Ecological Similarity and Coexistence of Epiphytic Ice-Nucleating (Ice + ) Pseudomonas syringae Strains and a Non-Ice-Nucleating (Ice - ) Biological Control Agent

Influence of Co-occurring Weakly Pathogenic Bacterial Species on Bacterial Spot Disease Dynamics on Tomato

Mixed infections caused by multiple pathogenic and weakly pathogenic strains inhabiting the same host plants are common in nature and may modify pathogen dynamics. However, traditional plant pathogen studies have mostly focused on the binary interaction between a single host and a single pathogen. In this study, we have looked beyond this binary interaction and evaluated the impact of coinfection on disease dynamics on tomato using the bacterial spot pathogen Xanthomonas perforans (Xp), the co-occurring weakly pathogenic strain of X. arboricola (Xa), and the co-occurring potential weak pathogenic strain of Pseudomonas capsici (Pc). Time-series coinfection experiments monitoring disease severity and within-host population dynamics revealed higher disease severity in coinfection by three species compared with infection by Xp alone. However, coinfection by dual species, Xp and Pc, or Xa resulted in lower disease severity compared with Xp alone. Thus, coinfection outcomes depend on interacting species. Weak pathogens could exploit Xp to colonize the host plant as indicated by their higher populations in coinfection. However, Xp population dynamics were dependent on the coinfecting partner. While resource competition might be a possible explanation for lower Xp population in dual coinfection, interaction of Pc with the host was found to influence Xp population. Interestingly, Xp population was higher in the presence of three-species interaction compared with Xp and Xa coinfection, suggesting potential modulation of cooperative interactions among Xp and Xa in three-species coinfection rather than competitive interactions. Humidity played a significant role in population dynamics of the three species. Overall, this study highlighted the importance of coinfection dynamics in studying plant disease outbreaks.

Metabolic resource overlap impacts competition among phyllosphere bacteria

The phyllosphere is densely colonised by microbial communities, despite sparse and heterogeneously distributed resources. The limitation of resources is expected to drive bacterial competition resulting in exclusion or coexistence based on fitness differences and resource overlap between individual colonisers. We studied the impact of resource competition by determining the effects of different bacterial colonisers on the growth of the model epiphyte Pantoea eucalypti 299R (Pe299R). Resource overlap was predicted based on genome-scale metabolic modelling. By combining results of metabolic modelling and pairwise competitions in the Arabidopsis thaliana phyllosphere and in vitro, we found that ten resources sufficed to explain fitness of Pe299R. An effect of both resource overlap and phylogenetic relationships was found on competition outcomes in vitro as well as in the phyllosphere. However, effects of resource competition were much weaker in the phyllosphere when compared to in vitro experiments. When investigating growth dynamics and reproductive success at the single-cell resolution, resource overlap and phylogenetic relationships are only weakly correlated with epiphytic Pe299R reproductive success, indicating that the leaf's spatial heterogeneity mitigates resource competition. Although the correlation is weak, the presence of competitors led to the development of Pe299R subpopulations that experienced different life histories and cell divisions. In some in planta competitions, Pe299R benefitted from the presence of epiphytes despite high resource overlap to the competitor strain suggesting other factors having stronger effects than resource competition. This study provides fundamental insights into how bacterial communities are shaped in heterogeneous environments and a framework to predict competition outcomes.

History of Discovery and Environmental Role of Ice Nucleating Bacteria

The phenomenon of biological ice nucleation that is exhibited by a variety of bacteria is a fascinating phenotype, which has been shown to incite frost damage to frost-sensitive plants and has been proposed to contribute to atmospheric processes that affect the water cycle and earth's radiation balance. This review explores the several possible drivers for the evolutionary origin of the ice nucleation phenotype. These bacteria and the gene required for this phenotype have also been exploited in processes as diverse as reporter gene assays to assess environmentally responsive gene expression in various plant pathogenic and environmental bacteria and in the detection of foodborne human pathogens when coupled with host-specific bacteriophage, whereas ice nucleating bacteria themselves have been exploited in the production of artificial snow for recreation and oil exploration and in the process of freezing of various food products. This review also examines the historical development of our understanding of ice nucleating bacteria, details of the genetic determinants of ice nucleation, and features of the aggregates of membrane-bound ice nucleation protein necessary for catalyzing ice. Lastly, this review also explores the role of these bacteria in limiting the supercooling ability of plants and the strategies and limitations of avoiding plant frost damage by managing these bacterial populations by bactericides, antagonistic bacteria, or cultural control strategies.

Use of Ecological Theory to Understand the Efficacy and Mechanisms of Multistrain Biological Control

Since the 1970s, over 6,500 articles have been published about microbial biocontrols and over 200 microbial isolates have been registered for commercial use. However, many of these solutions have seen limited use due to limitations with their in-field efficacy. Even when multiple biocontrol agents are combined to create multistrain biocontrols, the resulting combinations can be less effective than the individual agents. One likely contributor is due to how multistrain microbial biocontrols are created. Multistrain microbial biocontrols are generally produced under controlled settings that are divorced from the ecological conditions they will need to function under. Traditionally, researchers culture, identify, and screen isolates for pathogen suppression traits. Then these researchers will combine the most promising isolates in an attempt to create more effective solutions. This approach, while effective for identifying suppressive isolates and determining the mechanisms of pathogen suppression, does not take into consideration the variability of natural environments, nor the complex ecological interactions that occur between plant hosts, pathogens, and component biocontrol agents, thus limiting the range of circumstances that these multistrain solutions can reliably succeed. To address these limitations, we suggest the application of relevant ecological theory to determine which isolates should be combined to create more reliable multistrain biocontrols. In this synthesis, we build on prior work focused on addressing plant pathogens through the use of multistrain microbial biocontrols, but we argue that viewing this work through the lens of ecology reveals key "design principles" from natural communities that are stable, functioning, and comprise multiple species.

Insect Freeze-Tolerance Downunder: The Microbial Connection

Insects that are freeze-tolerant start freezing at high sub-zero temperatures and produce small ice crystals. They do this using ice-nucleating agents that facilitate intercellular ice growth and prevent formation of large crystals where they can damage tissues. In Aotearoa/New Zealand the majority of cold adapted invertebrates studied survive freezing at any time of year, with ice formation beginning in the rich microbiome of the gut. Some freeze-tolerant insects are known to host symbiotic bacteria and/or fungi that produce ice-nucleating agents and we speculate that gut microbes of many New Zealand insects may provide ice-nucleating active compounds that moderate freezing. We consider too the possibility that evolutionary disparate freeze-tolerant insect species share gut microbes that are a source of ice-nucleating agents and so we describe potential transmission pathways of shared gut fauna. Despite more than 30 years of research into the freeze-tolerant mechanisms of Southern Hemisphere insects, the role of exogenous ice-nucleating agents has been neglected. Key traits of three New Zealand freeze-tolerant lineages are considered in light of the supercooling point (temperature of ice crystal formation) of microbial ice-nucleating particles, the initiation site of freezing, and the implications for invertebrate parasites. We outline approaches that could be used to investigate potential sources of ice-nucleating agents in freeze-tolerant insects and the tools employed to study insect microbiomes.

