Two Genomic Regions Involved in Catechol Siderophore Production by Erwinia carotovora

Siderophore profiling of co-habitating soil bacteria by ultra-high resolution mass spectrometry

Calcareous soil microbes complete for scarce iron by synthesizing diverse suites of siderophores detectable by ultra-high resolution mass spectrometry.

Diversity and evolution of the phenazine biosynthesis pathway

Phenazines are versatile secondary metabolites of bacterial origin that function in biological control of plant pathogens and contribute to the ecological fitness and pathogenicity of the producing strains. In this study, we employed a collection of 94 strains having various geographic, environmental, and clinical origins to study the distribution and evolution of phenazine genes in members of the genera Pseudomonas, Burkholderia, Pectobacterium, Brevibacterium, and Streptomyces. Our results confirmed the diversity of phenazine producers and revealed that most of them appear to be soil-dwelling and/or plant-associated species. Genome analyses and comparisons of phylogenies inferred from sequences of the key phenazine biosynthesis (phzF) and housekeeping (rrs, recA, rpoB, atpD, and gyrB) genes revealed that the evolution and dispersal of phenazine genes are driven by mechanisms ranging from conservation in Pseudomonas spp. to horizontal gene transfer in Burkholderia spp. and Pectobacterium spp. DNA extracted from cereal crop rhizospheres and screened for the presence of phzF contained sequences consistent with the presence of a diverse population of phenazine producers in commercial farm fields located in central Washington state, which provided the first evidence of United States soils enriched in indigenous phenazine-producing bacteria.

Pseudomonas corrugata contains a conserved N-acyl homoserine lactone quorum sensing system; its role in tomato pathogenicity and tobacco hypersensitivity response

Pseudomonas corrugata is a phytopathogenic bacterium, causal agent of tomato pith necrosis, yet it is an ubiquitous bacterium that is part of the microbial community in the soil and in the rhizosphere of different plant species. Although it is a very heterogeneous species, all the strains tested were able to produce short chain acyl homoserine lactone (AHL) quorum sensing signal molecules. The main AHL produced was N-hexanoyl-L-homoserine lactone (C(6)-AHL). An AHL quorum sensing system, designated PcoI/PcoR, was identified and characterized. The role of the quorum sensing system in the expression of a variety of traits was evaluated. Inactivation of pcoI abolished the production of AHLs. The pcoR mutant, but not the pcoI mutant, was impaired in swarming, unable to cause a hypersensitivity response on tobacco and resulted in a reduced tomato pith necrosis phenotype. The pcoI mutant showed a reduced antimicrobial activity against various fungi and bacteria when assayed on King's B medium. These results demonstrate that the AHL quorum sensing in Ps. corrugata regulates traits that contribute to virulence, antimicrobial activity and fitness. This is the first report of genes of Ps. corrugata involved in the disease development and biological control activity.

Mutations affecting predation ability of the soil bacterium Myxococcus xanthus

Myxococcus xanthus genetic mutants with characterized phenotypes were analysed for the ability to prey on susceptible bacteria. Quantification of predatory ability was scored by a newly developed method under conditions in which prey bacteria provided the only source of nutrients. These results were corroborated by data derived using a previously published protocol that measures predation in the presence of limited external nutrients. First, early developmental regulatory mutants were examined, because their likely functions in assessing the local nutrient status were predicted to be also important for predation. The results showed that predation efficiency is reduced by 64-80 % for mutants of three A-signalling components, AsgA, AsgC and AsgE, but not for AsgB. This suggests that an Asg regulon function that is separate from A-signal production is needed for predation. Besides the Asg components, mutations in the early developmental genes sdeK and csgA were also consistently observed to reduce predatory efficacy by 36 and 33 %, respectively. In contrast, later developmental components, such as DevRS, 4406 and PhoP4, did not appear to play significant roles in predation. The predatory abilities of mutants defective for motility were also tested. The data showed that adventurous, but not social, motility is required for predation in the assay. Also, mutants for components in the chemotaxis-like Frz system were found to be reduced in predation efficiency by between 62 and 85 %. In sum, it was demonstrated here that defects in development and development-related processes affect the ability of M. xanthus to prey on other bacteria.

Role of iron in rhizobacteria-mediated induced systemic resistance of cucumber

ABSTRACT Seed treatment with the rhizosphere bacterium Serratia marcescens strain 90-166 suppressed anthracnose of cucumber, caused by Colleto-trichum orbiculare, through induced systemic resistance (ISR). When the iron concentration of a planting mix was decreased by addition of an iron chelator, suppression of cucumber anthracnose by strain 90-166 was significantly improved. Strain 90-166 produced 465 +/- 70 mg/liter of catechol siderophore, as determined by the Rioux assay in deferrated King's medium B. The hypothesis that a catechol siderophore produced by strain 90-166 may be responsible for induction of systemic resistance by this strain was tested by evaluating disease suppression by a mini-Tn5-phoA mutant deficient in siderophore production. Sequence analysis of genomic DNA flanking the mini-Tn5-phoA insertion identified the target gene as entA, which encodes an enzyme in the catechol siderophore biosynthetic pathways of several bacteria. Severity of anthracnose of cucumbers treated with the entA mutant was not significantly different (P = 0.05) from the control, whereas plants treated with wild-type 90-166 had significantly less disease (P = 0.05) than the control. Total (internal and external) population sizes of 90-166 and the entA mutant on roots did not differ significantly (P = 0.05) at any sample time, whereas internal population sizes of the entA mutant were significantly lower (P = 0.05) than those of the wild-type strain at two sampling times. These data suggest that catechol siderophore biosynthesis genes in Serratia marcescens 90-166 are associated with ISR but that this role may be indirect via a reduction in internal root populations.

WITHHOLDING AND EXCHANGING IRON: Interactions Between Erwinia spp. and Their Plant Hosts

The critical role of iron in plant host-parasite relationships has been elucidated in diseases as different as the soft rot and fire blight incited by Erwinia chrysanthemi and E. amylovora, respectively. As in animal infections, the role of iron and its ligands in the virulence of plant pathogens seems to be more subtle than might be expected, and is intimately related to the life cycle of the pathogen within its host. This review discusses how iron, because of its unique position in biological systems, controls the activities of these plant pathogens. Molecular studies illustrating the key question of iron acquisition and homeostasis during pathogenesis are described. The production of siderophores by pathogens not only represents a powerful strategy to acquire iron from host tissues but may also act as a protective agent against iron toxicity. The need of the host to bind and possibly sequester the metal during pathogenesis is another central issue. Possible modes of iron competition between plant host and pathogen are considered.