Xanthomonas axonopodis pv. phaseoli var. fuscans is aggregated in stable biofilm population sizes in the phyllosphere of field-grown beans

Comparative transcriptomics reveals a highly polymorphic Xanthomonas HrpG virulence regulon

BACKGROUND

Bacteria of the genus Xanthomonas cause economically significant diseases in various crops. Their virulence is dependent on the translocation of type III effectors (T3Es) into plant cells by the type III secretion system (T3SS), a process regulated by the master response regulator HrpG. Although HrpG has been studied for over two decades, its regulon across diverse Xanthomonas species, particularly beyond type III secretion, remains understudied.

RESULTS

In this study, we conducted transcriptome sequencing to explore the HrpG regulons of 17 Xanthomonas strains, encompassing six species and nine pathovars, each exhibiting distinct host and tissue specificities. We employed constitutive expression of plasmid-borne hrpG*, which encodes a constitutively active form of HrpG, to induce the regulon. Our findings reveal substantial inter- and intra-specific diversity in the HrpG* regulons across the strains. Besides 21 genes directly involved in the biosynthesis of the T3SS, the core HrpG* regulon is limited to only five additional genes encoding the transcriptional activator HrpX, the two T3E proteins XopR and XopL, a major facility superfamily (MFS) transporter, and the phosphatase PhoC. Interestingly, genes involved in chemotaxis and genes encoding enzymes with carbohydrate-active and proteolytic activities are variably regulated by HrpG*.

CONCLUSIONS

The diversity in the HrpG* regulon suggests that HrpG-dependent virulence in Xanthomonas might be achieved through several distinct strain-specific strategies, potentially reflecting adaptation to diverse ecological niches. These findings enhance our understanding of the complex role of HrpG in regulating various virulence and adaptive pathways, extending beyond T3Es and the T3SS.

Whole Genome Sequences of Nine Xanthomonas Strains Responsible for Common Bacterial Blight of Bean

We report the complete and circularized genome sequences of nine strains of Xanthomonas phaseoli pv. phaseoli and Xanthomonas citri pv. fuscans, which cause common bacterial blight of bean. These assemblies provide high-quality material for functional and evolutionary studies of these legume pathogens.

Microbe Related Chemical Signalling and Its Application in Agriculture

The agriculture sector has been put under tremendous strain by the world’s growing population. The use of fertilizers and pesticides in conventional farming has had a negative impact on the environment and human health. Sustainable agriculture attempts to maintain productivity, while protecting the environment and feeding the global population. The importance of soil-dwelling microbial populations in overcoming these issues cannot be overstated. Various processes such as rhizospheric competence, antibiosis, release of enzymes, and induction of systemic resistance in host plants are all used by microbes to influence plant-microbe interactions. These processes are largely founded on chemical signalling. Producing, releasing, detecting, and responding to chemicals are all part of chemical signalling. Different microbes released distinct sorts of chemical signal molecules which interacts with the environment and hosts. Microbial chemicals affect symbiosis, virulence, competence, conjugation, antibiotic production, motility, sporulation, and biofilm growth, to name a few. We present an in-depth overview of chemical signalling between bacteria-bacteria, bacteria-fungi, and plant-microbe and the diverse roles played by these compounds in plant microbe interactions. These compounds’ current and potential uses and significance in agriculture have been highlighted.

Common bacterial blight of bean: a model of seed transmission and pathological convergence

BACKGROUND

Xanthomonas citri pv. fuscans (Xcf) and Xanthomonas phaseoli pv. phaseoli (Xpp) are the causal agents of common bacterial blight of bean (CBB), an important disease worldwide that remains difficult to control. These pathogens belong to distinct species within the Xanthomonas genus and have undergone a dynamic evolutionary history including the horizontal transfer of genes encoding factors probably involved in adaptation to and pathogenicity on common bean. Seed transmission is a key point of the CBB disease cycle, favouring both vertical transmission of the pathogen and worldwide distribution of the disease through global seed trade.

TAXONOMY

Kingdom: Bacteria; phylum: Proteobacteria; class: Gammaproteobacteria; order: Lysobacterales (also known as Xanthomonadales); family: Lysobacteraceae (also known as Xanthomonadaceae); genus: Xanthomonas; species: X. citri pv. fuscans and X. phaseoli pv. phaseoli (Xcf-Xpp).