Identification of novel aphid-killing bacteria to protect plants

Aphids, including the peach-potato aphid, Myzus persicae, are major insect pests of agriculture and horticulture, and aphid control measures are limited. There is therefore an urgent need to develop alternative and more sustainable means of control. Recent studies have shown that environmental microbes have varying abilities to kill insects. We screened a range of environmental bacteria isolates for their abilities to kill target aphid species. Tests demonstrated the killing aptitude of these bacteria against six aphid genera (including Myzus persicae). No single bacterial strain was identified that was consistently toxic to insecticide-resistant aphid clones than susceptible clones, suggesting resistance to chemicals is not strongly correlated with bacterial challenge. Pseudomonas fluorescens PpR24 proved the most toxic to almost all aphid clones whilst exhibiting the ability to survive for over three weeks on three plant species at populations of 5-6 log CFU cm^-2 leaf. Application of PpR24 to plants immediately prior to introducing aphids onto the plants led to a 68%, 57% and 69% reduction in aphid populations, after 21 days, on Capsicum annuum, Arabidopsis thaliana and Beta vulgaris respectively. Together, these findings provide new insights into aphid susceptibility to bacterial infection with the aim of utilizing bacteria as effective biocontrol agents.

Conjugation Dynamics of Self-Transmissible and Mobilisable Plasmids into E. coli O157:H7 on Arabidopsis thaliana Rosettes

Many antibiotic resistance genes present in human pathogenic bacteria are believed to originate from environmental bacteria. Conjugation of antibiotic resistance conferring plasmids is considered to be one of the major reasons for the increasing prevalence of antibiotic resistances. A hotspot for plasmid-based horizontal gene transfer is the phyllosphere, i.e., the surfaces of aboveground plant parts. Bacteria in the phyllosphere might serve as intermediate hosts with transfer capability to human pathogenic bacteria. In this study, the exchange of mobilisable and self-transmissible plasmids via conjugation was evaluated. The conjugation from the laboratory strain Escherichia coli S17-1, the model phyllosphere coloniser Pantoea eucalypti 299R, and the model pathogen E. coli O157:H7 to the recipient strain E. coli O157:H7::MRE103 (EcO157:H7red) in the phyllosphere of Arabidopsis thaliana was determined. The results suggest that short-term occurrence of a competent donor is sufficient to fix plasmids in a recipient population of E. coli O157:H7red. The spread of self-transmissible plasmids was limited after initial steep increases of transconjugants that contributed up to 10% of the total recipient population. The here-presented data of plasmid transfer will be important for future modelling approaches to estimate environmental spread of antibiotic resistance in agricultural production environments.

Genomics of Aerobic Photoheterotrophs in Wheat Phyllosphere Reveals Divergent Evolutionary Patterns of Photosynthetic Genes in Methylobacterium spp

Phyllosphere is a habitat to a variety of viruses, bacteria, fungi, and other microorganisms, which play a fundamental role in maintaining the health of plants and mediating the interaction between plants and ambient environments. A recent addition to this catalogue of microbial diversity was the aerobic anoxygenic phototrophs (AAPs), a group of widespread bacteria that absorb light through bacteriochlorophyll α (BChl a) to produce energy without fixing carbon or producing molecular oxygen. However, culture representatives of AAPs from phyllosphere and their genome information are lacking, limiting our capability to assess their potential ecological roles in this unique niche. In this study, we investigated the presence of AAPs in the phyllosphere of a winter wheat (Triticum aestivum L.) in Denmark by employing bacterial colony based infrared imaging and MALDI-TOF mass spectrometry (MS) techniques. A total of ∼4,480 colonies were screened for the presence of cellular BChl a, resulting in 129 AAP isolates that were further clustered into 21 groups based on MALDI-TOF MS profiling, representatives of which were sequenced using the Illumina NextSeq and Oxford Nanopore MinION platforms. Seventeen draft and four complete genomes of AAPs were assembled belonging in Methylobacterium, Rhizobium, Roseomonas, and a novel Alsobacter. We observed a diverging pattern in the evolutionary rates of photosynthesis genes among the highly homogenous AAP strains of Methylobacterium (Alphaproteobacteria), highlighting an ongoing genomic innovation at the gene cluster level.

Leaf mineral content govern microbial community structure in the phyllosphere of spinach (Spinacia oleracea) and rocket (Diplotaxis tenuifolia)

The plant microbiome is an important factor for plant health and productivity. While the impact of nitrogen (N) availability for plant growth and development is well established, its influence on the microbial phyllosphere community structure is unknown. We hypothesize that nitrogen impacts the growth and abundance of several microorganisms on the leaf surface. The bacterial and fungal communities of baby leaf spinach (Spinacia oleracea), and rocket (Diplotaxis tenuifolia) were investigated in a field trial for two years in a commercial setting. Nitrogen fertilizer was tested in four doses (basic nitrogen, basic + suboptimal, basic + commercial, basic + excess) with six replicates in each. Culture-independent (Illumina sequencing) and culture-dependent (viable count and identification of bacterial isolates) community studies were combined with monitoring of plant physiology and site weather conditions. This study found that alpha diversity of bacterial communities decreased in response to increasing nitrogen fertilizer dose, whereas viable counts showed no differences. Correspondingly, fungal communities of the spinach phyllosphere showed a decreasing pattern, whereas the decreasing diversity of fungal communities of rocket was not significant. Plant species and effects of annual variations on microbiome structure were observed for bacterial and fungal communities on both spinach and rocket. This study provides novel insights on the impact of nitrogen fertilizer regime on a nutrient scarce habitat, the phyllosphere.

Is Exploitation Competition Involved in a Multitrophic Strategy for the Biocontrol of Fusarium Head Blight?

Trichoderma gamsii T6085 was used in combination with a Fusarium oxysporum isolate (7121) in order to evaluate, in a multitrophic approach, their competitive ability against F. graminearum, one of the main causal agents of Fusarium head blight (FHB) on wheat. The two antagonists and the pathogen were coinoculated on two different natural substrates, wheat and rice kernels. Both T6085 and 7121, alone and coinoculated, significantly reduced the substrate colonization and mycotoxin production by the pathogen. The two antagonists did not affect each other. Using a metabolic approach (Biolog), we investigated whether exploitation competition could explain this antagonistic activity. The aim was to define whether the three fungi coexist or if one isolate nutritionally dominates another. Results obtained from Biolog suggest that no exploitative competition occurs between the antagonists and the pathogen during the colonization of the natural substrates. Interference competition was then preliminarily evaluated to justify the reduction in the pathogen's growth and to better explain mechanisms. A significant reduction of F. graminearum growth was observed when the pathogen grew in the cultural filtrates of T. gamsii T6085, both alone and cocultured with F. oxysporum 7121, thus suggesting the involvement of secondary metabolites. As far as we know, this is the first time that an ecological study has been performed to explain how and which kind of competition could be involved in a multitrophic biocontrol of FHB.

