According to the World Health Organization, foodborne illness related to microbial contamination of fruits and vegetables is increasing globally. Contamination can come from water sources before, during, and after harvest, and can be magnified by increasing pressures on water resources and occurrences of extreme weather. Agricultural processes create complex and dynamic aqueous environments in which bacteria may grow, attach, and proliferate on produce surfaces and compromise food safety. This study investigated the effects of solution chemistry and growth conditions on bacterial deposition on spinach leaf surfaces using a parallel plate flow cell. The microfluidic flow cell enables us to observe microbial adhesion to and release from substrates, under highly controllable flow conditions, in real time. Two pathogens, Escherichia coli O157:H7 and Salmonella Typhimurium, and two non-pathogen environmental E. coli isolates were grown in high- and low-nutrient media, and attachment was observed in 10 mM KCl, CaCl2, and artificial groundwater (AGW) suspensions. Cell surface charge, hydrophobicity, and extracellular polymer production and composition were measured for bacteria in each growth media and solution chemistry. Between the two simple salt solutions, bacterial suspension in CaCl2 enhanced attachment over KCl as expected, due to changes in surface charge and hydrophobicity. However, attachment trends in AGW varied by bacteria strain and growth conditions, and cannot be explained by electrostatic and hydrophobic interactions due to cell responses to the complex solution chemistry. When suspended in AGW, total extracellular polymer production and compositional sugar-to-protein ratio increased for all bacteria, which indicates changes in diversity and density of cell surface functional groups. Furthermore, low-nutrient media promoted pathogen attachment more than high-nutrient conditions (p &lt; 0.01), while this trend was not observed for the environmental isolates (p &gt; 0.01). E. coli O157:H7 attachment was always greater than or equal to that of Salmonella. Undernourished cells were smaller and had different surface structures, which may contribute to enhanced irreversible attachment and biofilm formation. These results show the importance of growth conditions, strains, and solution complexities in understanding mechanisms of bacterial adhesion at interfaces of water and food surfaces, and ultimately improving removal and inactivation of foodborne pathogens.

Attachment of foodborne microbes to spinach leaf surfaces: The role of solution chemistry and growth conditions

255th National Meeting (2018)

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