HOST RANGE

The main host of Xcf-Xpp is the common bean (Phaseolus vulgaris). Lima bean (Phaseolus lunatus) and members of the Vigna genus (Vigna aconitifolia, Vigna angularis, Vigna mungo, Vigna radiata, and Vigna umbellata) are also natural hosts of Xcf-Xpp. Natural occurrence of Xcf-Xpp has been reported for a handful of other legumes such as Calopogonium sp., Pueraria sp., pea (Pisum sativum), Lablab purpureus, Macroptilium lathyroides, and Strophostyles helvola. There are conflicting reports concerning the natural occurrence of CBB agents on tepary bean (Phaseolus acutifolius) and cowpea (Vigna unguiculata subsp. unguiculata).

SYMPTOMS

CBB symptoms occur on all aerial parts of beans, that is, seedlings, leaves, stems, pods, and seeds. Symptoms initially appear as water-soaked spots evolving into necrosis on leaves, pustules on pods, and cankers on twigs. In severe infections, defoliation and wilting may occur.

DISTRIBUTION

CBB is distributed worldwide, meaning that it is frequently encountered in most places where bean is cultivated in the Americas, Asia, Africa, and Oceania, except for arid tropical areas. Xcf-Xpp are regulated nonquarantine pathogens in Europe and are listed in the A2 list by the European and Mediterranean Plant Protection Organization (EPPO).

GENOME

The genome consists of a single circular chromosome plus one to four extrachromosomal plasmids of various sizes, for a total mean size of 5.27 Mb with 64.7% GC content and an average predicted number of 4,181 coding sequences.

DISEASE CONTROL

Management of CBB is based on integrated approaches that comprise measures aimed at avoiding Xcf-Xpp introduction through infected seeds, cultural practices to limit Xcf-Xpp survival between host crops, whenever possible the use of tolerant or resistant bean genotypes, and chemical treatments, mainly restricted to copper compounds. The use of pathogen-free seeds is essential in an effective management strategy and requires appropriate sampling, detection, and identification methods. USEFUL WEBSITES: https://gd.eppo.int/taxon/XANTPH, https://gd.eppo.int/taxon/XANTFF, and http://www.cost.eu/COST_Actions/ca/CA16107.

Genome sequence of Xanthomonas fuscans subsp. fuscans strain Xff49: a new isolate obtained from common beans in Southern Brazil

The genus Xanthomonas comprises Gram-negative bacteria, many of which are phytopathogens. Xanthomonas fuscans subsp. fuscans is one of the most devastating pathogens affecting the bean plant, resulting in the common bacterial blight of bean (CBB). The disease is mainly foliar and affects a wide variety of bean species, thus acting as the yield-limiting factor for the bean crop. Here, we report the whole-genome sequencing of a new strain of X. fuscans subsp. fuscans, named Xff49, isolated from the infected and symptomatic beans from Capão do Leão, Southern Brazil. The genetic analysis demonstrated the presence of single-nucleotide variants (SNVs) in this strain, potentially affecting the mobilome, cell mobility, and inorganic ion metabolism. In addition, the analysis resulted in the identification of a new plasmid similar to the pAX22 derived from Achromobacter denitrificans, which was named plX, along with plA and plC, previously reported in other strains of X. fuscans subsp. fuscans. Xff49 represents the first Brazilian genome of X. fuscans subsp. fuscans and might provide useful information applicable to the studies of phylogenetics, evolution, and pathogenomics, thereby allowing a better understanding of the genomic features present in the Brazilian strains.

Adaptation of genetically monomorphic bacteria: evolution of copper resistance through multiple horizontal gene transfers of complex and versatile mobile genetic elements

Copper-based antimicrobial compounds are widely used to control plant bacterial pathogens. Pathogens have adapted in response to this selective pressure. Xanthomonas citri pv. citri, a major citrus pathogen causing Asiatic citrus canker, was first reported to carry plasmid-encoded copper resistance in Argentina. This phenotype was conferred by the copLAB gene system. The emergence of resistant strains has since been reported in Réunion and Martinique. Using microsatellite-based genotyping and copLAB PCR, we demonstrated that the genetic structure of the copper-resistant strains from these three regions was made up of two distant clusters and varied for the detection of copLAB amplicons. In order to investigate this pattern more closely, we sequenced six copper-resistant X. citri pv. citri strains from Argentina, Martinique and Réunion, together with reference copper-resistant Xanthomonas and Stenotrophomonas strains using long-read sequencing technology. Genes involved in copper resistance were found to be strain dependent with the novel identification in X. citri pv. citri of copABCD and a cus heavy metal efflux resistance-nodulation-division system. The genes providing the adaptive trait were part of a mobile genetic element similar to Tn3-like transposons and included in a conjugative plasmid. This indicates the system's great versatility. The mining of all available bacterial genomes suggested that, within the bacterial community, the spread of copper resistance associated with mobile elements and their plasmid environments was primarily restricted to the Xanthomonadaceae family.

