Escherichia coli strain harboring carAB of Pseudomonas spp. strain G degrades DSF more rapidly than the parental strain, and overexpression of carAB in trans increased the ability of Pseudomonas spp. strain G to degrade as compared with the parental strain. Coinoculation of X. campestris pv. campestris with DSF-degrading bacteria into mustard and cabbage leaves reduced disease severity up to twofold compared with plants inoculated only with the pathogen. Likewise, disease incidence and severity in grape stems coinoculated with Xylella fastidiosa and DSF-degrading strains were significantly reduced compared with plants inoculated with the pathogen alone. Coinoculation of grape plants with a carAB mutant of Pseudomonas spp. strain G complemented with carAB in trans reduced disease severity as well or better than the parental strain. These results indicate that overexpression of carAB in other endophytes could be a useful strategy of biocontrol for the control of diseases caused by plant pathogens that produce DSF.     

The theft of host heme by Gram-positive pathogenic bacteria

The element iron is essential for bacteria and plays a key role in the virulence and pathology of bacterial diseases. The largest reservoir of iron within the human body is in protoporphyrin IX, the compound commonly referred to as heme and bound by hemoglobin. For many years, the study of heme uptake in bacteria was restricted to Gram-negative organisms. However, recent studies have shed light on how bacteria containing a thick peptidoglycan, such as Gram-positive bacteria, acquire and transport heme. This review summarizes old and new research covering the acquisition, transport, and utilization of heme in Gram-positive bacterial pathogens.     

Towards more accurate detection of pathogenic Gram-positive bacteria in waters

Medically important bacteria can persist in surface waters longer than was previously thought, by activating specific survival strategies and, thus, may represent a further threat to human health, in that they are non-detectable by the traditional culture methods currently used for the evaluation of microbiological quality. Combining microbial physiology, microbial biochemistry, microbial genetics, microbial ecology and molecular biology techniques allow us to achieve more accurate detection of human pathogens located in natural environments external to the human body.     

Virulence gene regulation by peptides in staphylococci and other Gram-positive bacteria

In staphylococci, autoinducing peptides activate agr. a global regulator of the expression of genes encoding virulence factors and other exoproteins. During the past year, there have been major advances in the structure-function analysis of these peptides and the regulation of a virulence factor by an autoinducing peptide in pneumococci has been demonstrated.     

Bacteriocins from plant pathogenic bacteria

Many bacteria produce antimicrobial substances such as nonribosomally synthesized antibiotics and ribosomally synthesized proteinaceous compounds referred to as bacteriocins. Secretion of antimicrobials is generally thought to contribute to the competitiveness of the producing organism, but there are indications that these compounds in some cases may have regulatory roles too. Bacteriocins most often act on closely related species only and are thus of interest for application as targeted narrow-spectrum antimicrobials with few side effects. Although the application of bacteriocins in plant disease control is an attractive option, very little is known about the occurrence and roles of these compounds in plant pathogenic bacteria and their natural competitors occurring in the same biotopes. This study presents an overview of current knowledge of bacteriocins from plant pathogenic bacteria.     

Iron uptake mechanisms of pathogenic bacteria

Abstract: Most of the iron in a mammalian body is complexed with various proteins. Moreover, in response to infection, iron availability is reduced in both extracellular and intracellular compartments. Bacteria need iron for growth and successful bacterial pathogens have therefore evolved to compete successfully for iron in the highly iron-stressed environment of the host's tissues and body fluids. Several strategies have been identified among pathogenic bacteria, including reduction of ferric to ferrous iron, occupation of intracellular niches, utilisation of host iron compounds, and production of siderophores. While direct evidence that high affinity mechanisms for iron acquisition function as bacterial virulence determinants has been provided in only a small number of cases, it is likely that many if not all such systems play a central role in the pathogenesis of infection.     

Threats and opportunities of plant pathogenic bacteria

Plant pathogenic bacteria can have devastating effects on plant productivity and yield. Nevertheless, because these often soil-dwelling bacteria have evolved to interact with eukaryotes, they generally exhibit a strong adaptivity, a versatile metabolism, and ingenious mechanisms tailored to modify the development of their hosts. Consequently, besides being a threat for agricultural practices, phytopathogens may also represent opportunities for plant production or be useful for specific biotechnological applications. Here, we illustrate this idea by reviewing the pathogenic strategies and the (potential) uses of five very different (hemi)biotrophic plant pathogenic bacteria: Agrobacterium tumefaciens, A. rhizogenes, Rhodococcus fascians, scab-inducing Streptomyces spp., and Pseudomonas syringae.     