Driving factors of epiphytic bacterial communities: A review

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The physicochemistry of leaves is unique and is a major driver of leaf colonisation.

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Competition and cooperation may be major drivers of bacterial colonisation.

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Leaves respond to bacterial colonisation locally and systemically.

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How leaf responses shape bacterial colonisation patterns is unclear.

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Plant-microbe interaction should be studied at the micrometer resolution.

Bacteria establish complex, compositionally consistent communities on healthy leaves. Ecological processes such as dispersal, diversification, ecological drift, and selection as well as leaf surface physicochemistry and topology impact community assembly. Since the leaf surface is an oligotrophic environment, species interactions such as competition and cooperation may be major contributors to shape community structure. Furthermore, the plant immune system impacts on microbial community composition, as plant cells respond to bacterial molecules and shape their responses according to the mixture of molecules present. Such tunability of the plant immune network likely enables the plant host to differentiate between pathogenic and non-pathogenic colonisers, avoiding costly immune responses to non-pathogenic colonisers. Plant immune responses are either systemically distributed or locally confined, which in turn affects the colonisation pattern of the associated microbiota. However, how each of these factors impacts the bacterial community is unclear. To better understand this impact, bacterial communities need to be studied at a micrometre resolution, which is the scale that is relevant to the members of the community. Here, current insights into the driving factors influencing the assembly of leaf surface-colonising bacterial communities are discussed, with a special focus on plant host immunity as an emerging factor contributing to bacterial leaf colonisation.

Multiple Pseudomonas species secrete exolysin-like toxins and provoke Caspase-1-dependent macrophage death

Pathogenic bacteria secrete protein toxins that provoke apoptosis or necrosis of eukaryotic cells. Here, we developed a live-imaging method, based on incorporation of a DNA-intercalating dye into membrane-damaged host cells, to study the kinetics of primary bone marrow-derived macrophages (BMDMs) mortality induced by opportunistic pathogen Pseudomonas aeruginosa expressing either Type III Secretion System (T3SS) toxins or the pore-forming toxin, Exolysin (ExlA). We found that ExlA promotes the activation of Caspase-1 and maturation of interleukin-1β. BMDMs deficient for Caspase-1 and Caspase-11 were resistant to ExlA-induced death. Furthermore, by using KO BMDMs, we determined that the upstream NLRP3/ASC complex leads to the Caspase-1 activation. We also demonstrated that Pseudomonas putida and Pseudomonas protegens and the Drosophila pathogen Pseudomonas entomophila, which naturally express ExlA-like toxins, are cytotoxic toward macrophages and provoke the same type of pro-inflammatory death as does ExlA⁺ P. aeruginosa. These results demonstrate that ExlA-like toxins of two-partner secretion systems from diverse Pseudomonas species activate the NLRP3 inflammasome and provoke inflammatory pyroptotic death of macrophages.

Nutrients and host attributes modulate the abundance and functional traits of phyllosphere microbiome in rice

The abundance of phyllosphere bacterial communities of seven genotypes of rice ADT- 38, ADT-43, CR-1009, PB-1, PS-5, P-44, and PB-1509 was investigated, in relation to nutrient dynamics of rhizosphere and leaves. P-44 genotype recorded highest pigment accumulation, while genotypes CR-1009 and P-44 exhibited most number of different bacterial morphotypes, Colony forming units in two media (Nutrient agar and R2A) varied significantly and ranged from 10⁶-10⁷ per g plant tissues. Among the selected 60 distinct morphotypes, IAA and siderophore producers were the dominant functional types. Biocontrol activity against Drechslera oryzae was shown by 38 isolates, while 17 and 9 isolates were potent against Rhizoctonia solani and Magnaporthe oryzae respectively. Principal Component Analysis (PCA) illustrated the significant effects of selected soil and leaf nutrients of seven rice varieties on the culturable phyllospheric population (log CFU), particularly in the R2A medium. Eigen values revealed that 83% of the variance observed could be assigned to Leaf-Fe, Leaf-Mn, chlorophyll b and soil organic carbon (OC). Quantitative PCR analyses of abundance of bacteria, cyanobacteria and archaebacteria revealed a host-specific response, with CR-1009 showing highest number of 16S rRNA copies of bacterial members, while both P-44 and PS-5 had higher cyanobacterial abundance, but lowest number of those belonging to archaebacteria. Nutritional aspects of leaf and soil influenced the abundance of bacteria and their functional attributes; this is of interest for enhancing the efficacy of foliar inoculants, thereby, improving plant growth and disease tolerance.

Independent Co-Option of a Tailed Bacteriophage into a Killing Complex in Pseudomonas

Competition between microbes is widespread in nature, especially among those that are closely related. To combat competitors, bacteria have evolved numerous protein-based systems (bacteriocins) that kill strains closely related to the producer. In characterizing the bacteriocin complement and killing spectra for the model strain Pseudomonas syringae B728a, we discovered that its activity was not linked to any predicted bacteriocin but is derived from a prophage. Instead of encoding an active prophage, this region encodes a bacteriophage-derived bacteriocin, termed an R-type syringacin. This R-type syringacin is striking in its convergence with the well-studied R-type pyocin of P. aeruginosa in both genomic location and molecular function. Genomic alignment, amino acid percent sequence identity, and phylogenetic inference all support a scenario where the R-type syringacin has been co-opted independently of the R-type pyocin. Moreover, the presence of this region is conserved among several other Pseudomonas species and thus is likely important for intermicrobial interactions throughout this important genus.

Evolutionary innovation is often achieved through modification of complexes or processes for alternate purposes, termed co-option. Notable examples include the co-option of a structure functioning in locomotion (bacterial flagellum) to one functioning in protein secretion (type three secretion system). Similar co-options can occur independently in distinct lineages. We discovered a genomic region in the plant pathogen Pseudomonas syringae that consists of a fragment of a bacteriophage genome. The fragment encodes only the tail of the bacteriophage, which is lethal toward strains of this species. This structure is similar to a previously described structure produced by the related species Pseudomonas aeruginosa. The two structures, however, are not derived from the same evolutionary event. Thus, they represent independent bacteriophage co-options. The co-opted bacteriophage from P. syringae is found in the genomes of many other Pseudomonas species, suggesting ecological importance across this genus.