The Xanthomonas type IV pilus

Type IV pili, a special class of bacterial surface filaments, are key behavioral mediators for many important human pathogens. However, we know very little about the role of these structures in the lifestyles of plant-associated bacteria. Over the past few years, several groups studying the extensive genus of Xanthomonas spp. have gained insights into the roles of played by type IV pili in bacteria-host interactions and pathogenesis, motility, biofilm formation, and interactions with bacteriophages. Protein-protein interaction studies have identified T4P regulators and these, along with structural studies, have begun to reveal some of the possible molecular mechanisms that may control the extension/retraction cycles of these dynamic filaments.

Surface polysaccharides and quorum sensing are involved in the attachment and survival of Xanthomonas albilineans on sugarcane leaves

Xanthomonas albilineans, the causal agent of sugarcane leaf scald, is a bacterial plant pathogen that is mainly spread by infected cuttings and contaminated harvesting tools. However, some strains of this pathogen are known to be spread by aerial means and are able to colonize the phyllosphere of sugarcane before entering the host plant and causing disease. The objective of this study was to identify the molecular factors involved in the survival or growth of X. albilineans on sugarcane leaves. We developed a bioassay to test for the attachment of X. albilineans on sugarcane leaves using tissue-cultured plantlets grown in vitro. Six mutants of strain XaFL07-1 affected in surface polysaccharide production completely lost their capacity to survive on the sugarcane leaf surface. These mutants produced more biofilm in vitro and accumulated more cellular poly-β-hydroxybutyrate than the wild-type strain. A mutant affected in the production of small molecules (including potential biosurfactants) synthesized by non-ribosomal peptide synthetases (NRPSs) attached to the sugarcane leaves as well as the wild-type strain. Surprisingly, the attachment of bacteria on sugarcane leaves varied among mutants of the rpf gene cluster involved in bacterial quorum sensing. Therefore, quorum sensing may affect polysaccharide production, or both polysaccharides and quorum sensing may be involved in the survival or growth of X. albilineans on sugarcane leaves.

Draft Genome Sequence of the Flagellated Xanthomonas fuscans subsp. fuscans Strain CFBP 4884

We report the draft genome sequence of the flagellated strain CFBP 4884 of Xanthomonas fuscans subsp. fuscans, which was isolated in an outbreak of common bacterial blight of beans along with non-flagellated strains. Comparative genomics will allow one to decipher the genomic diversity of strains cohabiting in epidemics.

Genome sequence of Xanthomonas fuscans subsp. fuscans strain 4834-R reveals that flagellar motility is not a general feature of xanthomonads

BACKGROUND

Xanthomonads are plant-associated bacteria responsible for diseases on economically important crops. Xanthomonas fuscans subsp. fuscans (Xff) is one of the causal agents of common bacterial blight of bean. In this study, the complete genome sequence of strain Xff 4834-R was determined and compared to other Xanthomonas genome sequences.

RESULTS

Comparative genomics analyses revealed core characteristics shared between Xff 4834-R and other xanthomonads including chemotaxis elements, two-component systems, TonB-dependent transporters, secretion systems (from T1SS to T6SS) and multiple effectors. For instance a repertoire of 29 Type 3 Effectors (T3Es) with two Transcription Activator-Like Effectors was predicted. Mobile elements were associated with major modifications in the genome structure and gene content in comparison to other Xanthomonas genomes. Notably, a deletion of 33 kbp affects flagellum biosynthesis in Xff 4834-R. The presence of a complete flagellar cluster was assessed in a collection of more than 300 strains representing different species and pathovars of Xanthomonas. Five percent of the tested strains presented a deletion in the flagellar cluster and were non-motile. Moreover, half of the Xff strains isolated from the same epidemic than 4834-R was non-motile and this ratio was conserved in the strains colonizing the next bean seed generations.

CONCLUSIONS

This work describes the first genome of a Xanthomonas strain pathogenic on bean and reports the existence of non-motile xanthomonads belonging to different species and pathovars. Isolation of such Xff variants from a natural epidemic may suggest that flagellar motility is not a key function for in planta fitness.

