Bacteria in the leaf ecosystem with emphasis on Pseudomonas syringae-a pathogen, ice nucleus, and epiphyte.

The extremely large number of leaves produced by terrestrial and aquatic plants provide habitats for colonization by a diversity of microorganisms. This review focuses on the bacterial component of leaf microbial communities, with emphasis on Pseudomonas syringae-a species that participates in leaf ecosystems as a pathogen, ice nucleus, and epiphyte. Among the diversity of bacteria that colonize leaves, none has received wider attention than P. syringae, as it gained notoriety for being the first recombinant organism (Ice(-) P. syringae) to be deliberately introduced into the environment. We focus on P. syringae to illustrate the attractiveness and somewhat unique opportunities provided by leaf ecosystems for addressing fundamental questions of microbial population dynamics and mechanisms of plant-bacterium interactions. Leaf ecosystems are dynamic and ephemeral. The physical environment surrounding phyllosphere microbes changes continuously with daily cycles in temperature, radiation, relative humidity, wind velocity, and leaf wetness. Slightly longer-term changes occur as weather systems pass. Seasonal climatic changes impose still a longer cycle. The physical and physiological characteristics of leaves change as they expand, mature, and senesce and as host phenology changes. Many of these factors influence the development of populations of P. syringae upon populations of leaves. P. syringae was first studied for its ability to cause disease on plants. However, disease causation is but one aspect of its life strategy. The bacterium can be found in association with healthy leaves, growing and surviving for many generations on the surfaces of leaves as an epiphyte. A number of genes and traits have been identified that contribute to the fitness of P. syringae in the phyllosphere. While still in their infancy, such research efforts demonstrate that the P. syringae-leaf ecosystem is a particularly attractive system with which to bridge the gap between what is known about the molecular biology of genes linked to pathogenicity and the ecology and epidemiology of associated diseases as they occur in natural settings, the field.     

Microbial and Functional Diversity within the Phyllosphere of Espeletia Species in an Andean High-Mountain Ecosystem

Microbial populations residing in close contact with plants can be found in the rhizosphere, in the phyllosphere as epiphytes on the surface, or inside plants as endophytes. Here, we analyzed the microbiota associated with Espeletia plants, endemic to the Páramo environment of the Andes Mountains and a unique model for studying microbial populations and their adaptations to the adverse conditions of high-mountain neotropical ecosystems. Communities were analyzed using samples from the rhizosphere, necromass, and young and mature leaves, the last two analyzed separately as endophytes and epiphytes. The taxonomic composition determined by performing sequencing of the V5-V6 region of the 16S rRNA gene indicated differences among populations of the leaf phyllosphere, the necromass, and the rhizosphere, with predominance of some phyla but only few shared operational taxonomic units (OTUs). Functional profiles predicted on the basis of taxonomic affiliations differed from those obtained by GeoChip microarray analysis, which separated community functional capacities based on plant microenvironment. The identified metabolic pathways provided insight regarding microbial strategies for colonization and survival in these ecosystems. This study of novel plant phyllosphere microbiomes and their putative functional ecology is also the first step for future bioprospecting studies in search of enzymes, compounds, or microorganisms relevant to industry or for remediation efforts.     

Summer season and long-term drought increase the richness of bacteria and fungi in the foliar phyllosphere of Quercus ilex in a mixed Mediterranean forest

We explored the changes in richness, diversity and evenness of epiphytic (on the leaf surface) and endophytic (within leaf tissues) bacteria and fungi in the foliar phyllosphere of Quercus ilex, the dominant tree species of Mediterranean forests. Bacteria and fungi were assessed during ontogenic development of the leaves, from the wet spring to the dry summer season in control plots and in plots subjected to drought conditions mimicking those projected for future decades. Our aim was to monitor succession in microbiota during the colonisation of plant leaves and its response to climate change. Ontogeny and seasonality exerted a strong influence on richness and diversity of the microbial phyllosphere community, which decreased in summer in the whole leaf and increased in summer in the epiphytic phyllosphere. Drought precluded the decrease in whole leaf phyllosphere diversity and increased the rise in the epiphytic phyllosphere. Both whole leaf bacterial and fungal richness decreased with the decrease in physiological activity and productivity of the summer season in control trees. As expected, the richness of epiphytic bacteria and fungi increased in summer after increasing time of colonisation. Under summer dry conditions, there was a positive relationship between TRF (terminal restriction fragments) richness and drought, both for whole leaf and epiphytic phyllosphere, and especially for fungal communities. These results demonstrate that changes in climate are likely to significantly alter microbial abundance and composition of the phyllosphere. Given the diverse functions and large number of phyllospheric microbes, the potential functional implications of such community shifts warrant exploration.     