Microbiology, genomics, and clinical significance of the Pseudomonas fluorescens species complex, an unappreciated colonizer of humans

Pseudomonas fluorescens is not generally considered a bacterial pathogen in humans; however, multiple culture-based and culture-independent studies have identified it at low levels in the indigenous microbiota of various body sites. With recent advances in comparative genomics, many isolates originally identified as the "species" P. fluorescens are now being reclassified as novel Pseudomonas species within the P. fluorescens "species complex." Although most widely studied for its role in the soil and the rhizosphere, P. fluorescens possesses a number of functional traits that provide it with the capability to grow and thrive in mammalian hosts. While significantly less virulent than P. aeruginosa, P. fluorescens can cause bacteremia in humans, with most reported cases being attributable either to transfusion of contaminated blood products or to use of contaminated equipment associated with intravenous infusions. Although not suspected of being an etiologic agent of pulmonary disease, there are a number of reports identifying it in respiratory samples. There is also an intriguing association between P. fluorescens and human disease, in that approximately 50% of Crohn's disease patients develop serum antibodies to P. fluorescens. Altogether, these reports are beginning to highlight a far more common, intriguing, and potentially complex association between humans and P. fluorescens during health and disease.

Immigrant Pantoea agglomerans embedded within indigenous microbial aggregates: a novel spatial distribution of epiphytic bacteria

Immigrant bacteria located on leaf surfaces are important to the health of plants as well as to people who consume fresh fruits and vegetables. However, the spatial distribution and organization of these immigrant bacteria on leaf surfaces are still poorly understood. To examine the spatial organization of these strains, two bacterial strains on tobacco leaves: (1) an indigenous strain, Pseudomonas stutzeri Nov. Y2011 labeled with green fluorescent protein, and (2) an immigrant strain Pantoea agglomerans labeled with cyan fluorescent protein isolated from pear, were studied. Under moist conditions, P. agglomerans cells quickly disappeared from direct observation by laser-scanning confocal microscopy, although elution results indicated that large amounts of live cells were still present on the leaves. Following exposure to desiccation stress, particles of cyan fluorescent protein-labeled P. agglomerans were visible within cracked aggregates of P. stutzeri Nov. Y2011. Detailed observation of sectioned aggregates showed that colonies of immigrant P. agglomerans were embedded within aggregates of P. stutzeri Nov. Y2011. Furthermore, carbon-resource partitioning studies suggested that these two species could coexist without significant nutritional competition. This is the first observation of an immigrant bacterium embedding within aggregates of indigenous bacteria on leaves to evade harsh conditions in the phyllosphere.

Single-cell versus population-level reproductive success of bacterial immigrants to pre-colonized leaf surfaces

We assessed how preemptive inoculation of plant leaves with bacteria affected the establishment of secondary colonizers. We quantified the latter in two ways: (i) at the population level, i.e. as counts of colony-forming units and (ii) at the level of single cells by tracking the reproductive success of individual bacteria. Both analyses showed that the ability of secondary immigrants to establish on the leaf was negatively correlated with the level of pre-population by primary colonizers. This effect was best described by an inverse dose-response curve with an apparent half-point inhibition efficacy of approximately 10(6) cells of primary colonizers per gram leaf. This efficacy was the same whether calculated from population- or average single-cell data. However, single-cell data revealed that even under conditions of heavy pre-population with primary colonizers, a small fraction of secondary immigrants still produced offspring, although the corresponding population measurement showed no increase in total population size. This observation has direct relevance for biocontrol strategies that are based on the principle of preemptive exclusion of foliar bacterial pathogens: even at seemingly saturating levels of primary inoculum, some secondary colonizers may still be able to reproduce and possibly reach a quorum to trigger behaviours that enhance survival or virulence.

Resident bacteria on leaves enhance survival of immigrant cells of Salmonella enterica

Although Salmonella enterica apparently has comparatively low epiphytic fitness on plants, external factors that would influence its ability to survive on plants after contamination would be of significance in the epidemiology of human diseases caused by this human pathogen. Viable population sizes of S. enterica applied to plants preinoculated with Pseudomonas syringae or either of two Erwinia herbicola strains was ≥10-fold higher than that on control plants that were not precolonized by such indigenous bacteria when assessed 24 to 72 h after the imposition of desiccation stress. The protective effect of P. fluorescens, which exhibited antibiosis toward S. enterica in vitro, was only ≈50% that conferred by other bacterial strains. Although S. enterica could produce small cellular aggregates after incubation on wet leaves for several days, and the cells in such aggregates were less susceptible to death upon acute dehydration than solitary cells (as determined by propidium iodide staining), most Salmonella cells were found as isolated cells when it was applied to leaves previously colonized by other bacterial species. The proportion of solitary cells of S. enterica coincident with aggregates of cells of preexisting epiphytic species that subsequently were judged as nonviable by viability staining on dry leaves was as much as 10-fold less than those that had landed on uncolonized portions of the leaf. Thus, survival of immigrant cells of S. enterica on plants appears to be strongly context dependent, and the presence of common epiphytic bacteria on plants can protect such immigrants from at least one key stress (i.e., desiccation) encountered on leaf surfaces.

Contribution of nitrate assimilation to the fitness of Pseudomonas syringae pv. syringae B728a on plants

The ability of Pseudomonas syringae pv. syringae to use nitrate as a nitrogen source in culture and on leaves was assessed. Substantial amounts of leaf surface nitrate were detected directly and by use of a bioreporter of nitrate on bean plants grown with a variety of nitrogen sources. While a nitrate reductase mutant, P. syringae ΔnasB, exhibited greatly reduced growth in culture with nitrate as the sole nitrogen source, it exhibited population sizes similar to those of the wild-type strain on leaves. However, the growth of the ΔnasB mutant was much less than that of the wild-type strain when cultured in bean leaf washings supplemented with glucose, suggesting that P. syringae experiences primarily carbon-limited and only secondarily nitrogen-limited growth on bean leaves. Only a small proportion of the cells of a green fluorescent protein (GFP)-based P. syringae nitrate reductase bioreporter, LK2(pOTNas4), exhibited fluorescence on leaves. This suggests that only a subset of cells experience high nitrate levels or that nitrate assimilation is repressed by the presence of ammonium or other nitrogenous compounds in many leaf locations. While only a subpopulation of P. syringae consumes nitrate at a given time on the leaves, the ability of those cells to consume this resource would be strongly beneficial to those cells, especially in environments in which nitrate is the most abundant form of nitrogen.

Microbiology for the masses: teaching concepts and skills for a general audience

Microbes are extraordinarily talented and diverse: they can perform a multitude of tasks and live in a variety of situations. The impact of diverse microbes upon society and the environment can be used to teach students critical skills for a variety of careers.