Thermo-regulation of genes mediating motility and plant interactions in Pseudomonas syringae

Pseudomonas syringae is an important phyllosphere colonist that utilizes flagellum-mediated motility both as a means to explore leaf surfaces, as well as to invade into leaf interiors, where it survives as a pathogen. We found that multiple forms of flagellum-mediated motility are thermo-suppressed, including swarming and swimming motility. Suppression of swarming motility occurs between 28° and 30°C, which coincides with the optimal growth temperature of P. syringae. Both fliC (encoding flagellin) and syfA (encoding a non-ribosomal peptide synthetase involved in syringafactin biosynthesis) were suppressed with increasing temperature. RNA-seq revealed 1440 genes of the P. syringae genome are temperature sensitive in expression. Genes involved in polysaccharide synthesis and regulation, phage and IS elements, type VI secretion, chemosensing and chemotaxis, translation, flagellar synthesis and motility, and phytotoxin synthesis and transport were generally repressed at 30°C, while genes involved in transcriptional regulation, quaternary ammonium compound metabolism and transport, chaperone/heat shock proteins, and hypothetical genes were generally induced at 30°C. Deletion of flgM, a key regulator in the transition from class III to class IV gene expression, led to elevated and constitutive expression of fliC regardless of temperature, but did not affect thermo-regulation of syfA. This work highlights the importance of temperature in the biology of P. syringae, as many genes encoding traits important for plant-microbe interactions were thermo-regulated.

Bean common bacterial blight: pathogen epiphytic life and effect of irrigation practices

In recent years, bean common bacterial blight (CBB) caused by Xanthomonas axonopodis pv. phaseoli (Xap) has caused serious yield losses in several countries. CBB is considered mainly a foliar disease in which symptoms initially appear as small water-soaked spots that then enlarge and become necrotic and usually bordered by a chlorotic zone. Xap epiphytic population community has a critical role in the development of the disease and subsequent epidemics. The epiphytic population of Xap in the field has two major parts; solitary cells (potentially planktonic) and biofilms which are sources for providing and refreshing the solitary cell components. Irrigation type has a significant effect on epiphytic population of Xap. The mean epiphytic population size in the field with an overhead sprinkler irrigation system is significantly higher than populations under furrow irrigation. A significant positive correlation between the epiphytic population size of Xap and disease severity has been reported in both the overhead irrigated (r=0.64) and the furrow irrigated (r= 0.44) fields.

Water relations in the interaction of foliar bacterial pathogens with plants

This review examines the many ways in which water influences the relations between foliar bacterial pathogens and plants. As a limited resource in aerial plant tissues, water is subject to manipulation by both plants and pathogens. A model is emerging that suggests that plants actively promote localized desiccation at the infection site and thus restrict pathogen growth as one component of defense. Similarly, many foliar pathogens manipulate water relations as one component of pathogenesis. Nonvascular pathogens do this using effectors and other molecules to alter hormonal responses and enhance intercellular watersoaking, whereas vascular pathogens use many mechanisms to cause wilt. Because of water limitations on phyllosphere surfaces, bacterial colonists, including pathogens, benefit from the protective effects of cellular aggregation, synthesis of hygroscopic polymers, and uptake and production of osmoprotective compounds. Moreover, these bacteria employ tactics for scavenging and distributing water to overcome water-driven barriers to nutrient acquisition, movement, and signal exchange on plant surfaces.

A multilocus sequence analysis of Xanthomonas campestris reveals a complex structure within crucifer-attacking pathovars of this species

Previous classification of Xanthomonas campestris has defined six pathovars (aberrans, armoraciae, barbareae, campestris, incanae, and raphani) that cause diseases on cruciferous plants. However, pathogenicity assays with a range of strains and different hosts identifies only three types of symptom: black rot, leaf spot and bacterial blight. These findings raise the question of the genetic relatedness between strains assigned to different pathovars or symptom phenotypes. Here we have addressed this issue by multilocus sequence analysis of 42 strains. The X. campestris species was polymorphic at the 8 loci analysed and had a high genetic diversity; 23 sequence types were identified of which 16 were unique. All strains that induce black rot (pathovars aberrans and campestris) were genetically close but split in two groups. Only three clonal complexes were found, all within pathovar campestris. The assignment of the genome-sequenced strain 756C to pathovar raphani suggested from disease symptoms was confirmed, although this group of strains was particularly polymorphic. Strains belonging to pathovars barbareae and incanae were closely related, but distinct from pathovar campestris. There is evidence of genetic exchanges of housekeeping genes within this species as deduced from a clear incongruence between individual gene phylogenies and from network structures from SplitsTree analysis. Overall this study showed that the high genetic diversity derived equally from recombination and point mutation accumulation. However, X. campestris remains a species with a clonal evolution driven by a differential adaptation to cruciferous hosts.