High Diversity Revealed in Leaf‐Associated Protists (Rhizaria: Cercozoa) of Brassicaceae

The largest biological surface on earth is formed by plant leaves. These leaf surfaces are colonized by a specialized suite of leaf‐inhabiting microorganisms, recently termed “phyllosphere microbiome”. Microbial prey, however, attract microbial predators. Protists in particular have been shown to structure bacterial communities on plant surfaces, but virtually nothing is known about the community composition of protists on leaves. Using newly designed specific primers targeting the 18S rDNA gene of Cercozoa, we investigated the species richness of this common protist group on leaves of four Brassicaceae species from two different locations in a cloning‐based approach. The generated sequences revealed a broad diversity of leaf‐associated Cercozoa, mostly bacterial feeders, but also including known plant pathogens and a taxon of potential endophytes that were recently described as algal predators in freshwater systems. This initial study shows that protists must be regarded as an integral part of the microbial diversity in the phyllosphere of plants.     

Bacterial Succession on the Leaf Surface: A Novel System for Studying Successional Dynamics

Succession is a widely studied process in plant and animal systems, but succession in microbial communities has received relatively little attention despite the ubiquity of microorganisms in natural habitats. One important microbial habitat is the phyllosphere, or leaf surface, which harbors large, diverse populations of bacteria and offers unique opportunities for the study of succession and temporal community assembly patterns. To explore bacterial community successional patterns, we sampled phyllosphere communities on cottonwood (Populus deltoides) trees multiple times across the growing season, from leaf emergence to leaf fall. Bacterial community composition was highly variable throughout the growing season; leaves sampled as little as a week apart were found to harbor significantly different communities, and the temporal variability on a given tree exceeded the variability in community composition between individual trees sampled on a given day. The bacterial communities clearly clustered into early-, mid-, and late-season clusters, with early- and late-season communities being more similar to each other than to the mid-season communities, and these patterns appeared consistent from year to year. Although we observed clear and predictable changes in bacterial community composition during the course of the growing season, changes in phyllosphere bacterial diversity were less predictable. We examined the species–time relationship, a measure of species turnover rate, and found that the relationship was fundamentally similar to that observed in plant and invertebrate communities, just on a shorter time scale. The temporal dynamics we observed suggest that although phyllosphere bacterial communities have high levels of phylogenetic diversity and rapid turnover rates, these communities follow predictable successional patterns from season to season.     

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.     

叶际微生物及其与外界互作的研究进展

叶际微生物及其生存环境共同形成了一个复杂的生态系统。建立在纯种分离和纯培养技术基础之上的传统研究方法只能了解其中部分叶际微生物,但对物种组成、种群结构和生态学作用等方面的认识都比较片面。近年来随着分子生物学和生物信息学的进步,人们对叶际微生物总群落的分析逐渐揭示了叶际微生物组成的多样性及其特点,以及与外界互相作用的复杂性。研究表明,植物种类、地理位置和季节差异等都不同程度地影响着叶际微生物群落的构成。本文综述了近年来国内外叶际微生物群落结构组成及其与外界互作方面的研究进展,有利于加深对叶际微生物的了解,也有助于深入理解叶际微生物与植物生长和植物病虫害防治的关联关系。     

Protection of tomato seedlings against infection by Pseudomonas syringae pv. tomato by using the plant growth-promoting bacterium Azospirillum brasilense