Forward genetic in planta screen for identification of plant-protective traits of Sphingomonas sp. strain Fr1 against Pseudomonas syringae DC3000

Sphingomonas sp. strain Fr1 has recently been shown to protect Arabidopsis thaliana against the bacterial leaf pathogen Pseudomonas syringae DC3000. Here, we describe a forward genetic in planta screen to identify genes in Sphingomonas sp. Fr1 necessary for this effect. About 5,000 Sphingomonas sp. Fr1 mini-Tn5 mutants were assayed for a defect in plant protection against a luxCDABE-tagged P. syringae DC3000 derivative in a space-saving 24-well plate system. The bioluminescence of the pathogen was used as the indicator of pathogen proliferation and allowed for the identification of Sphingomonas sp. Fr1 mutants that had lost the ability to restrict pathogen growth before disease symptoms were visible. Potential candidates were validated using the same miniaturized experimental system. Of these mutants, 10 were confirmed as plant protection defective yet colonization competent. The mutants were subsequently evaluated in a previously described standard microbox system, and plants showed enhanced disease phenotypes after pathogen infection relative to those inoculated with the parental strain as a control. However, the disease severities were lower than those observed for control plants that were grown axenically prior to pathogen challenge, which suggests that several traits may contribute to plant protection. Transposon insertion sites of validated mutants with defects in plant protection were determined and mapped to 7 distinct genomic regions. In conclusion, the established screening protocol allowed us to identify mutations that affect plant protection, and it opens the possibility to uncover traits important for in planta microbe-microbe interactions.

Development of species-, strain- and antibiotic biosynthesis-specific quantitative PCR assays for Pantoea agglomerans as tools for biocontrol monitoring

Pantoea agglomerans is a cosmopolitan plant epiphytic bacterium that includes some of the most effective biological antagonists against the fire blight pathogen Erwinia amylovora, a major threat to pome fruit production worldwide. Strain E325 is commercially available as Bloomtime Biological™ in the USA and Canada. New quantitative PCR (qPCR) assays were developed for species- and strain -specific detection in the environment, and for detection of indigenous strains carrying the biocontrol antibacterial peptide biosynthesis gene paaA. The qPCR assays were highly specific, efficient and sensitive, detecting fewer than three cells per reaction or 700 colony forming units per flower, respectively. The qPCR assays were tested on field samples, giving first indications to the incidence of P. agglomerans E325 related strains, total P. agglomerans and pantocin A producing bacteria in commercial orchards. These assays will facilitate monitoring the environmental behavior of biocontrol P. agglomerans after orchard application for disease protection, proprietary strain-tracking, and streamlined screening for discovery of new biocontrol strains.

Variation in local carrying capacity and the individual fate of bacterial colonizers in the phyllosphere

Using a phyllosphere model system, we demonstrated that the term ‘carrying capacity', as it is commonly used in microbial ecology, needs to be understood as the sum of many ‘local carrying capacities' in order to better explain and predict the course and outcome of bacterial colonization of an environment. Using a green fluorescent protein-based bioreporter system for the quantification of reproductive success (RS) in individual Erwinia herbicola cells, we were able to reconstruct the contribution of individual immigrants to bacterial population sizes on leaves. Our analysis revealed that plant foliage represents to bacteria an environment where individual fate is determined by the local carrying capacity of the site where an immigrant cell lands. With increasing inoculation densities, the RS of most immigrants declined, suggesting that local carrying capacity under the tested conditions was linked to local nutrient availability. Fitting the observed experimental data to an adapted model of phyllosphere colonization indicated that there might exist three types of sites on leaves, which differ in their frequency of occurrence and local carrying capacity. Specifically, our data were consistent with a leaf environment that is characterized by few sites where individual immigrants can produce high numbers of offspring, whereas the remainder of the leaf offered an equal number of sites with low and medium RS. Our findings contribute to a bottom–up understanding of bacterial colonization of leaf surfaces, which includes a quantifiable role of chance in the experience at the individual level and in the outcome at the population level.

Isolates of Thermoanaerobacter thermohydrosulfuricus from decaying wood compost display genetic and phenotypic microdiversity

In this study, 12 strains of Thermoanaerobacter were isolated from a single decaying wood compost sample and subjected to genetic and phenotypic profiling. The 16S rRNA encoding gene sequences suggested that the isolates were most similar to strains of either Thermoanaerobacter pseudethanolicus or Thermoanaerobacter thermohydrosulfuricus. Examination of the lesser conserved chaperonin-60 (cpn60) universal target showed that some isolates shared the highest sequence identity with T. thermohydrosulfuricus; however, others to Thermoanaerobacter wiegelii and Thermoanaerobacter sp. Rt8.G4 (formerly Thermoanaerobacter brockii Rt8.G4). BOX-PCR fingerprinting profiles identified differences in the banding patterns not only between the isolates and the reference strains, but also among the isolates themselves. To evaluate the extent these genetic differences were manifested phenotypically, the utilization patterns of 30 carbon substrates were examined and the niche overlap indices (NOI) calculated. Despite showing a high NOI (> 0.9), significant differences existed in the substrate utilization capabilities of the isolates suggesting that either a high degree of niche specialization or mechanisms allowing for non-competitive co-existence, were present within this ecological context. Growth studies showed that the isolates were physiologically distinct in both growth rate and the fermentation product ratios. Our data indicate that phenotypic diversity exists within genetically microdiverse Thermoanaerobacter isolates from a common environment.

Protection of Arabidopsis thaliana against leaf-pathogenic Pseudomonas syringae by Sphingomonas strains in a controlled model system

Diverse bacterial taxa live in association with plants without causing deleterious effects. Previous analyses of phyllosphere communities revealed the predominance of few bacterial genera on healthy dicotyl plants, provoking the question of whether these commensals play a particular role in plant protection. Here, we tested two of them, Methylobacterium and Sphingomonas, with respect to their ability to diminish disease symptom formation and the proliferation of the foliar plant pathogen Pseudomonas syringae pv. tomato DC3000 on Arabidopsis thaliana. Plants were grown under gnotobiotic conditions in the absence or presence of the potential antagonists and then challenged with the pathogen. No effect of Methylobacterium strains on disease development was observed. However, members of the genus Sphingomonas showed a striking plant-protective effect by suppressing disease symptoms and diminishing pathogen growth. A survey of different Sphingomonas strains revealed that most plant isolates protected A. thaliana plants from developing severe disease symptoms. This was not true for Sphingomonas strains isolated from air, dust, or water, even when they reached cell densities in the phyllosphere comparable to those of the plant isolates. This suggests that plant protection is common among plant-colonizing Sphingomonas spp. but is not a general trait conserved within the genus Sphingomonas. The carbon source profiling of representative isolates revealed differences between protecting and nonprotecting strains, suggesting that substrate competition plays a role in plant protection by Sphingomonas. However, other mechanisms cannot be excluded at this time. In conclusion, the ability to protect plants as shown here in a model system may be an unexplored, common trait of indigenous Sphingomonas spp. and may be of relevance under natural conditions.