Transmission of plant-pathogenic bacteria by nonhost seeds without induction of an associated defense reaction at emergence

An understanding of the mechanisms involved in the different steps of bacterial disease epidemiology is essential to develop new control strategies. Seeds are the passive carriers of a diversified microbial cohort likely to affect seedling physiology. Among seed-borne plant-pathogenic bacteria, seed carriage in compatible situations is well evidenced. The aims of our work are to determine the efficiency of pathogen transmission to seeds of a nonhost plant and to evaluate bacterial and plant behaviors at emergence. Bacterial transmission from flowers to seeds and from seeds to seedlings was measured for Xanthomonas campestris pv. campestris in incompatible interactions with bean. Transmissions from seeds to seedlings were compared for X. campestris pv. campestris, for Xanthomonas citri pv. phaseoli var. fuscans in compatible interactions with bean, and for Escherichia coli, a human pathogen, in null interactions with bean. The induction of defense responses was monitored by using reverse transcription and quantitative PCR (RT-qPCR) of genes representing the main signaling pathways and assaying defense-related enzymatic activities. Flower inoculations resulted in a high level of bean seed contamination by X. campestris pv. campestris, which transmitted efficiently to seedlings. Whatever the type of interaction tested, dynamics of bacterial population sizes were similar on seedlings, and no defense responses were induced evidencing bacterial colonization of seedlings without any associated defense response induction. Bacteria associated with the spermosphere multiply in this rich environment, suggesting that the colonization of seedlings relies mostly on commensalism. The transmission of plant-pathogenic bacteria to and by nonhost seeds suggests a probable role of seeds of nonhost plants as an inoculum source.

Molecular detection of transcriptionally active bacteria from failed prosthetic hip joints removed during revision arthroplasty

The purpose of this study was to use microbiological culture and bacterial 16S rRNA gene sequencing methods to detect transcriptionally active bacteria present on the surface of failed prosthetic hip joints removed during revision arthroplasty. Five failed prosthetic hip joints were sonicated to dislodge adherent bacteria and subjected to microbiological culture. Bacterial RNA was extracted from each sonicate, cDNA prepared by reverse transcription and the 16S rRNA gene amplified using universal primers. Polymerase chain reaction (PCR) products were cloned, assigned to distinct groups by restriction fragment length polymorphism (RFLP) analysis and one representative clone from each group was sequenced. Bacteria were identified by comparison of the obtained 16S rRNA gene sequences with those deposited in public access sequence databases. All five specimens were positive for the presence of bacteria by both culture and PCR. Culture methods identified species from eight genera. Molecular detection of transcriptionally active bacteria identified a wider range of species. A total of 42 phylotypes were identified, of which Lysobacter gummosus was the most abundant (31.6%). Thirty-four clones (14.5%) represented uncultivable phylotypes. No potentially novel species were identified. It is concluded that a diverse range of transcriptionally active bacterial species are present within biofilms on the surface of failed prosthetic hip joints.

Occurrence of Xanthomonas axonopodis pv. phaseoli, the causal agent of common bacterial blight disease, on seeds of common bean (Phaseolus vulgaris L.) in upper Egypt

Common bean seed lots collected from different seed dealers and Malawii agriculture station were screened for the presence of Xanthomonas axonopodis pv. phaseoli. In the laboratory the pathogen was isolated following the routine laboratory assay method, i.e. direct plating method using yeast extract-dextrose-calcium carbonate agar medium (YDC). Yellow, convex, mucoid colonies of Xanthomonas were consistently isolated on YDC from seed samples. The presumptive pathogen was confirmed by isolation on semiselective medium, such as mTBM and MD5A. Further, the pathogen was confirmed by biochemical, physiological and, finally, the pathogenicity tests. Five samples out of seven were positive for Xanthomonas. The isolates were found to cause common blight of 3-week-old common bean plants by 7 d after inoculation. Bacteria with the same characteristics as those inoculated were re-isolated from the infected plants.

Transcriptional studies of the hrpM/opgH gene in Pseudomonas syringae during biofilm formation and in response to different environmental challenges

Pseudomonas syringae pv. syringae strain FF5 is a phytopathogen that causes a rapid dieback on ornamental pear trees. In the present study, the transcriptional expression of hrpM/opgH, algD, hrpR and rpoD was evaluated in P. syringae FF5 and FF5.M2 (hrpM/opgH mutant). The temporal expression of these genes was evaluated during biofilm formation, the hypersensitive reaction (HR) on tobacco plants, and when the bacteria were subjected to different environmental stresses. The results indicate that mutations in hrpM negatively impair several traits including biofilm formation, the ability to cause disease in host plants and the HR in non-host plants, and the expression of hrpR, a regulatory gene modulating the latter two traits. Furthermore, FF5.M2 was decreased in swarming motility and unable to respond to different environmental challenges. Interestingly, FF5.M2 showed an exponential increase in the expression of algD, which is the first gene to be transcribed during the biosynthesis of the alginate, a virulence factor in P. syringae. The expression of both hrpM and algD were required for biofilm formation, and hrpM was expressed earlier than algD during biofilm development. These findings indicate that hrpM expression is required for several traits in P. syringae and plays an important role in how this bacterium responds to environmental challenges.

