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

Efficacy of a prototype, food-grade alkaline surfactant washing solution an d 1% NaCl-NaHCO3 (pH 10.0) against Escherichia coli O157:H7 cells on lettuc e leaves was evaluated. Lettuce was inoculated with 10(9) CFU/ml of E. coli O157:H7 for 24 +/- 1 hr at 4 degreesC. Samples were rinsed and treated wit h the prototype washing solution containing lauryl sodium sulfate or NaCl-N aHCO3 for 3 min at 22 degreesC. Viability of E. coli O157:H7 cells was exam ined by plate counts at the surface and cut edge, and by confocal scanning microscopic (CSLM) observation of samples stained with Sytox green and Alex a 594 conjugated antibody against E. coli O157:H7 at intact leaf surface, s tomata, and damaged tissue (0 to 10, 30 to 40, and 0 to 40 tm 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/cm(2)) than for NaCl-NaHCO3 (0.2 to 0 .4 log CFU/cm(2)) (P < 0.05). Percentage of viability determined by CSLM fo r prototype washing solution was significantly greater at 30 to 40 Lm from cut surfaces than at 0 to 10 and 0 to 40 mu Lm 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 o f E. coli O157:H7 cells remained viable at all sites except at the surface. The percent viabilities determined by CSLM were not significantly differen t from those determined by plate counts for NaCl-NaHCO3 treatment (P <great er than> 0.05). However, CSLM indicated significantly greater percent viabi lity than plate counts for lettuce treated with the prototype washing solut ion (P < 0.05). Surfactant-containing washing solutions warrant additional testing for decontamination of fresh produce.