Mechanistically compatible mixtures of bacterial antagonists improve biological control of fire blight of pear

Mixtures of biological control agents can be superior to individual agents in suppressing plant disease, providing enhanced efficacy and reliability from field to field relative to single biocontrol strains. Nonetheless, the efficacy of combinations of Pseudomonas fluorescens A506, a commercial biological control agent for fire blight of pear, and Pantoea vagans strain C9-1 or Pantoea agglomerans strain Eh252 rarely exceeds that of individual strains. A506 suppresses growth of the pathogen on floral colonization and infection sites through preemptive exclusion. C9-1 and Eh252 produce peptide antibiotics that contribute to disease control. In culture, A506 produces an extracellular protease that degrades the peptide antibiotics of C9-1 and Eh252. We hypothesized that strain A506 diminishes the biological control activity of C9-1 and Eh252, thereby reducing the efficacy of biocontrol mixtures. This hypothesis was tested in five replicated field trials comparing biological control of fire blight using strain A506 and A506 aprX::Tn5, an extracellular protease-deficient mutant, as individuals and combined with C9-1 or Eh252. On average, mixtures containing A506 aprX::Tn5 were superior to those containing the wild-type strain, confirming that the extracellular protease of A506 diminished the biological control activity of C9-1 and Eh252 in situ. Mixtures of A506 aprX::Tn5 and C9-1 or Eh252 were superior to oxytetracycline or single biocontrol strains in suppressing fire blight of pear. These experiments demonstrate that certain biological control agents are mechanistically incompatible, in that one strain interferes with the mechanism by which a second strain suppresses plant disease. Mixtures composed of mechanistically compatible strains of biological control agents can suppress disease more effectively than individual biological control agents.

Control of fire blight by Pseudomonas fluorescens A506 and Pantoea vagans C9-1 applied as single strains and mixed inocula

The biological control agents Pseudomonas fluorescens A506 and Pantoea vagans C9-1 were evaluated individually and in combination for the suppression of fire blight of pear or apple in 10 field trials inoculated with the pathogen Erwinia amylovora. The formulation of pathogen inoculum applied to blossoms influenced establishment of the pathogen and the efficacy of biological control. Pantoea vagans C9-1 suppressed fire blight in all five trials in which the pathogen was applied as lyophilized cells but in none of the trials in which the pathogen was applied as freshly harvested cells. In contrast, Pseudomonas fluorescens A506 reduced disease significantly in only one trial. A mixture of the two strains also suppressed fire blight, but the magnitude of disease suppression over all field trials (averaging 32%) was less than that attained by C9-1 alone (42%). The two biological control agents did not antagonize one another on blossom surfaces, and application of the mixture of A506 and C9-1 to blossoms resulted in a greater proportion of flowers having detectable populations of at least one bacterial antagonist than the application of individual strains. Therefore, the mixture of A506 and C9-1 provided less disease control than expected based upon the epiphytic population sizes of the antagonists on blossom surfaces. We speculate that the biocontrol mixture was less effective than anticipated due to incompatibility between the mechanisms by which A506 and C9-1 suppress disease.

Influence of immigration on epiphytic bacterial populations on navel orange leaves

Factors that influenced the increase in epiphytic bacterial population size on navel orange leaves during winter months were investigated to test the assumption that such populations were the result of multiplication on orange leaves. The population sizes of bacteria of different kinds, including ice nucleation-active (Ice(sup+)) bacteria, were from 6- to 30-fold larger on leaves of navel orange trees adjacent to other plant species than on trees growing near other citrus species. Total and Ice(sup+) bacterial population sizes on other plant species growing near navel orange trees were from 18- to 60-fold and 2- to 18,000-fold larger, respectively, than on navel orange trees. About twice the number of bacterial cells of a given type were deposited onto petri dishes opened simultaneously in navel orange orchards with other plant species nearby as in orchards surrounded by citrus trees. Epiphytic bacteria and airborne bacteria were more numerous near the upwind edge of orchards bordering on other plant species, but not in orchards adjacent to other citrus trees, and decreased with distance from other plant species. Navel orange leaves also exhibited progressive increases in the ability to supercool as a function of increasing distance from the upwind edge of orchards adjacent to other plant species but not in orchards adjacent to other citrus trees. While the population size of three different bacterial strains remained nearly constant for 60 days after inoculation, total bacterial populations increased more than 50-fold during this period. These results suggest that immigration of bacteria from plants having high epiphytic bacterial populations could account for most, if not all, of the seasonal increase in bacterial populations on navel orange leaves and have important implications for procedures to modify bacterial communities on leaves.

Coexistence among Epiphytic Bacterial Populations Mediated through Nutritional Resource Partitioning

The levels of coexistence between Pseudomonas syringae and various nonpathogenic epiphytic species in the phyllosphere of beans (Phaseolus vulgaris) were assessed by using replacement series. The epiphytic species Pseudomonas fluorescens, Pantoea agglomerans, Stenotrophomonas maltophilia, and Methylobacterium organophilum were all capable of exhibiting higher levels of coexistence with P. syringae than was observed with a near-isogenic P. syringae strain pair. The ecological similarity of the epiphytes was estimated with niche overlap indices derived from in vitro carbon source utilization profiles. The level of coexistence of the epiphytes was inversely correlated with the ecological similarity of the strains. Hence, the level of coexistence between the epiphytes was proportional to the degree of niche differentiation, defined as the ability to utilize carbon sources not utilized by a competing strain. Comparisons of utilization profiles for groups of carbon sources (amino acids, organic acids, and carbohydrates) indicated the types of carbon sources for which the strains likely competed in the bean phyllosphere. P. fluorescens and P. syringae strains probably competed for most carbon sources. S. maltophilia and M. organophilum strains probably competed with P. syringae for most organic acids but few amino acids or carbohydrates. P. agglomerans strains probably competed with P. syringae for most amino acids and organic acids but few carbohydrates. A variable level of coexistence observed between P. agglomerans and P. syringae probably reflected the variability in abundance in the bean phyllosphere of the carbohydrates that P. agglomerans utilized exclusively.

Raindrop Momentum Triggers Growth of Leaf-Associated Populations of Pseudomonas syringae on Field-Grown Snap Bean Plants

Observational and microclimate modification experiments were conducted under field conditions to determine the role of the physical environment in effecting large increases in phyllosphere population sizes of Pseudomonas syringae pv. syringae, the causal agent of bacterial brown spot disease of snap bean (Phaseolus vulgaris L.). Comparisons of daily changes in population sizes of P. syringae on three plantings of snap bean cultivar Cascade and one of cultivar Eagle with weather conditions indicated a strong association of rainfalls with periods of 1 to 3 days in duration during which increases in bacterial population sizes were greater than 10-fold and up to 1,000-fold. The effects of rain on populations of P. syringae were explored further by modifying the microclimate of bean plants in the field with polyethylene shelters to shield plants from rain and fine-mesh inert screens to modify the momentum of raindrops. After each of three separate intense rains, the greater-than-10-fold increases in population sizes of P. syringae observed on plants exposed to the rains did not occur on plants in the shelters or under the screens. The screens decreased the velocity and, thus, the momentum of raindrops but not the volume or quality of rainwater that fell on plants under the screens. Thus, the absence of increases in population sizes of P. syringae on plants under the screens suggests that raindrop momentum plays a role in the growth-triggering effect of intense rains on populations of P. syringae on bean plants under field conditions.