A "repertoire for repertoire" hypothesis: repertoires of type three effectors are candidate determinants of host specificity in Xanthomonas

Background

The genetic basis of host specificity for animal and plant pathogenic bacteria remains poorly understood. For plant pathogenic bacteria, host range is restricted to one or a few host plant species reflecting a tight adaptation to specific hosts.

Methodology/Principal Findings

Two hypotheses can be formulated to explain host specificity: either it can be explained by the phylogenetic position of the strains, or by the association of virulence genes enabling a pathological convergence of phylogenically distant strains. In this latter hypothesis, host specificity would result from the interaction between repertoires of bacterial virulence genes and repertoires of genes involved in host defences. To challenge these two hypotheses, we selected 132 Xanthomonas axonopodis strains representative of 18 different pathovars which display different host range. First, the phylogenetic position of each strain was determined by sequencing the housekeeping gene rpoD. This study showed that many pathovars of Xanthomonas axonopodis are polyphyletic. Second, we investigated the distribution of 35 type III effector genes (T3Es) in these strains by both PCR and hybridization methods. Indeed, for pathogenic bacteria T3Es were shown to trigger and to subvert host defences. Our study revealed that T3E repertoires comprise core and variable gene suites that likely have distinct roles in pathogenicity and different evolutionary histories. Our results showed a correspondence between composition of T3E repertoires and pathovars of Xanthomonas axonopodis. For polyphyletic pathovars, this suggests that T3E genes might explain a pathological convergence of phylogenetically distant strains. We also identified several DNA rearrangements within T3E genes, some of which correlate with host specificity of strains.

Conclusions/Significance

These data provide insight into the potential role played by T3E genes for pathogenic bacteria and support a “repertoire for repertoire” hypothesis that may explain host specificity. Our work provides resources for functional and evolutionary studies aiming at understanding host specificity of pathogenic bacteria, functional redundancy between T3Es and the driving forces shaping T3E repertoires.

Populations of Xanthomonas citri pv. mangiferaeindicae from asymptomatic mango leaves are primarily endophytic

Epiphytic survival of several Xanthomonas pathovars has been reported, but most studies failed to determine whether such populations were resident epiphytes, resulting from latent infections, or casual epiphytes. This study aimed at understanding the nature of Xanthomonas citri pv. mangiferaeindicae populations associated with asymptomatic leaves. When spray-inoculated on mango leaves cv. Maison Rouge, the pathogen multiplied markedly in association with juvenile leaves, but was most often detected as low population sizes (<1 x 10(3) cfu g(-1)) in association with mature leaves. Our results suggest a very low biological significance of biofilm-associated populations of X. citri pv. mangiferaeindicae, while saprophytic microbiota associated with mango leaves survived frequently as biofilms. A chloroform vapor-based disinfestation assay which kills cells specifically located on the leaf surface and not those located within the leaf mesophyll was developed. When applied to spray-inoculated leaves maintained under controlled environmental conditions, 155 out of the 168 analyzed datasets collected over three assessment dates for seven bacterial strains representative of the genetic diversity of the pathogen failed to demonstrate a significant X. citri pv. mangiferaeindicae population decrease on chloroform treated leaves up to 13 days after inoculation. We conclude that an efficient survival of X. citri pv. mangiferaeindicae present on mango leaf surfaces following a limited dissemination event is largely dependent on the availability of juvenile plant tissues. The bacterium gains access to protected sites (e.g., mesophyll) through stomata where it becomes endophytic and eventually causes disease. Chloroform vapor-based disinfestation assays should be useful for further studies aiming at evaluating survival sites of bacteria associated with the phyllosphere.

Adhesion and fitness in the bean phyllosphere and transmission to seed of Xanthomonas fuscans subsp. fuscans

Deciphering the mechanisms enabling plant-pathogenic bacteria to disperse, colonize, and survive on their hosts provides the necessary basis to set up new control methods. We evaluated the role of bacterial attachment and biofilm formation in host colonization processes for Xanthomonas fuscans subsp. fuscans on its host. This bacterium is responsible for the common bacterial blight of bean (Phaseolus vulgaris), a seedborne disease. The five adhesin genes (pilA, fhab, xadA1, xadA2, and yapH) identified in X. fuscans subsp. fuscans CFBP4834-R strain were mutated. All mutants were altered in their abilities to adhere to polypropylene or seed. PilA was involved in adhesion and transmission to seed, and mutation of pilA led to lower pathogenicity on bean. YapH was required for adhesion to seed, leaves, and abiotic surfaces but not for in planta transmission to seed or aggressiveness on leaves. Transmission to seed through floral structures did not require any of the known adhesins. Conversely, all mutants tested, except in yapH, were altered in their vascular transmission to seed. In conclusion, we showed that adhesins are implicated in the various processes leading to host phyllosphere colonization and transmission to seed by plant-pathogenic bacteria.

