Purpose: To evaluate the effect of habituation of Listeria monocytogenes and Salmonella spp. on tomato or lettuce, on their subsequent resistance to lethal stresses.
Methods: Fresh cut lettuce or whole cherry tomatoes were inoculated (106 CFU/cm2) with three strains of L. monocytogenes serotypes 1/2a, 4b or Salmonella spp. serovars Typhimurium, Enteritidis and incubated for 1 or 5 days at 5°C. Adapted cells were harvested and exposed in Tryptic Soy Broth to (i) pH 3.5 (lactate, acetate or HCl) and pH 1.5 (HCl) for 6 h; (ii) 20% NaCl for 14 days and (iii) 60°C for 150 s. L. monocytogenes grown overnight (TSBYE) at 30°C or Salmonella at 37°C (TSB) and at 5°C for 1 or 5 days were used as in vitro controls.
Results: Habituation on lettuce for 1 day sensitized L. monocytogenes to pH 3.5 compared to control (30°C); however after 5 days, resistance of cells to acetic acid and pH 1.5 increased (P < 0.05). Cells adapted on tomato surface survived at pH 1.5 for 5 h, while control cells only for 1 h. During osmotic stress, cells habituated on tomato surface for 5 days reduced to <0.7 log CFU/ml after 14 days, while those habituated for 1 day after 8 days; lettuce-adapted and control cells survived only for 4 days. Under thermal stress, tomato-adapted cells decreased below the limit of 0.7 log CFU/ml, while control cells and those adapted to lettuce remained at 2.5 log CFU/ml after 150 s. Salmonella adapted to lettuce or tomato surface, died-off after one day in 20% NaCl, whereas control cells (37°C) survived for 10 days.
Significance: Pathogen habituation on vegetables may enhance their stress tolerance and in vitro models are insufficient to assess such a risk.