Direct microscopic observation of lettuce leaf decontamination with a prototype fruit and vegetable washing solution and 1% NaCL-NaHCO3

Abstract

Efficacy of a prototype, food-grade alkaline surfactant washing solution and 1% NaCl-NaHCO3 (pH 10.0) against Escherichia coli O157:H7 cells on lettuce leaves was evaluated. Lettuce was inoculated with 109 CFU/ml of E. coli O157:H7 for 24 ± 1 h at 4°C. Samples were rinsed and treated with the prototype washing solution containing lauryl sodium sulfate or NaCl-NaHCO3 for 3 min at 22°C. Viability of E. coli O157:H7 cells was examined by plate counts at the surface and cut edge, and by confocal scanning microscopic (CSLM) observation of samples stained with Sytox green and Alexa 594 conjugated antibody against E. coli O157:H7 at intact leaf surface, stomata, and damaged tissue (0 to 10, 30 to 40, and 0 to 40 μm from the cut surface). Although both treatments caused significant log reductions in CFU at the surface and cut edge, log reductions were greater for the prototype washing solution (0.7 to 1.1 log CFU/cm2) than for NaCl-NaHCO3 (0.2 to 0.4 log CFU/cm2) (P < 0.05). Percentage of viability determined by CSLM for prototype washing solution was significantly greater at 30 to 40 μm from cut surfaces than at 0 to 10 and 0 to 40 μm from cut surfaces and intact surfaces (P <0.05). Stomata provided moderate protection. NaCl-NaHCO3 was less effective than the prototype washing solution, and high percentages of E. coli O157:H7 cells remained viable at all sites except at the surface. The percent viabilities determined by CSLM were not significantly different from those determined by plate counts for NaCl-NaHCO3 treatment (P > 0.05). However, CSLM indicated significantly greater percent viability than plate counts for lettuce treated with the prototype washing solution (P < 0.05). Surfactant-containing washing solutions warrant additional testing for decontamination of fresh produce.