Pseudomonas syringae pv. tomato, the causal agent of bacterial speck of tomato, and the plant growth-promoting bacterium Azospirillum brasilense were inoculated onto tomato plants, either alone, as a mixed culture, or consecutively. The population dynamics in the rhizosphere and foliage, the development of bacterial speck disease, and their effects on plant growth were monitored. When inoculated onto separate plants, the A. brasilense population in the rhizosphere of tomato plants was 2 orders of magnitude greater than the population of P. syringae pv. tomato (10(7) versus 10(5) CFU/g [dry weight] of root). Under mist chamber conditions, the leaf population of P. syringae pv. tomato was 1 order of magnitude greater than that of A. brasilense (10(7) versus 10(6) CFU/g [dry weight] of leaf). Inoculation of seeds with a mixed culture of the two bacterial strains resulted in a reduction of the pathogen population in the rhizosphere, an increase in the A. brasilense population, the prevention of bacterial speck disease development, and improved plant growth. Inoculation of leaves with the mixed bacterial culture under mist conditions significantly reduced the P. syringae pv. tomato population and significantly decreased disease severity. Challenge with P. syringae pv. tomato after A. brasilense was established in the leaves further reduced both the population of P. syringae pv. tomato and disease severity and significantly enhanced plant development. Both bacteria maintained a large population in the rhizosphere for 45 days when each was inoculated separately onto tomato seeds (10(5) to 10(6) CFU/g [dry weight] of root). However, P. syringae pv. tomato did not survive in the rhizosphere in the presence of A. brasilense. Foliar inoculation of A. brasilense after P. syringae pv. tomato was established on the leaves did not alleviate bacterial speck disease, and A. brasilense did not survive well in the phyllosphere under these conditions, even in a mist chamber. Several applications of a low concentration of buffered malic acid significantly enhanced the leaf population of A. brasilense (>10(8) CFU/g [dry weight] of leaf), decreased the population of P. syringae pv. tomato to almost undetectable levels, almost eliminated disease development, and improved plant growth to the level of uninoculated healthy control plants. Based on our results, we propose that A. brasilense be used in prevention programs to combat the foliar bacterial speck disease caused by P. syringae pv. tomato.     

Diversity and Activity of PAH-Degrading Bacteria in the Phyllosphere of Ornamental Plants

Phyllosphere bacteria on ornamental plants were characterized based on their diversity and activity towards the removal of polycyclic aromatic hydrocarbons (PAHs), the major air pollutants in urban area. The amounts of PAH-degrading bacteria were about 1–10% of the total heterotrophic phyllosphere populations and consisted of diverse bacterial species such as Acinetobacter, Pseudomonas, Pseudoxanthomonas, Mycobacterium, and uncultured bacteria. Bacterial community structures analyzed by polymerase chain reaction–denaturing gradient gel electrophoresis from each plant species showed distinct band patterns. The uniqueness of these phyllosphere bacterial communities was partly due to the variation in leaf morphology and chemical properties of ornamental plants. The PAH degradation activity of these bacteria was monitored in gas-tight systems containing sterilized or unsterilized leaves. The results indicated that phyllosphere bacteria on unsterilized leaves were able to enhance the activity of leaves for phenanthrene removal. When compared between plant species, phenanthrene removal efficiency corresponded to the size of phenanthrene-degrading bacteria. In addition, phyllosphere bacteria on Wrightia religiosa were able to reduce other PAHs such as acenaphthylene, acenaphthene, and fluorine in 60-ml glass vials and in a 14-l glass chamber. Thus, phyllosphere bacteria on ornamental plants may play an important role in natural attenuation of airborne PAHs in urban areas.     

Both leaf properties and microbe-microbe interactions influence within-species variation in bacterial population diversity and structure in the lettuce (Lactuca Species) phyllosphere

Morphological and chemical differences between plant genera influence phyllosphere microbial populations, but the factors driving within-species variation in phyllosphere populations are poorly understood. Twenty-six lettuce accessions were used to investigate factors controlling within-species variation in phyllosphere bacterial populations. Morphological and physiochemical characteristics of the plants were compared, and bacterial community structure and diversity were investigated using terminal restriction fragment length polymorphism (T-RFLP) profiling and 16S rRNA gene clone libraries. Plant morphology and levels of soluble carbohydrates, calcium, and phenolic compounds (which have long been associated with plant responses to biotic stress) were found to significantly influence bacterial community structure. Clone libraries from three representative accessions were found to be significantly different in terms of both sequence differences and the bacterial genera represented. All three libraries were dominated by Pseudomonas species and the Enterobacteriaceae family. Significant differences in the relative proportions of genera in the Enterobacteriaceae were detected between lettuce accessions. Two such genera (Erwinia and Enterobacter) showed significant variation between the accessions and revealed microbe-microbe interactions. We conclude that both leaf surface properties and microbial interactions are important in determining the structure and diversity of the phyllosphere bacterial community.     