Differential effect of environmental conditions on the growth and regulation of the fumonisin biosynthetic gene FUM1 in the maize pathogens and fumonisin producers Fusarium verticillioides and Fusarium proliferatum

The effects of ecophysiological factors, temperature and solute potential, on both the growth and the regulation of the fumonisin biosynthetic FUM1 gene were studied and compared in one isolate each of the two closely related fumonisin-producing and maize pathogens Fusarium verticillioides and Fusarium proliferatum. The effect of solute potential and temperature was examined on in vitro mycelia growth and on the expression of the FUM1 gene, quantified by species-specific real-time reverse transcriptase-PCR assays. Although both isolates showed similar two-dimensional profiles of growth, for F. verticillioides, optimal growth conditions were maintained at higher temperatures and lower solute potential values. FUM1 gene expression was markedly induced at 20 degrees C in both isolates, under suboptimal conditions for growth; however, their expression patterns differed in relation to solute potential. Whereas FUM1 expression was induced in response to increasing water stress in the isolate of F. verticillioides, the F. proliferatum one showed a stable expression pattern regardless of water potential conditions. These results suggest a differential regulation of fumonisin biosynthesis in these isolates of the two species that might be related to their different host range, and play an ecological role. Additionally, environmental conditions leading to water stress (drought) might result in increased risk of fumonisin contamination of maize caused by F. verticillioides.

Responses of 2 epiphytic yeasts to foliar infection by Rhizoctonia solani or mechanical wounding on the phylloplane of tall fescue

A growth-chamber experiment was conducted to determine how foliar disease or wounding affects the ability of 2 phylloplane yeasts (Rhodotorula glutinis and Cryptococcus laurentii) to colonize leaves of tall fescue (Festuca arundinacea). Yeasts were applied separately and together onto healthy leaves, leaves infected with Rhizoctonia solani (diseased), and mechanically bruised (wounded) leaves. In all 3 trials, the leaf disturbance treatment significantly affected the abundance of yeast on the phylloplane of tall fescue. Yeast abundance on the diseased or wounded leaves was significantly greater than on the nontreated, healthy leaves. In 2 of the 3 trials, the yeast species applied also had a significant affect on yeast abundance. Typically, R. glutinis was significantly more abundant than C. laurentii when applied individually, but not significantly greater than the total yeast colony-forming units of the co-inoculated treatment. When the 2 yeasts were co-inoculated onto the leaves, R. glutinis comprised 89.7%, 75.4%, and 67.6% of the recovered yeast colony-forming units on healthy, diseased, and wounded leaves, respectfully. Our data suggest that these 2 species of yeasts will differentially colonize compromised leaf tissue with disease or wounds favoring populations of R. glutinis over C. laurentii.

Implications of pathogenesis by Erwinia amylovora on rosaceous stigmas to biological control of fire blight

As a prerequisite to infection of flowers, Erwinia amylovora grows epiphytically on stigmas, which provide a conducive habitat for bacterial growth. Stigmas also support growth of several other bacterial genera, which allows for biological control of fire blight; although, in practice, it is very difficult to exclude E. amylovora completely from this habitat. We investigated the dynamics of growth suppression of E. amylovora by comparing the ability of virulent and avirulent strains of E. amylovora to compete with each other on stigmas of pear, apple, and blackberry, and to compete with a co-inoculated mixture of effective bacterial antagonists. When strains were inoculated individually, virulent E. amylovora strain Ea153N attained the highest population size on stigmas, with population sizes that were approximately double those of an avirulent hrpL mutant of Ea153 or the bacterial antagonists. In competition experiments, growth of the avirulent derivative was suppressed by the antagonist mixture to a greater extent than the virulent strain. Unexpectedly, the virulent strain enhanced the population size of the antagonist mixture. Similarly, a small dose of virulent Ea153N added to inoculum of an avirulent hrpL mutant of Ea153 significantly increased the population size of the avirulent strain. A pathogenesis-gene reporter strain, Ea153 dspE::gfp, was applied to flowers and a subset of the population expressed the green fluorescent protein while growing epiphytically on stigmas of apple. These results are consistent with the hypothesis that virulent E. amylovora modifies the epiphytic habitat presented by the stigma through a pathogenesis-related process, which increases host resources available to itself and, coincidentally, to nonpathogenic competitors. Over nine orchard trials, avirulent Ea153 hrpL significantly suppressed the incidence of fire blight four times compared with six for the antagonist mixture. The degree of biological control achievable with an avirulent strain of E. amylovora likely is limited by its inability to utilize the stigmatic habitat to the same degree as a virulent strain.

Leaf age as a risk factor in contamination of lettuce with Escherichia coli O157:H7 and Salmonella enterica

Outbreaks of Escherichia coli O157:H7 infections have been linked increasingly to leafy greens, particularly to lettuce. We present here the first evidence that this enteric pathogen can multiply on the leaves of romaine lettuce plants. The increases in population size of E. coli O157:H7 in the phyllosphere of young lettuce plants ranged from 16- to 100-fold under conditions of warm temperature and the presence of free water on the leaves and varied significantly with leaf age. The population size was consistently ca. 10-fold higher on the young (inner) leaves than on the middle leaves. The growth rates of Salmonella enterica and of the natural bacterial microflora were similarly leaf age dependent. Both enteric pathogens also achieved higher population sizes on young leaves than on middle leaves harvested from mature lettuce heads, suggesting that leaf age affects preharvest as well as postharvest colonization. Elemental analysis of the exudates collected from the surfaces of leaves of different ages revealed that young-leaf exudates were 2.9 and 1.5 times richer in total nitrogen and carbon, respectively, than middle-leaf exudates. This trend mirrored the nitrogen and carbon content of the leaf tissue. Application of ammonium nitrate, but not glucose, to middle leaves enhanced the growth of E. coli O157:H7 significantly, suggesting that low nitrogen limits its growth on these leaves. Our results indicate that leaf age and nitrogen content contribute to shaping the bacterial communities of preharvest and postharvest lettuce and that young lettuce leaves may be associated with a greater risk of contamination with E. coli O157:H7.

The influence of exogenous nutrients on the abundance of yeasts on the phylloplane of turfgrass

Four experiments were conducted to assess the effect of foliar applications of various nutrient solutions on the phylloplane yeast community of tall fescue (Festuca arundinacea Schreb.). In the first three experiments, increasing concentrations of sucrose (2-16%), yeast extract (0.5-2.5%), and sucrose plus yeast extract (2.5-18.5% total) were applied and the yeast colony forming units (cfu) enumerated 14 h later by dilution plating. Significant positive linear relationships were observed between the number of yeast cfu and applications of both yeast extract and sucrose plus yeast extract. Foliar applications of sucrose alone had no significant effect on yeast community abundance, indicating that phylloplane yeasts of turfgrass are not limited by the amount or availability of carbohydrates. In the fourth experiment, five different solutions were applied to tall fescue to investigate the response of the yeast community to organic and inorganic nitrogen sources. Tryptone or yeast extract, both with considerable amino acid composition, significantly increased the yeast population, while yeast nitrogen base (with or without amino acids) and ammonium sulfate had no affect on yeast abundance. These results suggest that organic nitrogen stimulate yeast community growth and development on the phylloplane of tall fescue, while carbohydrates, inorganic nitrogen, and non-nitrogenous nutrients have little positive effect.

