Comparison of replica leaf surface materials for phyllosphere microbiology

Mircrofabricating double-sided polydimethylsiloxane (PDMS) artificial phylloplane for microbial food safety research

Leafy green surface microbiology studies often experience significant variations in results due to the heterogeneous nature of leaf surfaces. To provide a precise and controllable substitute, we microfabricated double-sided artificial leafy green phylloplanes using polydimethylsiloxane (PDMS) with a vinyl-terminated polyethylene glycol chain-based hydrophobicity modifier (PDMS-PEG) to modify PDMS hydrophobicity. We further tested the properties and applications of these artificial leaves, by examining the function of epicuticular wax, growth and survival of E. coli O157:H7 87-23 on the surface, and removal of attached E. coli cells via sanitation. The double-sided PDMS-PDMS-PEG leaves well-replicated their natural counterparts in macroscopic and microscopic structure, hydrophobicity, and E. coli O157:H7 87-23 attachment. After depositing natural epicuticular wax onto artificial leaves, the leaf surface wetting ability decreased, while E. coli O157:H7 87-23 surface retention increased. The artificial leaves supplied with lettuce lysate or bacterial growth media supported E. coli O157:H7 87-23 growth and survival similarly to those on natural leaves. In the sanitation test, the artificial lettuce leaves also displayed patterns similar to those of natural leaves regarding sanitizer efficiency. Overall, this study showcased the microfabrication and applications of double-sided PDMS-PDMS-PEG leaves as a replicable and controllable platform for future leafy green food safety studies.

Exogenous double-stranded RNA inhibits the infection physiology of rust fungi to reduce symptoms in planta

Rust fungi (Pucciniales) are a diverse group of plant pathogens in natural and agricultural systems. They pose ongoing threats to the diversity of native flora and cause annual crop yield losses. Agricultural rusts are predominantly managed with fungicides and breeding for resistance, but new control strategies are needed on non-agricultural plants and in fragile ecosystems. RNA interference (RNAi) induced by exogenous double-stranded RNA (dsRNA) has promise as a sustainable approach for managing plant-pathogenic fungi, including rust fungi. We investigated the mechanisms and impact of exogenous dsRNA on rust fungi through in vitro and whole-plant assays using two species as models, Austropuccinia psidii (the cause of myrtle rust) and Coleosporium plumeriae (the cause of frangipani rust). In vitro, dsRNA either associates externally or is internalized by urediniospores during the early stages of germination. The impact of dsRNA on rust infection architecture was examined on artificial leaf surfaces. dsRNA targeting predicted essential genes significantly reduced germination and inhibited development of infection structures, namely appressoria and penetration pegs. Exogenous dsRNA sprayed onto 1-year-old trees significantly reduced myrtle rust symptoms. Furthermore, we used comparative genomics to assess the wide-scale amenability of dsRNA to control rust fungi. We sequenced genomes of six species of rust fungi, including three new families (Araucariomyceaceae, Phragmidiaceae, and Skierkaceae) and identified key genes of the RNAi pathway across 15 species in eight families of Pucciniales. Together, these findings indicate that dsRNA targeting essential genes has potential for broad-use management of rust fungi across natural and agricultural systems.

Biotemplate Replication of Novel Mangifera indica Leaf (MIL) for Atmospheric Water Harvesting: Intrinsic Surface Wettability and Collection Efficiency

Water shortage has become a global crisis that has posed and still poses a serious threat to the human race, especially in developing countries. Harvesting moisture from the atmosphere is a viable approach to easing the world water crisis due to its ubiquitous nature. Inspired by nature, biotemplate surfaces have been given considerable attention in recent years though these surfaces still suffer from intrinsic trade-offs making replication more challenging. In the design of artificial surfaces, maximizing their full potential and benefits as that of the natural surface is difficult. Here, we conveniently made use of Mangifera indica leaf (MIL) and its replicated surfaces (RMIL) to collect atmosphere water. This research provides a novel insight into the facile replication mechanism of a wettable surface made of Polydimethylsiloxane (PDMS), which has proven useful in collecting atmospheric water. This comparative study shows that biotemplate surfaces (RMIL) with hydrophobic characteristics outperform natural hydrophilic surfaces (DMIL and FMIL) in droplet termination and water collection abilities. Water collection efficiency from the Replicated Mangifera indica leaf (RMIL) surface was shown to be superior to that of the Dry Mangifera indica leaf (DMIL) and Fresh Mangifera indica leaf (FMIL) surfaces. Furthermore, the wettability of the DMIL, FMIL, and RMIL was thoroughly investigated, with the apices playing an important role in droplet roll-off.

