Viable but non-culturable Listeria monocytogenes on parsley leaves and absence of recovery to a culturable state

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 (10(9) L. monocytogenes per leaf on TSA) was reduced by 6 log(10) scales in 15 days, whereas number of viable L. monocytogenes counted under the microscope was reduced by 3-4 log(10) 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.