Viable but non-culturableListeria monocytogeneson parsley leaves and absence of recovery to a culturable state

Abstract

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.

Induction of Viable but Nonculturable Escherichia coli O157:H7 in the Phyllosphere of Lettuce: a Food Safety Risk Factor

“…Viability was reduced by a 2 log 10 scale within the first 2 days and then reached a level that remained stable until the end of experiment (12 days postinoculation) for both strains. Similar results were reported by Dreux et al (15) when parsley plants inoculated with 10 9 culturable L. monocytogenes bacteria per leaf were exposed to low relative humidity (47 to 69%), and the viable population detected decreased based on the same biphasic pattern (15). In a more stressful challenge (8°C) the entire E. coli population evolved toward a VBNC state, but the time required to induce the VBNC response was different for selected strains.…”

“…Recently, Brandl et al (7,8) demonstrated that E. coli O157:H7 and Salmonella enterica can multiply on the leaf surface at incubation temperatures of between 16 and 30°C (7,8). Dreux et al (15) studied the fate of Listeria monocytogenes on parsley leaves under high and low relative humidity (47 to 69%) and reported induction of the VBNC state due to the stress caused by changes in water availability (15). In our study, exposure of E. coli O157:H7 to low temperatures of 8°C and 16°C on the surface of a coldseason plant was considered comparable for natural conditions.…”

“…Recently, the first report was published that provides strong evidence of the induction of the VBNC state for the human pathogen L. monocytogenes when it is exposed to low relative humidity on the plant surface (15). Dreux et al investigated the survival of L. monocytogenes on parsley leaves under dry conditions and evaluated the ability of the bacteria to recover culturability under saturated humidity (15,16). The authors noted the difference between enumeration of L. monocytogenes populations by standard direct plate counts from vegetables exposed to a high relative humidity and under stress conditions, where both culture and enrichment methods underestimated the total number of viable cells.…”

“…Also, pathogenic bacteria linked with the consumption of the fresh produce, including Salmonella spp. and Listeria monocytogenes, have been shown to evolve toward the VBNC state in the environment and the food chain (3,15,29,30). Recently, the first report was published that provides strong evidence of the induction of the VBNC state for the human pathogen L. monocytogenes when it is exposed to low relative humidity on the plant surface (15).…”

“…and Listeria monocytogenes, have been shown to evolve toward the VBNC state in the environment and the food chain (3,15,29,30). Recently, the first report was published that provides strong evidence of the induction of the VBNC state for the human pathogen L. monocytogenes when it is exposed to low relative humidity on the plant surface (15). Dreux et al investigated the survival of L. monocytogenes on parsley leaves under dry conditions and evaluated the ability of the bacteria to recover culturability under saturated humidity (15,16).…”