Spatial persistence of Escherichia coli O157:H7 flowing on micropatterned structures inspired by stomata and microgrooves of leafy greens

This study investigated the persistence of Escherichia coli O157:H7 on the surface of leafy greens, and penetration into intercellular spaces through stomatal pores. This was done by determining the role of leafy green-mimicking microgrooves (normal and fully open stomata and micro-sized crevices) and the roles of pore size and orientation on proliferation of E. coli. Microgroove-embedded microfluidics were fabricated using soft-lithography. Natural wax-polydimethylsiloxane (PDMS) substrates were used for mimicking the surface of leafy greens, such as spinach and lettuce. Spatial microcolonization of E. coli was characterized after cell suspensions (10⁹ CFU/mL) were exposed to static and laminar flow conditions for 36 h. Under fluid shear, higher numbers of microcolonies were seen at vortices and edges, particularly located on downstream areas of individual stomata microstructures. With crevice arrays, colonies existed predominantly on crevices at the sides of arrays rather than those in center-lines, due to higher fluid shear at these locations. These results suggested that spatial changes of bacterial colonization in response to shear flow can affect contamination of leafy greens and show the need and possibilities to design irrigation and washing systems to mitigate such growth.