Top 10 plant pathogenic bacteria in molecular plant pathology

Many plant bacteriologists, if not all, feel that their particular microbe should appear in any list of the most important bacterial plant pathogens. However, to our knowledge, no such list exists. The aim of this review was to survey all bacterial pathologists with an association with the journal Molecular Plant Pathology and ask them to nominate the bacterial pathogens they would place in a 'Top 10' based on scientific/economic importance. The survey generated 458 votes from the international community, and allowed the construction of a Top 10 bacterial plant pathogen list. The list includes, in rank order: (1) Pseudomonas syringae pathovars; (2) Ralstonia solanacearum; (3) Agrobacterium tumefaciens; (4) Xanthomonas oryzae pv. oryzae; (5) Xanthomonas campestris pathovars; (6) Xanthomonas axonopodis pathovars; (7) Erwinia amylovora; (8) Xylella fastidiosa; (9) Dickeya (dadantii and solani); (10) Pectobacterium carotovorum (and Pectobacterium atrosepticum). Bacteria garnering honourable mentions for just missing out on the Top 10 include Clavibacter michiganensis (michiganensis and sepedonicus), Pseudomonas savastanoi and Candidatus Liberibacter asiaticus. This review article presents a short section on each bacterium in the Top 10 list and its importance, with the intention of initiating discussion and debate amongst the plant bacteriology community, as well as laying down a benchmark. It will be interesting to see, in future years, how perceptions change and which bacterial pathogens enter and leave the Top 10.     

Molecular-genetic mechanisms of antigenic variability of pathogenic bacteria

The literature on the molecular genetic mechanisms for antigenic variability of pathogenic bacteria is reviewed. Ability to antigenic variability in any case discussed is considered to be a pathogenicity factor permitting efficient struggle against the immune system of the host-organism. The molecular basis for such variability is instability of the genome structure, coding for highly immunogenic bacterial proteins.     

Neuronal mechanisms of Caenorhabditis elegans and pathogenic bacteria interactions

Individuals interact with environment through different neuronal functions, such as olfaction and mechanosensation; experience shapes these physiological functions. It is not well understood how an individual senses and processes multiple cues of natural stimuli in the environment and how experience modulates these physiological mechanisms. Recent molecular genetics and behavioral studies on the interactions of the genetic model organism Caenorhabditis elegans with pathogenic bacteria have provided insights on the molecular and cellular mechanisms underlying these regulatory processes.     

Determinants of pathogenicity and avirulence in plant pathogenic bacteria

Many plant pathogenic bacteria possess a conserved protein secretion system that is thought to transfer Avr (avirulence) proteins, with potential activities in both parasitism and defense elicitation, into plant cells. avr genes may be acquired horizontally by these bacteria, and avr gene compositions are highly variable. In the past year, heterologous expression experiments have revealed that the products of avr genes can be interchanged among different genera of bacteria with retention of secretion, pathogenicity, and avirulence activities, suggesting mechanisms for rapid coevolution of these parasites with changing plant hosts.     

Amyloidogenesis of type III-dependent harpins from plant pathogenic bacteria

Harpins are heat-stable, glycine-rich type III-secreted proteins produced by plant pathogenic bacteria, which cause a hypersensitive response (HR) when infiltrated into the intercellular space of tobacco leaves; however, the biochemical mechanisms by which harpins cause plant cell death remain unclear. In this study, we determined the biochemical characteristics of HpaG, the first harpin identified from a Xanthomonas species, under plant apoplast-like conditions using electron microscopy and circular dichroism spectroscopy. We found that His(6)-HpaG formed biologically active spherical oligomers, protofibrils, and beta-sheet-rich fibrils, whereas the null HR mutant His(6)-HpaG(L50P) did not. Biochemical analysis and HR assay of various forms of HpaG demonstrated that the transition from an alpha-helix to beta-sheet-rich fibrils is important for the biological activity of protein. The fibrillar form of His(6)-HpaG is an amyloid protein based on positive staining with Congo red to produce green birefringence under polarized light, increased protease resistance, and beta-sheet fibril structure. Other harpins, such as HrpN from Erwinia amylovora and HrpZ from Pseudomonas syringae pv. syringae, also formed curvilinear protofibrils or fibrils under plant apoplast-like conditions, suggesting that amyloidogenesis is a common feature of harpins. Missense and deletion mutagenesis of HpaG indicated that the rate of HpaG fibril formation is modulated by a motif present in the C terminus. The plant cytotoxicity of HpaG is unique among the amyloid-forming proteins that occur in several microorganisms. Structural and morphological analogies between HpaG and disease-related amyloidogenic proteins, such as Abeta protein, suggest possible common biochemical characteristics in the induction of plant and animal cell death.     

