The survival and transfer of Listeria innocua and Clostridium sporogenes, used as surrogates of the food borne pathogens Listeria monocytogenes and Clostridium botulinum, were quantitatively assessed under field conditions. In the soil, spores of C. sporogenes declined by less than 0.7 log cycles within 16 months and were detected on parsley leaves throughout the experiment. In contrast, L. innocua in the soil declined by 7 log cycles in 90 days and was detected on leaves in low numbers (>0.04 MPN g(-1)) during the first 30 days. Rates of decline in soil were similar in the laboratory at 20 degrees C for two strains of L. innocua and L. monocytogenes ; and in the field for L. innocua over two different years. L. innocua survived better in winter, indicating an important influence of temperature. The major cause of transfer of L. innocua from soil to parsley leaves was splashing due to rain and irrigation. As few as 1 CFU g(-1) Listeria in soil led to contamination of parsley leaves. Internalisation of Listeria through parsley roots was not observed. Under the conditions of soil and climate studied, a delay of 90 days between application of potentially contaminated fertilizer and harvest should be sufficient to eliminate L. monocytogenes.

Aims:  To investigate the presence of viable but non‐culturable Listeria monocytogenes during survival on parsley leaves under low relative humidity (RH) and to evaluate the ability of L. monocytogenes to recover from VBNC to culturable state under satured humidity. Methods and Results:  Under low RH (47–69%) on parsley leaves, the initial number of L. monocytogenes populations counted on non selective media (109 L. monocytogenes per leaf on TSA) was reduced by 6 log10 scales in 15 days, whereas number of viable L. monocytogenes counted under the microscope was reduced by 3–4 log10 scales, indicating the presence of VBNC cells. This was demonstrated on three L. monocytogenes strains (EGDe, Bug 1995 and LmP60). Changing from low to 100% RH permitted an increase of the culturable counts of L. monocytogenes and this growth was observed only when residual culturable cells were present. Moreover, VBNC L. monocytogenes inoculated on parsley leaves did not become culturable after incubation under 100% RH. Conclusions:  Dry conditions induced VBNC L. monocytogenes on parsley leaves but these VBNC were likely unable to recover culturability after transfer to satured humidity. Significance and Impact of Study:  Enumeration on culture media presumably under‐estimates the number of viable L. monocytogenes on fresh produce after exposure to low RH.

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