Abstract
Pathogens found on fresh produce may encounter low temperatures, high acidity and limited nutrient availability. The aim of this study was to evaluate the effect of habituation of Listeria monocytogenes on cherry tomatoes or lettuce leaves on its subsequent response to inhibitory levels of acid, osmotic and heat stress. Habituation was performed by inoculating lettuce coupons, whole cherry tomatoes or tryptic soy broth (TSB) with a three-strains composite of L. monocytogenes, which were further incubated at 5°C for 24 hours or 5 days. Additionally, cells grown overnight in TSB supplemented with 0.6% yeast extract (TSBYE) at 30°C were used as control cells. Following habituation, L. monocytogenes cells were harvested and exposed to: (i) pH 3.5 adjusted with lactic acid, acetic acid or hydrochloric acid (HCl), and pH 1.5 (HCl) for 6 h; (ii) 20% NaCl and (iii) 60°C for 150 s. Results showed that tomato-habituated L. monocytogenes cells were more tolerant (P < 0.05) to acid or osmotic stress than those habituated on lettuce, and habituation on both foods resulted in more stress resistant cells than prior growth in TSB. On the contrary, the highest resistance to heat stress (P < 0.05) was exhibited by the lettuce-habituated L. monocytogenes cells followed by TSB-grown cells at 5°C for 24 h, whereas tomato-habituated cells were highly sensitized. Prolonged starvation on fresh produce (5 days vs. 24 h) increased resistance to osmotic and acid stress, but reduced thermotolerance, regardless of the pre-exposure environment (i.e., tomatoes, lettuce or TSB). These results indicate that L. monocytogenes cells habituated on fresh produce at low temperatures might acquire resistance to subsequent antimicrobial treatments raising important food safety implications.
Highlights
Fresh fruits and vegetables are an integral part of a healthy diet, rich in nutrients, fibers and vitamins
In tryptic soy broth (TSB) stored at 5°C, L. monocytogenes population increased between 24 h and 5 days of incubation by 1.3 log CFU/mL; the pH values were 7.4 and 7.3, respectively
The results presented here demonstrate that L. monocytogenes cells previously habituated on fresh lettuce leaves or cherry tomatoes under cold temperatures exhibit a significantly altered response during subsequent exposure to osmotic, acid and heat stresses compared to control cultures grown in synthetic laboratory media
Summary
Fresh fruits and vegetables are an integral part of a healthy diet, rich in nutrients, fibers and vitamins. Listeria monocytogenes is a foodborne pathogen, able to survive under a wide range of environmental conditions (pH, temperature, and aw) [4]. It is the causative agent of listeriosis, a severe foodborne disease with high mortality rates among the immune-compromised individuals, pregnant women, neonates, and the elderly. Due to its widespread occurrence in the environment, ability to attach to surfaces and tolerance to stress factors, the incidence rate of L. monocytogenes in fresh produce and the food-processing environment is remarkably high [5,6,7,8]. In 2014, data provided by 15 EU member states for 3.272 units of ready-to-eat (RTE) fruit and vegetables showed that 2.8% were positive for L. monocytogenes detection [9]
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