Factors influencing the transport and deposition of Salmonella and colloids from evaporating sessile droplets on polydimethylsiloxane surfaces of fresh produce and simple micropatterns
Date
2019
Authors
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Publisher
University of Delaware
Abstract
The transport and deposition of colloids and biocolloids (e.g., bacteria) are important processes that occur in environmental (e.g., the vadose zone) and engineered (e.g., wastewater treatment) systems. Although the major mechanisms surrounding colloid transport and retention are known, less is known about these mechanisms in the context of unsaturated systems (e.g., porous media and on biological surfaces) compared to saturated systems, particularly for biocolloids such as bacteria. Importantly, bacteria that are pathogenic to humans, such as E. coli O157:H7 and Salmonella spp., are found in agricultural systems throughout the farm-to-fork continuum, including soil and manure, irrigation water, and on crop surfaces. Human pathogenic bacteria attach to and survive on the surfaces of ready-to-eat fresh produce, which can lead to large-scale outbreaks of foodborne illness and even death. The most recent outbreak involving E. coli O157:H7 on Romaine lettuce in 2018 underscores the value of fresh produce safety research: there is a dire need to understand how fresh produce contamination with human bacterial pathogens occurs and to develop effective mitigation strategies. Currently, the most widely-used approach, chlorine sanitation, achieves just 1-2 log reduction of colony-forming units (CFU) per gram of produce and is even less effective at removing bacteria attached in biofilms. Researchers have developed alternative methods to remove contaminants, but these methods do not achieve significantly higher sanitation rates. To date, how human pathogens are deposited and retained on plant surfaces as well as how these pathogens survive throughout processing are research areas not fully elucidated. ☐ The focus of this research was to improve the understanding of how (bio)colloids interact on produce surfaces. To do this, polydimethylsiloxane (PDMS) surfaces of spinach and lettuce were used in addition to micropatterned surfaces of simple geometries fabricated via standard lithography procedures. A major effort of this work was to visualize the transport and deposition of (bio)colloids from contact lines of evaporating droplets on different surfaces using confocal microscopy, and to correlate the qualitative data to the mechanisms involved in colloid transport theory considering the influence of surface, solution, and (bio)colloid properties. PDMS is an optically-clear and inert material that faithfully replicates surface features to nanometer scales. Using PDMS mimics of the surfaces also reduced the confounding variations found on natural plant surfaces and is a suitable material for imaging purposes. ☐ Evaporating droplets in either water or surfactant Tween 80 were used as the system to deposit Salmonella enterica sv. Enteritidis, an important foodborne pathogen, and polystyrene latex microspheres (diameter = 2 μm), used as an ideal particle for comparison to the bacteria, on the different surfaces. Tween 80, a nonionic surfactant, allowed for the manipulation of surface tension of the droplet suspension to consider the effect of capillary forces on particle transport and deposition. Tween 80 is also approved by the FDA for use in washing fruits and vegetable surfaces and has been researched as an alternative method to chlorine but with limited success. ☐ In Chapter 2 of this work, transport and deposition of bio(colloids) is considered in the context of fresh produce safety by using PDMS Lettuce and PDMS Spinach surfaces compared to smooth, flat PDMS and Glass surfaces. In Chapter 3, PDMS micropatterns of raised pillars, depressed dots, and grooved features were compared to smooth, flat PDMS and Glass surfaces to further explore the role of surface topography/roughness in addition to the other factors influencing particle transport and deposition. In addition to the visualization of evaporation and resulting patterns, the physicochemical characteristics, surface roughness and hydrophobicity, along with the evaporation characteristics, evaporation time, contact line behavior, and droplet contact areas, were quantified. Taken together, it was demonstrated in this work that: ☐ 1) The surface properties (roughness and hydrophobicity) strongly influenced the spatiotemporal deposition of colloids and Salmonella enterica sv. Enteritidis. Deposition pattern morphology of the bacteria and colloids were clearly different based on surface architecture and roughness. Surface roughness features caused contact lines of the evaporating droplets to pin at the features, where transport to these regions and ultimately deposition was observed. Additionally, surface hydrophobicity, which is influenced by surface roughness, influenced the thickness of water films on the surfaces, which in turn altered transport behavior and resulting deposition patterns. ☐ 2) The transport behavior between colloids and Salmonella observed in this study demonstrated that bacteria better mobilize with the contact line and form distinct rings around the final contact line area, where the bacteria were rapidly transported during the last stages of droplet evaporation. Although a detailed quantitative comparison was not explored in this work, (bio)colloid size and shape likely play a role. When choosing a colloid as surrogate bacteria, researchers should use particles that have similar properties such as size and shape; ☐ 3) The addition of surfactant Tween 80 altered the transport and deposition behavior of Salmonella and colloids due to a reduction in capillary forces by decreasing the surface tension and film thickness, which changed the way the airwater- interface interacted on the particles and solid surfaces. In effect, the contact line could not mobilize the particles and thus lead to deposition of particles over a larger area on the surfaces used. ☐ 4) Finally, no single parameter was solely responsible for the transport and deposition of particles in these studies. The combination of surface, solution, and particle properties all influenced the hydrodynamic flow, film thickness, and capillary forces in the evaporating droplets that dictated how particles were transported and deposited on the surfaces. ☐ Visualizing the transport and deposition of (bio)colloids at contact lines, here by using evaporating droplets, improved the understanding of the factors influencing (bio)colloid interactions on surfaces. The food industry can devise improved strategies for decontamination of produce surfaces with the fundamental knowledge explored in this work. Future research can expand upon the work developed here with modeling and more detailed experiments to further develop these contributing factors in (bio)colloid transport and deposition, including ways to manipulate these surface-particle interactions.