Quorum sensing and virulence regulation in Xanthomonas campestris

It is now clear that cell-cell communication, often referred to as quorum sensing (QS), is the norm in the prokaryotic kingdom and this community-wide genetic regulatory mechanism has been adopted for regulation of many important biological functions. Since the 1980s, several types of QS signals have been identified, which are associated commonly with different types of QS mechanisms. Among them, the diffusible signal factor (DSF)-dependent QS system, originally discovered from bacterial pathogen Xanthomonas campestris pv. campestris, is a relatively new regulatory mechanism. The rapid research progress over the last few years has identified the chemical structure of the QS signal DSF, established the DSF regulon, and unveiled the general signaling pathways and mechanisms. Particular noteworthy are that DSF biosynthesis is modulated by a novel posttranslational autoinduction mechanism involving protein-protein interaction between the DSF synthase RpfF and the sensor RpfC, and that QS signal sensing is coupled to intracellular regulatory networks through a second messenger cyclic-di-GMP and a global regulator Clp. Genomic and genetic analyses show that the DSF QS-signaling pathway regulates diverse biological functions including virulence, biofilm dispersal, and ecological competence. Moreover, evidence is emerging that the DSF QS system is conserved in a range of plant and human bacterial pathogens.

Causes and consequences of plant-associated biofilms

The rhizosphere is the critical interface between plant roots and soil where beneficial and harmful interactions between plants and microorganisms occur. Although microorganisms have historically been studied as planktonic (or free-swimming) cells, most are found attached to surfaces, in multicellular assemblies known as biofilms. When found in association with plants, certain bacteria such as plant growth promoting rhizobacteria not only induce plant growth but also protect plants from soil-borne pathogens in a process known as biocontrol. Contrastingly, other rhizobacteria in a biofilm matrix may cause pathogenesis in plants. Although research suggests that biofilm formation on plants is associated with biological control and pathogenic response, little is known about how plants regulate this association. Here, we assess the biological importance of biofilm association on plants.

The Type III secretion system of Xanthomonas fuscans subsp. fuscans is involved in the phyllosphere colonization process and in transmission to seeds of susceptible beans

Understanding the survival, multiplication, and transmission to seeds of plant pathogenic bacteria is central to study their pathogenesis. We hypothesized that the type III secretion system (T3SS), encoded by hrp genes, could have a role in host colonization by plant pathogenic bacteria. The seed-borne pathogen Xanthomonas fuscans subsp. fuscans causes common bacterial blight of bean (Phaseolus vulgaris). Directed mutagenesis in strain CFBP4834-R of X. fuscans subsp. fuscans and bacterial population density monitoring on bean leaves showed that strains with mutations in the hrp regulatory genes, hrpG and hrpX, were impaired in their phyllospheric growth, as in the null interaction with Escherichia coli C600 and bean. In the compatible interaction, CFBP4834-R reached high phyllospheric population densities and was transmitted to seeds at high frequencies with high densities. Strains with mutations in structural hrp genes maintained the same constant epiphytic population densities (1 x 10(5) CFU g(-1) of fresh weight) as in the incompatible interaction with Xanthomonas campestris pv. campestris ATCC 33913 and the bean. Low frequencies of transmission to seeds and low bacterial concentrations were recorded for CFBP4834-R hrp mutants and for ATCC 33913, whereas E. coli C600 was not transmitted. Moreover, unlike the wild-type strain, strains with mutations in hrp genes were not transmitted to seeds by vascular pathway. Transmission to seeds by floral structures remained possible for both. This study revealed the involvement of the X. fuscans subsp. fuscans T3SS in phyllospheric multiplication and systemic colonization of bean, leading to transmission to seeds. Our findings suggest a major contribution of hrp regulatory genes in host colonization processes.

