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International audience; Mutable bacterial cells are defective in their DNA repair system and they often have a phenotype different than their wild-type counterparts. In human bacterial pathogens, the mutable and hypermutable phenotype are often associated with general antibiotic resistance. Here, we quantified the occurrence of mutable cells in Pseudomonas viridiflava, a phytopathogenic bacterium in the P. syringae complex with a broad host range and capacity to live as saprophyte. Two phenotypic variants (transparent and mucoid) were produced by this bacterium. The transparent variant had a mutator phenotype, showed general antibiotic resistance and could not induce disease on the plant species tested (bean). In contrast, the mucoid variant did not display mutability nor resistance to antibiotics and was capable of inducing disease on bean. Both the transparent and mucoid variants were less fit when grown in vitro, while in planta both of the variants and wild-types attained similar population densities. Given the importance of the methyl-directed mismatch repair system (MMR) in the occurrence of mutable and hypermutable cells in human bacterial pathogens, we investigated whether mutations in mut genes were associated with mutator transparent cells in P. viridiflava. Our results showed no mutations in MMR genes in any of the P. viridiflava cells tested. Here we report that high mutation rate and antibiotic resistance are inversely correlated with pathogenicity in P. viridiflava but are not associated with mutations in MMR. In addition, P. viridiflava variants differ from variants produced by other phytopathogenic bacteria in the absence of reversion to the wild-type phenotype.

Bacterial blight of fruit trees caused by Pseudomonas syringae causes significant economic losses worldwide. With the expansion of bacterial canker of kiwi caused by P. syringae pv. actinidiae (Psa) and bacterial canker of apricot caused by P. syringae pv. syringae (Pss), identification of reservoirs of pathogenic strains is required. One potential reservoir is ground covers in orchards. A solution could be the development of ecological engineering practices, particularly ground cover management in order to reduce their impact as a source of inoculum for bacterial diseases of fruit trees caused by P. syringae and increase their role as a reserve for microorganisms that are antagonistic to pathogens of fruit trees. However, with the recent discovery of the complexity of the phylogeny of P. syringae and the existence of phylogroups containing more aggressive strains than others (Psa in phylogroup 1, Pss in phylogroup 2), one of the first goals is the development of a specific molecular detection method, by PCR, allowing rapid and accurate identification of the different phylogroups of P. syringae. This would be much more efficient than the only method currently available - sequencing of specific conserved genes used in phylogenetic identification. The simple implementation of this new method of genotyping makes it possible to screen samples of very large size with little effort. This method can be deployed to develop methods of control of bacterial blight, and can be used as a generic mean of detecting and monitoring orchards and crops. Indeed, the use of a technique targeting a single pathovar may be insufficient, as it is not uncommon for plants to be simultaneously attacked by several different strains of P. syringae. The method of detection and identification of phylogroups will be presented and a concrete example for specific samples (ground cover, buds, twigs) from orchards of apricot and kiwi will be described.

International audience; As a species complex, Pseudomonas syringae exists in both agriculture and natural aquatic habitats. P. viridiflava, a member of this complex, has been reported to be phenotypically largely homogenous. We characterized strains from different habitats, selected based on their genetic similarity to previously described P. viridiflava strains. We revealed two distinct phylogroups and two different kinds of variability in phenotypic traits and genomic content. The strains exhibited phase variation in phenotypes including pathogenicity and soft rot on potato. We showed that the presence of two configurations of the Type III Secretion System [single (S-PAI) and tripartite (T-PAI) pathogenicity islands] are not correlated with pathogenicity or with the capacity to induce soft rot in contrast to previous reports. The presence/absence of the avrE effector gene was the only trait we found to be correlated with pathogenicity of P. viridiflava. Other Type III secretion effector genes were not correlated with pathogenicity. A genomic region resembling an exchangeable effector locus (EEL) was found in S-PAI strains, and a probable recombination between the two PAIs is described. The ensemble of the variability observed in these phylogroups of P. syringae likely contributes to their adaptability to alternating opportunities for pathogenicity or saprophytic survival.