Factors affecting ethylene production by some plant pathogenic bacteria

Summary Methionine, up to 10^–3 M, added to a basal medium enhanced bacterial ethylene production in 14 of the 20 bacteria tested. The effects of substrate, cofactors, light, and temperature on ethylene production byPseudomonas solanacearum #25 revealed that the greatest effect occurred when 10^–5 M methionine and 10^–4 M FMN were combined, from which 4.10l/l of ethylene were produced. Higher levels of methionine resulted in production of high levels of non-enzymically produced ethylene and death of the bacteria. This non-enzymic production of ethylene was eliminated in the dark. Copper had no effect upon ethylene production. Twenty-nine and 35°C were inhibitory, whereas 19°C appeared to be near optimum for ethylene production.Pseudomonas solanacaerum #25 and some other bacteria are capable of ethylene production and methionine and FMN enhance this production.This work was supported by the Fred C. Gloeckner Foundation and the University of Minnesota Graduate School Grant in Aid #496-0307-4909-02.     

Innovative tools for detection of plant pathogenic viruses and bacteria

Detection of harmful viruses and bacteria in plant material, vectors or natural reservoirs is essential to ensure safe and sustainable agriculture. The techniques available have evolved significantly in the last few years to achieve rapid and reliable detection of pathogens, extraction of the target from the sample being important for optimising detection. For viruses, sample preparation has been simplified by imprinting or squashing plant material or insect vectors onto membranes. To improve the sensitivity of techniques for bacterial detection, a prior enrichment step in liquid or solid medium is advised. Serological and molecular techniques are currently the most appropriate when high numbers of samples need to be analysed. Specific monoclonal and/or recombinant antibodies are available for many plant pathogens and have contributed to the specificity of serological detection. Molecular detection can be optimised through the automatic purification of nucleic acids from pathogens by columns or robotics. New variants of PCR, such as simple or multiplex nested PCR in a single closed tube, co-operative-PCR and real-time monitoring of amplicons or quantitative PCR, allow high sensitivity in the detection of one or several pathogens in a single assay. The latest development in the analysis of nucleic acids is micro-array technology, but it requires generic DNA/RNA extraction and pre-amplification methods to increase detection sensitivity. The advances in research that will result from the sequencing of many plant pathogen genomes, especially now in the era of proteomics, represent a new source of information for the future development of sensitive and specific detection techniques for these microorganisms.     

Antimicrobial activity of LFchimera synthetic peptide against plant pathogenic bacteria

The present study was designed to evaluate potential antibacterial activities of synthetic LFchimera against five plant pathogenic bacteria such as Ralstonia solanacearum, Erwinia amylovora, Xanthomonas campestris, Pseudomonas syringae and Pectobacterium carotovorum. The agar disc-diffusion method with different concentrations (0.2, 0.4, 0.6 and 0.8 μM) of peptide was used to study the antibacterial activity of LFchimera against bacteria. The Minimum Inhibitory Concentration (MIC) of the LFchimera peptide were tested using serial dilution method at concentration ranging from 0 to 10 μM. The Results from agar disc-diffusion method revealed that LFchimera was effective against all bacterial strain in a dose-dependent manner. LFchimera showed highest activity in 0.8 μM which was significant compared to the standard antibiotic. LFchimera pepetide showed low MIC values (4 μM) against all tested bacteria. LFchimera peptide was found to show antibacterial activity against important phytopathogenic bacteria and can improve the potential of an antimicrobial peptide in plant disease management.     

Activity and mechanisms of action of selected biocidal agents on Gram-positive and -negative bacteria

AIMS: This study investigates the antimicrobial activity and mode of action of two natural products, eugenol and thymol, a commonly utilized biostatic agent, triclocarban (TCC), and two surfactants, didecyldimethylammonium chloride (DDDMAC) and C10-C16 alkyldimethyl amine N-oxides (ADMAO). METHODS AND RESULTS: Methods used included: determination of minimum inhibitory concentrations (MICs), lethal effect studies with suspension tests and the investigation of sub-MIC concentrations on growth of E. coli, Staph. aureus and Ps. aeruginosa using a Bioscreen microbiological analyser. Leakage of intracellular constituents and the effects of potentiating agents were also investigated. Only DDDMAC was bactericidal against all of the organisms tested. Eugenol, thymol and ADMAO showed bacteriostatic and bactericidal activity, but not against Ps. aeruginosa. TCC was only bacteristatic against Staph. aureus, but like the other agents, it did affect the growth of the other organisms in the Bioscreen experiments. All of the antimicrobial agents tested were potentiated by the permeabilizers to some extent and leakage of potassium was seen with all of the agents except TCC. CONCLUSIONS: DDDMAC was bactericidal against all organisms tested and all compounds had some bacteriostatic action. Low level static effects on bacterial growth were seen with sub-MIC concentrations. Membrane damage may account for at least part of the mode of action of thymol, eugenol, DDDMAC and ADMAO. SIGNIFICANCE AND IMPACT OF THE STUDY: The ingredients evaluated demonstrated a range of bactericidal and bacteriostatic properties against the Gram-negative and -positive organisms evaluated and the membrane (leakage of intracellular components) was implicated in the mode of action for most (except TCC). Sub-MIC levels of all ingredients did induce subtle effects on the organisms which impacted bacterial growth, even for those which had no true inhibitory effects.