Identification of bacteria on the surface of clinically infected and non-infected prosthetic hip joints removed during revision arthroplasties by 16S rRNA gene sequencing and by microbiological culture

It has been postulated that bacteria attached to the surface of prosthetic hip joints can cause localised inflammation, resulting in failure of the replacement joint. However, diagnosis of infection is difficult with traditional microbiological culture methods, and evidence exists that highly fastidious or non-cultivable organisms have a role in implant infections. The purpose of this study was to use culture and culture-independent methods to detect the bacteria present on the surface of prosthetic hip joints removed during revision arthroplasties. Ten consecutive revisions were performed by two surgeons, which were all clinically and radiologically loose. Five of the hip replacement revision surgeries were performed because of clinical infections and five because of aseptic loosening. Preoperative and perioperative specimens were obtained from each patient and subjected to routine microbiological culture. The prostheses removed from each patient were subjected to mild ultrasonication to dislodge adherent bacteria, followed by aerobic and anaerobic microbiological culture. Bacterial DNA was extracted from each sonicate and the 16S rRNA gene was amplified with the universal primer pair 27f/1387r. All 10 specimens were positive for the presence of bacteria by both culture and PCR. PCR products were then cloned, organised into groups by RFLP analysis and one clone from each group was sequenced. Bacteria were identified by comparison of the 16S rRNA gene sequences obtained with those deposited in public access sequence databases. A total of 512 clones were analysed by RFLP analysis, of which 118 were sequenced. Culture methods identified species from the genera Leifsonia (54.3%), Staphylococcus (21.7%), Proteus (8.7%), Brevundimonas (6.5%), Salibacillus (4.3%), Methylobacterium (2.2%) and Zimmermannella (2.2%). Molecular detection methods identified a more diverse microflora. The predominant genus detected was Lysobacter, representing 312 (60.9%) of 512 clones analysed. In all, 28 phylotypes were identified: Lysobacter enzymogenes was the most abundant phylotype (31.4%), followed by Lysobacter sp. C3 (28.3%), gamma proteobacterium N4-7 (6.6%), Methylobacterium SM4 (4.7%) and Staphylococcus epidermidis (4.7%); 36 clones (7.0%) represented uncultivable phylotypes. We conclude that a diverse range of bacterial species are found within biofilms on the surface of clinically infected and non-infected prosthetic hip joints removed during revision arthroplasties.

Biofilm formation by plant-associated bacteria

Plants support a diverse array of bacteria, including parasites, mutualists, and commensals on and around their roots, in the vasculature, and on aerial tissues. These microbes have a profound influence on plant health and productivity. Bacteria physically interact with surfaces to form complex multicellular and often multispecies assemblies, including biofilms and smaller aggregates. There is growing appreciation that the intensity, duration, and outcome of plant-microbe interactions are significantly influenced by the conformation of adherent microbial populations. Biofilms on different tissues have unique properties, reflecting the prevailing conditions at those sites. Attachment is required for biofilm formation, and bacteria interact with plant tissues through adhesins including polysaccharides and surface proteins, with initial contact often mediated by active motility. Recognition between lectins and their cognate carbohydrates is a common means of specificity. Biofilm development and the resulting intimate interactions with plants often require cell-cell communication between colonizing bacteria.

The HD-GYP domain, cyclic di-GMP signaling, and bacterial virulence to plants

Cyclic di-GMP is an almost ubiquitous second messenger in bacteria that was first described as an allosteric activator of cellulose synthase but is now known to regulate a range of functions, including virulence in human and animal pathogens. Two protein domains, GGDEF and EAL, are implicated in the synthesis and degradation, respectively, of cyclic di-GMP. These domains are widely distributed in bacteria, including plant pathogens. The majority of proteins with GGDEF and EAL domains contain additional signal input domains, suggesting that their activities are responsive to environmental cues. Recent studies have demonstrated that a third domain, HD-GYP, is also active in cyclic di-GMP degradation. In the plant pathogen Xanthomonas campestris pv. campestris, a two-component signal transduction system comprising the HD-GYP domain regulatory protein RpfG and cognate sensor RpfC positively controls virulence. The signals recognized by RpfC may include the cell-cell signal DSF, which also acts to regulate virulence in X. campestris pv. campestris. Here, we review these recent advances in our understanding of cyclic di-GMP signaling with particular reference to one or more roles in the bacterial pathogenesis of plants.

Fitness of human enteric pathogens on plants and implications for food safety

The continuous rise in the number of outbreaks of foodborne illness linked to fresh fruit and vegetables challenges the notion that enteric pathogens are defined mostly by their ability to colonize the intestinal habitat. This review describes the epidemiology of produce-associated outbreaks of foodborne disease and presents recently acquired knowledge about the behavior of enteric pathogens on plants, with an emphasis on Salmonella enterica, Escherichia coli O157:H7, and Listeria monocytogenes. The growth and survival of enteric pathogens on plants are discussed in the light of knowledge and concepts in plant microbial ecology, including epiphytic fitness, the physicochemical nature of plant surfaces, biofilm formation, and microbe-microbe and plant-microbe interactions. Information regarding the various stresses that affect the survival of enteric pathogens and the molecular events that underlie their interactions in the plant environment provides a good foundation for assessing their role in the infectious dose of the pathogens when contaminated fresh produce is the vehicle of illness.