Relationship of total viable and culturable cells in epiphytic populations of Pseudomonas syringae.

The direct viable count method, used to detect viable but nonculturable bacteria in aquatic systems, was modified to examine epiphytic populations of Pseudomonas syringae. Viable-population sizes determined from the number of cells that elongated when incubated with yeast extract and nalidixic acid were compared with those determined by the conventional plate count method. The plate count method accurately determined the number of viable cells in epiphytic P. syringae populations in a state of active growth under conditions of high relative humidity. The plate count method also accurately determined the number of viable cells in P. syringae inoculum, or a growing P. syringae population, subject to desiccation stress under conditions of low relative humidity. In epiphytic populations of P. syringae older than 80 h, however, the plate count underestimated the viable-population size by about two- to fourfold, suggesting that up to 75% of the P. syringae population was nonculturable. These nonculturable cells may have entered a starvation-survival state, induced by low nutrient availability in the phyllosphere environment. Epiphytic P. syringae populations undergoing rapid size changes due to growth and death under fluctuating environmental conditions in the field should be accurately enumerated by the plate count method. However, the possible underestimation of viable-population size under some circumstances should be considered in epidemiological studies of phytopathogenic bacteria and when genetically engineered microorganisms in terrestrial ecosystems are monitored.     

Colonization of tomato plants by Salmonella enterica is cultivar dependent, and type 1 trichomes are preferred colonization sites

Nontyphoid salmonellosis caused by Salmonella enterica is the most common bacterial food-borne illness in humans, and fresh produce, including tomatoes, is a common vehicle. Accumulating data indicate that human enteric pathogenic bacteria, including S. enterica, interact actively with plants. Tomato plants were inoculated with S. enterica to evaluate plausible contamination routes and to determine if the tomato cultivar affects S. enterica colonization. S. enterica population levels on tomato leaves were cultivar dependent. S. enterica levels on Solanum pimpinellifolium (West Virginia 700 [WVa700]) were lower than on S. lycopersicum cultivars. S. enterica preferentially colonized type 1 trichomes and rarely interacted with stomata, unlike what has been reported for cut lettuce leaves. Early S. enterica leaf colonization led to contamination of all fruit, with levels as high as 10(5) CFU per fruit. Reduced bacterial speck lesion formation correlated with reduced S. enterica populations in the phyllosphere. Tomato pedicels and calyxes also harbored large S. enterica populations following inoculation via contaminated water postharvest. WVa700 green fruit harbored significantly smaller S. enterica populations than did red fruit or S. lycopersicum fruit. We found that plants irrigated with contaminated water had larger S. enterica populations than plants grown from seeds planted in infested soil. However, both routes of contamination resulted in detectable S. enterica populations in the phyllosphere. Phyllosphere S. enterica populations pose a risk of fruit contamination and subsequent human disease. Restricting S. enterica phyllosphere populations may result in reduced fruit contamination. We have identified WVa700 as a tomato cultivar that can restrict S. enterica survival in the phyllosphere.     

Relationship of total viable and culturable cells in epiphytic populations of Pseudomonas syringae.