Micro-replication platform for studying the structural effect of seed surfaces on wetting properties

Biological surfaces in plants are critical for controlling essential functions such as wettability, adhesion, and light management, which are linked to their adaptation, survival, and reproduction. Biomimetically patterned surfaces replicating the microstructures of plant surfaces have become an emerging tool for understanding plant–environment interactions. In this study, we developed a two-step micro-replication platform to mimic the microstructure of seed surfaces and demonstrated that this initial platform can be used to study seed surface–environment interactions. The two-step process involved the extraction of a simplified seed surface model from real seeds and micro-replication of the simplified seed surface model using nanoimprint lithography. Using Allium seeds collected from Mongolia and Central Asia as the model system, we studied the wettability of biological and synthetic seed surfaces. We could independently control the material properties of a synthetic seed surface while maintaining the microstructures and, thereby, provide clear evidence that Allium seed surfaces were highly wettable owing to the high surface energy in the epidermal material rather than a microstructural effect. We expect that this platform can facilitate study of the independent effect of microstructure on the interaction of seed surfaces with their surroundings and contribute to research on the evolution of plant–environment interactions.

A microfluidic approach for studying microcolonization of Escherichia coli O157:H7 on leaf trichome-mimicking surfaces under fluid shear stress

Escherichia coli O157:H7 have previously been associated with disease outbreaks associated with leafy green vegetables. However, the physical mechanisms that determine the spatial organization of bacteria onto leafy greens are still not clear. Microfluidics with embedded trichome-mimicking microposts were employed to investigate the role of shear flow and configuration of trichomes on E. coli O157:H7 microcolonization. We characterized the three-dimensional microcolonization of green fluorescent protein (GFP)-tagged E. coli O157:H7 using multiphoton fluorescence microscopy and compared their differences under static (no flow; incubated for 36 h at 37°C) and fluid shear conditions (750 nl/min for 36 h at 37°C). For micropatterned trichome arrays, we demonstrated that natural wax-mixed polydimethylsiloxane retains similar topographies and contact angles to the surface of trichome-bearing leafy greens. Our results showed that E. coli O157:H7 under fluid shear stress aligned their colonization parallel to the direction of flow. In a static condition, their colonization had no preferential alignment, with statistically similar angular distributions in all directions. In addition, depending on dimensions of the trichome arrays and flow conditions, different bacterial microcolonization patterns grew radially from initial attachment; they formed into filamentous structures and developed into bridges by surface hydrophobicity and flow-induced shear with a nutrient-rich medium. Collectively, these results demonstrate how the consequences of bacterial colonization in response to shear flow can affect pathogenic bacterial contamination of leafy greens and biofilm architectures.

Controlling Surface-Induced Platelet Activation by Agarose and Gelatin-Based Hydrogel Films

Platelet-surface interaction is of paramount importance in biomedical applications as well as in vitro studies. However, controlling platelet-surface activation is challenging and still requires more effort as they activate immediately when contacting with any nonphysiological surface. As hydrogels are highly biocompatible, in this study, we developed agarose and gelatin-based hydrogel films to inhibit platelet-surface adhesion. We found promising agarose films that exhibit higher surface wettability, better controlled-swelling properties, and greater stiffness compared to gelatin, resulting in a strong reduction of platelet adhesion. Mechanical properties and surface wettability of the hydrogel films were varied by adding magnetite (Fe3O4) nanoparticles. While all of the films prevented platelet spreading, films formed by agarose and its nanocomposite repelled platelets and inhibited platelet adhesion and activation stronger than those of gelatin. Our results showed that platelet-surface activation is modulated by controlling the properties of the films underneath platelets and that the bioinert agarose can be potentially translated to the development of platelet storage and other medical applications.