Bacterial degradation of airborne phenol in the phyllosphere

Despite the vast surface area of terrestrial plant leaves and the large microbial communities they support, little is known of the ability of leaf-associated microorganisms to access and degrade airborne pollutants. Here, we examined bacterial acquisition and degradation of phenol on leaves by an introduced phenol degrader and by natural phyllosphere communities. Whole-cell gfp-based Pseudomonas fluorescens bioreporter cells detected phenol on leaves that had previously been transiently exposed to gaseous phenol, indicating that leaves accumulated phenol; moreover, they accumulated it in sites that were accessible to epiphytic bacteria and to concentrations that were at least 10-fold higher than those in the air. After inoculated leaves were exposed to gaseous 14C-phenol, leaves harbouring the phenol-degrading Pseudomonas sp. strain CF600 released eight times more 14CO2 than did leaves harbouring a non-degrading mutant, demonstrating that CF600 actively mineralized phenol on leaves. We evaluated phenol degradation by natural microbial communities on green ash leaves that were collected from a field site rich in airborne organic pollutants. We found that significantly more phenol was mineralized by these leaves when the communities were present than by these leaves following surface sterilization. Thus, phenol-degrading organisms were present in these natural communities and were metabolically capable of phenol degradation. Collectively, these results provide the first direct evidence that bacteria on leaves can degrade an organic pollutant from the air, and indicate that bacteria on leaves could potentially contribute to the natural attenuation of organic air pollutants.

Role of 2,4-diacetylphloroglucinol-producing fluorescent Pseudomonas spp. in the defense of plant roots

Plants have evolved strategies of stimulating and supporting specific groups of antagonistic microorganisms in the rhizosphere as a defense against diseases caused by soilborne plant pathogens owing to a lack of genetic resistance to some of the most common and widespread soilborne pathogens. Some of the best examples of natural microbial defense of plant roots occur in disease suppressive soils. Soil suppressiveness against many different diseases has been described. Take-all is an important root disease of wheat, and soils become suppressive to take-all when wheat or barley is grown continuously in a field following a disease outbreak; this phenomenon is known as take-all decline (TAD). In Washington State, USA and The Netherlands, TAD results from the enrichment during monoculture of populations of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing Pseudomonas fluorescens to a density of 10 (5) CFU/g of root, the threshold required to suppress the take-all pathogen, Gaeumannomyces graminis var. tritici. 2,4-DAPG-producing P. fluorescens also are enriched by monoculture of other crops such as pea and flax, and evidence is accumulating that 2,4-DAPG producers contribute to the defense of plant roots in many different agroecosystems. At this time, 22 distinct genotypes of 2,4-DAPG producers (designated A - T, PfY and PfZ) have been defined by whole-cell repetitive sequence-based (rep)-PCR analysis, restriction fragment length polymorphism (RFLP) analysis of PHLD, and phylogenetic analysis of PHLD, but the number of genotypes is expected to increase. The genotype of an isolate is predictive of its rhizosphere competence on wheat and pea. Multiple genotypes often occur in a single soil and the crop species grown modulates the outcome of the competition among these genotypes in the rhizosphere. 2,4-DAPG producers are highly effective biocontrol agents against a variety of plant diseases and ideally suited for serving as vectors for expressing other biocontrol traits in the rhizosphere.

Microbial colonization and competition on the marine alga Ulva australis

Pseudalteromonas tunicata and Roseobacter gallaeciensis are biofilm-forming marine bacteria that are often found in association with the surface of the green alga Ulva australis. They are thought to benefit the plant host by producing inhibitory compounds that are active against common fouling organisms. We investigated factors that influence the ability of P. tunicata and R. gallaeciensis to attach to and colonize the plant surface and also the competitive interactions that occur between these organisms and other isolates from U. australis during biofilm formation on the plant surface. A surprisingly high number of P. tunicata cells, at least 10(8) cells ml(-1), were required for colonization and establishment of a population of cells that persists on axenic surfaces of U. australis. Factors that enhanced colonization of P. tunicata included inoculation in the dark and pregrowth of inocula in medium containing cellobiose as the sole carbon source (cellulose is a major surface polymer of U. australis). It was also found that P. tunicata requires the presence of a mixed microbial community to colonize effectively. In contrast, R. gallaeciensis effectively colonized the plant surface under all conditions tested. Studies of competitive interactions on the plant surface revealed that P. tunicata was numerically dominant compared with all other bacterial isolates tested (except R. gallaeciensis), and this dominance was linked to production of the antibacterial protein AlpP. Generally, P. tunicata was able to coexist with competing strains, and each strain existed as microcolonies in spatially segregated regions of the plant. R. gallaeciensis was numerically dominant compared with all strains tested and was able to invade and disperse preestablished biofilms. This study highlighted the fact that microbial colonization of U. australis surfaces is a dynamic process and demonstrated the differences in colonization strategies exhibited by the epiphytic bacteria P. tunicata and R. gallaeciensis.

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.

In vitro influence of bacterial mixtures on Fusarium verticillioides growth and fumonisin B1 production: effect of seeds treatment on maize root colonization

AIMS

Enterobacter cloacae, Microbacterium oleovorans, Pseudomonas solanacearum and Bacillus subtilis were investigated in order to evaluate: (i) the inoculum size of two bacterial mixtures on Fusarium verticillioides growth and fumonisin B1 production in vitro at different water activities and (ii) the efficacy of a seed treatment with the best bacterial mixture on F. verticillioides root colonization in greenhouse studies.

METHODS AND RESULTS

The influence of bacterial mixtures (1 = E. cloacae and M. oleovorans and 2 = P. solanacearum and B. subtilis) to antagonize 13 F. verticillioides strains at different inoculum concentrations (10(8), 10(9) and 10(10) cells ml(-1)) and water activities (0.937, 0.955 and 0.982 aW) were examined. Antibiosis, growth rate and fumonisin B1 production were determined. Bacterial mixture 1 proved to exert the most effective control. Seed treatment with mixture 1 at 10(8) cells ml(-1) had the best inhibitory effect on F. verticillioides root colonization.

CONCLUSIONS

These results suggest that the combination E. cloacae and M. oleovorans has the potential for the biological control of F. verticillioides as a maize seed inoculant.

SIGNIFICANCE AND IMPACT OF THE STUDY

The application of this knowledge contributes to prevent the vertical transmission of F. verticillioides.