The direct viable count method, used to detect viable but nonculturable bacteria in aquatic systems, was modified to examine epiphytic populations of Pseudomonas syringae. Viable-population sizes determined from the number of cells that elongated when incubated with yeast extract and nalidixic acid were compared with those determined by the conventional plate count method. The plate count method accurately determined the number of viable cells in epiphytic P. syringae populations in a state of active growth under conditions of high relative humidity. The plate count method also accurately determined the number of viable cells in P. syringae inoculum, or a growing P. syringae population, subject to desiccation stress under conditions of low relative humidity. In epiphytic populations of P. syringae older than 80 h, however, the plate count underestimated the viable-population size by about two- to fourfold, suggesting that up to 75% of the P. syringae population was nonculturable. These nonculturable cells may have entered a starvation-survival state, induced by low nutrient availability in the phyllosphere environment. Epiphytic P. syringae populations undergoing rapid size changes due to growth and death under fluctuating environmental conditions in the field should be accurately enumerated by the plate count method. However, the possible underestimation of viable-population size under some circumstances should be considered in epidemiological studies of phytopathogenic bacteria and when genetically engineered microorganisms in terrestrial ecosystems are monitored.     

Fungal Phyllosphere Communities are Altered by Indirect Interactions Among Trophic Levels

Trophic interactions involving predators, herbivores, and plants have been described in terrestrial systems. However, there is almost no information on the effect of trophic interactions on microbial phyllosphere community abundance, diversity, or structure. In this study, the interaction between a parasitoid, an insect herbivore, and the fungal phyllosphere community is examined. Parasitoid wasps have an indirect negative impact on fungal community diversity. On the citrus phyllosphere, the exotic wasp species, Amitus hesperidum and Encarsia opulenta, may parasitize the citrus blackfly (Aleurocanthus woglumi). If parasitism levels are low, the blackfly may produce significant amounts of honeydew secretions on the surface of the leaf. Honeydew deposition provides a carbon-rich substrate for the development of fungal growth persisting as sooty mold on the leaves. Leaves from sooty mold-infested grapefruit (Citrus paradisi) trees were collected from multiple orchards in south Texas. The effect of different levels of exotic parasite activity, citrus blackfly, and sooty mold infestation on phyllosphere mycobiota community structure and diversity was examined. Our results suggest the presence of the parasitoid may lead to a top–down trophic cascade affecting phyllosphere fungal community diversity and structure. Additionally, persistent sooty mold deposits that have classically been referred to as Capnodium citri (and related asexual morphological forms) actually comprise a myriad of fungal species including many saprophytes and potential fruit and foliar pathogens of citrus.     

北京龙形水系三种沉水植物根际及叶际微生物群落特征

水生植物及植物表面附着微生物在人工湿地水体净化过程中发挥着重要的作用。以北京奥林匹克公园龙形水系为研究对象,通过高通量测序技术,对其底泥、水体及3种沉水植物——苦草Vallisneria natans、狐尾藻Myriophyllum verticillatum、龙须眼子菜Potamogeton pectinatus——的根际及叶际微生物群落的结构及功能进行了研究。结果表明,微生物多样性从高到低分别为底泥样品、植物根际样品、植物叶际样品和水体样品,植物叶际微生物种类要显著高于水体中微生物种类。LEfSe分析结果显示不同生境富集不同的微生物类群,其中底泥主要富集厌氧微生物类群,水体及植物叶际主要富集好氧微生物类群,植物根际则两者兼具。功能预测结果显示植物叶际样品的反硝化标志基因丰度要高于根际样品及底泥和水体样品,且狐尾藻和龙须眼子菜叶际样品反硝化标志基因丰度要高于苦草叶际样品。本研究可以为人工湿地构建时对沉水植物及功能微生物的选择提供指导意义。     

Effect of phenotypic plasticity on epiphytic survival and colonization by Pseudomonas syringae.

The bacterial epiphyte Pseudomonas syringae MF714R was cultured on agar or in broth or collected from colonized leaves; it was then inoculated onto greenhouse-grown bean plants incubated in a growth chamber at low relative humidity or in the field or onto field-grown bean plants. Cells cultured in liquid medium survived the least well after inoculation of leaf surfaces under all conditions. Cells cultured in solid medium exhibited the highest percent survival and desiccation tolerance in the growth chamber but generally survived less well in the field than did cells harvested from plants. Cells harvested from plants and inoculated onto plants in the field usually exhibited the highest percent survival, started to increase in population earlier, and reached a higher number than did cells cultured in vitro. Differences in field survival were apparently not attributable to differential UV tolerance. The observed effects of phenotypic plasticity on epiphytic survival and colonization should be considered in risk assessment studies, in studies of bacterial epidemiology, and in the use of microbial antagonists for biological pest control.     

