Adaptive Response of Listeria monocytogenes to Heat, Salinity and Low pH, after Habituation on Cherry Tomatoes and Lettuce Leaves

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.

Antioxidant and antimicrobial activity and potential of heather (Erica spp.) extracts in the control of Listeria monocytogenes

“…Listeria monocytogenes is a pathogen responsible for listeriosis, a foodborne disease with high mortality rates among the immunecompromised individuals, pregnant women, neonates and the elderly. In healthy individuals, it generally can cause self-limiting febrile gastroenteritis (Ferreira & Domingues, 2016;Poimenidou et al, 2016). There is a broad range of food products that pose a risk of L. monocytogenes dissemination, which, together with its ability to survive and develop tolerance to stress, makes it possible to persist in diverse environments.…”

Short communication: Long-term −20°C survival of Listeria monocytogenes in artificially and process-contaminated ice cream involved in an outbreak of listeriosis

“…Low temperatures are commonly used in food production, and the adaptive mechanisms developed by L. monocytogenes that allow its proliferation at low temperatures have been reviewed (Tasara and Stephan, 2006). Poimenidou et al (2016) showed that the habituation of L. monocytogenes on different fresh produce surfaces resulted in differential responses to acid, osmotic, and temperature stressors, and that the habituated cells demonstrated increased resistance to stressors compared with cells grown in laboratory media. In the current study, we assessed the long-term survival of artificial and in-process L. monocytogenes contaminants in ice cream scoops at −20°C.…”

Colonisation of lettuce by Listeria Monocytogenes

“…Listeria monocytogenes can survive and persist on lettuce leaf surfaces in different conditions (Poimenidou et al ., ). Murphy et al .…”

“…The ability of many strains of L . monocytogenes to be able to withstand harsh food environments such as high salt concentration, low oxygen and high acidity has been reported (Poimenidou et al ., ). After L .…”

“…L . monocytogenes cells on lettuce leaf surfaces for 5 days at 5 °C were able to survive for 360 min after lactic acid treatment but cells that were on the leaves just 24 h became inactivated after 265 min (Poimenidou et al ., ). This suggests that, under acidic stress, L .…”