Fate of Listeria spp. on parsley leaves grown in laboratory and field cultures

Aims: To investigate the population dynamics of Listeria monocytogenes and Listeria innocua on the aerial surfaces of parsley. Methods and Results: Under 100% relative humidity ( RH) in laboratory and regardless of the inoculum tested ( 10(3) - 10(8) CFU per leaf), counts of L. monocytogenes EGDe, LO28, LmP60 and L. innocua CIP 80- 12 tended towards approx. 10 5 CFU per leaf. Under low RH, Listeria spp. populations declined regardless to the inoculum size ( 10(4) - 10(8) CFU per leaf). L. innocua CIP 80- 12 survived slightly better than L. monocytogenes in the laboratory and was used in field cultures. Under field cultures, counts of L. innocua decreased more rapidly than in the laboratory, representing a decrease of 9 log(10) in 2 days in field conditions compared to a decrease of 4 center dot 5 log10 in 8 days in the laboratory. Counts of L. innocua on tunnel parsley cultures were always higher ( at least by 100 times) than those on unprotected parsley culture. Conclusions: Even with a high inoculum and under protected conditions ( i. e. plastic tunnels), population of L. monocytogenes on the surface of parsley on the field would decrease by several log10 scales within 2 days. Significance and Impact of the Study: Direct contamination of aerial surfaces of parsley with L. monocytogenes ( i. e. through contaminated irrigation water) will not lead to contaminated produce unless it occurs very shortly before harvest.