The ecology of the phyllosphere: geographic and phylogenetic variability in the distribution of bacteria on tree leaves

Large populations of bacteria live on leaf surfaces and these phyllosphere bacteria can have important effects on plant health. However, we currently have a limited understanding of bacterial diversity on tree leaves and the inter‐ and intra‐specific variability in phyllosphere community structure. We used a barcoded pyrosequencing technique to characterize the bacterial communities from leaves of 56 tree species in Boulder, Colorado, USA, quantifying the intra‐ and inter‐individual variability in the bacterial communities from 10 of these species. We also examined the geographic variability in phyllosphere communities on Pinus ponderosa from several locations across the globe. Individual tree species harboured high levels of bacterial diversity and there was considerable variability in community composition between trees. The bacterial communities were organized in patterns predictable from the relatedness of the trees as there was significant correspondence between tree phylogeny and bacterial community phylogeny. Inter‐specific variability in bacterial community composition exceeded intra‐specific variability, a pattern that held even across continents where we observed minimal geographic differentiation in the bacterial communities on P. ponderosa needles.     

Resilience of the Natural Phyllosphere Microbiota of the Grapevine to Chemical and Biological Pesticides

The phyllosphere is colonized by complex microbial communities, which are adapted to the harsh habitat. Although the role and ecology of nonpathogenic microorganisms of the phyllosphere are only partially understood, leaf microbiota could have a beneficial role in plant growth and health. Pesticides and biocontrol agents are frequently applied to grapevines, but the impact on nontarget microorganisms of the phyllosphere has been marginally considered. In this study, we investigated the effect of a chemical fungicide (penconazole) and a biological control agent (Lysobacter capsici AZ78) on the leaf microbiota of the grapevine at three locations. Amplicons of the 16S rRNA gene and of the internal transcribed spacer were sequenced for bacterial and fungal identification, respectively. Pyrosequencing analysis revealed that the richness and diversity of bacterial and fungal populations were only minimally affected by the chemical and biological treatments tested, and they mainly differed according to grapevine locations. Indigenous microbial communities of the phyllosphere are adapted to environmental and biotic factors in the areas where the grapevines are grown, and they are resilient to the treatments tested. The biocontrol properties of phyllosphere communities against downy mildew differed among grapevine locations and were not affected by treatments, suggesting that biocontrol communities could be improved with agronomic practices to enrich beneficial populations in vineyards.     

Fitness of Salmonella enterica serovar Thompson in the cilantro phyllosphere

The epiphytic fitness of Salmonella enterica was assessed on cilantro plants by using a strain of S. enterica serovar Thompson that was linked to an outbreak resulting from cilantro. Salmonella serovar Thompson had the ability to colonize the surface of cilantro leaves, where it was detected by confocal laser scanning microscopy (CLSM) at high densities on the veins and in natural lesions. The population sizes of two common colonizers of plant surfaces, Pantoea agglomerans and Pseudomonas chlororaphis, were 10-fold higher than that of the human pathogen on cilantro incubated at 22 degrees C. However, Salmonella serovar Thompson achieved significantly higher population levels and accounted for a higher proportion of the total culturable bacterial flora on cilantro leaves when the plants were incubated at warm temperatures, such as 30 degrees C, after inoculation, indicating that the higher growth rates exhibited by Salmonella serovar Thompson at warm temperatures may increase the competitiveness of this organism in the phyllosphere. The tolerance of Salmonella serovar Thompson to dry conditions on plants at 60% relative humidity was at least equal to that of P. agglomerans and P. chlororaphis. Moreover, after exposure to low humidity on cilantro, Salmonella serovar Thompson recovered under high humidity to achieve its maximum population size in the cilantro phyllosphere. Visualization by CLSM of green fluorescent protein-tagged Salmonella serovar Thompson and dsRed-tagged P. agglomerans inoculated onto cilantro revealed that the human pathogen and the bacterial epiphyte formed large heterogeneous aggregates on the leaf surface. Our studies support the hypothesis that preharvest contamination of crops by S. enterica plays a role in outbreaks linked to fresh fruits and vegetables.