Art-on-a-Chip: Preserving Microfluidic Chips for Visualization and Permanent Display

"After a certain high level of technical skill is achieved, science and art tend to coalesce in aesthetics, plasticity, and form. The greatest scientists are always artists as well." said Albert Einstein. Currently, photographic images bridge the gap between microfluidic/lab-on-a-chip devices and art. However, the microfluidic chip itself should be a form of art. Here, novel vibrant epoxy dyes are presented in combination with a simple process to fill and preserve microfluidic chips, to produce microfluidic art or art-on-a-chip. In addition, this process can be used to produce epoxy dye patterned substrates that preserve the geometry of the microfluidic channels-height within 10% of the mold master. This simple approach for preserving microfluidic chips with vibrant, colorful, and long-lasting epoxy dyes creates microfluidic chips that can easily be visualized and photographed repeatedly, for at least 11 years, and hence enabling researchers to showcase their microfluidic chips to potential graduate students, investors, and collaborators.

Leaf Surface Topography Contributes to the Ability of Escherichia coli on Leafy Greens to Resist Removal by Washing, Escape Disinfection With Chlorine, and Disperse Through Splash

The attachment of foodborne pathogens to leaf surfaces is a complex process that involves multiple physical, chemical, and biological factors. Here, we report the results from a study designed to specifically determine the contribution of spinach leaf surface topography as it relates to leaf axis (abaxial and adaxial) and leaf age (15, 45, and 75 days old) to the ability of Escherichia coli to resist removal by surface wash, to avoid inactivation by chlorine, and to disperse through splash impact. We used fresh spinach leaves, as well as so-called "replicasts" of spinach leaf surfaces in the elastomer polydimethylsiloxane to show that leaf vein density correlated positively with the failure to recover E. coli from surfaces, not only using a simple water wash and rinse, but also a more stringent wash protocol involving a detergent. Such failure was more pronounced when E. coli was surface-incubated at 24°C compared to 4°C, and in the presence, rather than absence, of nutrients. Leaf venation also contributed to the ability of E. coli to survive a 50 ppm available chlorine wash and to laterally disperse by splash impact. Our findings suggest that the topographical properties of the leafy green surface, which vary by leaf age and axis, may need to be taken into consideration when developing prevention or intervention strategies to enhance the microbial safety of leafy greens.

Bacterial survival in microscopic surface wetness

Plant leaves constitute a huge microbial habitat of global importance. How microorganisms survive the dry daytime on leaves and avoid desiccation is not well understood. There is evidence that microscopic surface wetness in the form of thin films and micrometer-sized droplets, invisible to the naked eye, persists on leaves during daytime due to deliquescence - the absorption of water until dissolution - of hygroscopic aerosols. Here, we study how such microscopic wetness affects cell survival. We show that, on surfaces drying under moderate humidity, stable microdroplets form around bacterial aggregates due to capillary pinning and deliquescence. Notably, droplet-size increases with aggregate-size, and cell survival is higher the larger the droplet. This phenomenon was observed for 13 bacterial species, two of which - Pseudomonas fluorescens and P. putida - were studied in depth. Microdroplet formation around aggregates is likely key to bacterial survival in a variety of unsaturated microbial habitats, including leaf surfaces.

Replicating Arabidopsis Model Leaf Surfaces for Phyllosphere Microbiology

Artificial surfaces are commonly used in place of leaves in phyllosphere microbiology to study microbial behaviour on plant leaf surfaces. These surfaces enable a reductionist approach to be undertaken, to enable individual environmental factors influencing microorganisms to be studied. Commonly used artificial surfaces include nutrient agar, isolated leaf cuticles, and reconstituted leaf waxes. Recently, replica surfaces mimicking the complex topography of leaf surfaces for phyllosphere microbiology studies are appearing in literature. Replica leaf surfaces have been produced in agar, epoxy, polystyrene, and polydimethylsiloxane (PDMS). However, none of these protocols are suitable for replicating fragile leaves such as of the model plant Arabidopsis thaliana. This is of importance, as A. thaliana is a model system for molecular plant genetics, molecular plant biology, and microbial ecology. To overcome this limitation, we introduce a versatile replication protocol for replicating fragile leaf surfaces into PDMS. Here we demonstrate the capacity of our replication process using optical microscopy, atomic force microscopy (AFM), and contact angle measurements to compare living and PDMS replica A. thaliana leaf surfaces. To highlight the use of our replica leaf surfaces for phyllosphere microbiology, we visualise bacteria on the replica leaf surfaces in comparison to living leaf surfaces.

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.