Growth Potential of Listeria monocytogenes on Refrigerated Spinach and Rocket Leaves in Modified Atmosphere Packaging

Prevalence and growth potential of Listeria monocytogenes in innovative, pre-packed, plant-based ready-to-eat food products on the Belgian market

In recent years, pre-packed ready-to-eat (RTE) food products on the Belgian market have shifted to a more plant-based composition due to a variety of reasons, including consumer concerns about health, animal welfare, and sustainability. However, similar to animal-based RTE foods, plant-based RTE foods can be susceptible to the presence and outgrowth of Listeria monocytogenes (L. monocytogenes). Three innovative, pre-packed, plant-based RTE food product categories on the Belgian market were identified based upon data gaps regarding the prevalence and growth potential of this pathogen. These were vegetarian and vegan deli sandwich slices (category 1), fresh-cut (mixes of) leafy vegetables (category 2), and multi-ingredient salad bowls (category 3). Reports on associated listeriosis outbreaks and recalls were collected and a comprehensive literature review on the prevalence of L. monocytogenes (i.e. detection in 25 g food) was performed. In addition, the prevalence of L. monocytogenes was also determined through an exploratory retail survey of ca. 50 different RTE products of each category. A batch was considered positive if L. monocytogenes was detected in a food item, either on the day of purchase, at the end of shelf life, or both. During the retail survey, L. monocytogenes was not detected in category 2 (0 out of 51 batches), while 1 out of 51 and 6 out of 48 batches were found positive for respectively category 1 and 3. The observed L. monocytogenes concentration did not exceed 10 CFU/g at any point in time in any batch. Furthermore, challenge tests were performed to determine the growth potential of L. monocytogenes in nine pre-packed, plant-based RTE food products (two to four different products of each category, and three different batches per product). After inoculation, products were stored for half of their shelf life at 7 °C and half of their shelf life at 9 °C (simulation of resp. retail and consumer storage). In six of the nine challenge tests executed, growth of L. monocytogenes was supported (i.e. growth potential ≥0.50 log10 CFU/g during shelf life). The highest growth potential was observed for fresh-cut iceberg lettuce (3.60 log10 CFU/g in 9 days), but a large variation regarding the growth potential of L. monocytogenes was noted both between and within the three studied pre-packed, plant-based RTE food product categories. This variation was mainly caused by differences in product composition, physicochemical product characteristics, present (competitive) microbiota such as lactic acid bacteria, applied preservation techniques, and shelf life.

Cultivation Conditions of Spinach and Rocket Influence Epiphytic Growth of Listeria monocytogenes

Leafy vegetables are associated with Listeriosis outbreaks due to contamination with Listeria monocytogenes. To date, contradictory findings were reported on spinach, rocket, and kale, where some studies reported growth of L. monocytogenes, while others did not. Thus, the current study investigated the reason for conflicting findings by producing leafy vegetables, where cultivation factors were known for growth potential studies. Of all polytunnel produce, kale Nero di Toscana demonstrated the highest growth potential (2.56 log cfu g−1), followed by spinach F1 Cello (1.84 log cfu g−1), rocket Buzz (1.41 log cfu g−1), spinach F1 Trumpet (1.37 log cfu g−1), and finally rocket Esmee (1.23 log cfu g−1). Thus, plant species and variety influenced L. monocytogenes growth potentials. Moreover, significantly lower growth potentials of 0.3 log cfu g−1 were identified when rocket Buzz was cultivated in open fields (1.11 log cfu g−1) instead of a polytunnel. The opposite effect was observed for spinach F1 Trumpet, where growth potentials increased significantly by 0.84 log cfu g−1 when cultivated in open fields (2.21 log cfu g−1). Furthermore, a significant seasonality effect between batches was found (p < 0.05). This study revealed that spinach and rocket cultivation conditions are at least co-factors in the reporting of differing growth potentials of L. monocytogenes across literature and should be considered when conducting future growth potential studies.

Microbiological safety of spinach throughout commercial supply chains in Gauteng Province, South Africa and characterization of isolated multidrug-resistant Escherichia coli

AIM

To investigate the microbiological quality, potential foodborne pathogen presence, and to phenotypically (antimicrobial resistance [AMR] profiles) and genotypically (DNA fingerprints and diarrhoeagenic genes) characterize Escherichia coli isolated throughout spinach production systems from farm-to-sale.

METHODS AND RESULTS

Samples (n = 288) were collected from two commercial supply chains using either river or borehole irrigation water. E. coli was enumerated throughout the chain where river water was directly used for overhead irrigation at levels between 0.00 and 3.22 log colony forming unit (CFU) g^-1 . Following enrichment, isolation and matrix-assisted laser desorption ionization time-of-flight mass spectrometry identification, E. coli was isolated from 22.57% (n = 65/288) of all samples. Salmonella spp. were isolated from 3% (n = 9/288) of river and irrigation water samples on one farm, and no Listeria monocytogenes was detected throughout the study. Of the 80 characterized E. coli isolates, one harboured the stx2 virulence gene, while 43.75% (n = 35) were multidrug resistant. Overall, 26.30% of the multidrug-resistant E. coli isolates were from production scenario one that used river irrigation water, and 17.50% from the second production scenario that used borehole irrigation water. A greater percentage of resistance phenotypes were from water E. coli isolates (52.50%), than isolates from spinach (37.50%). E. coli isolates from spinach and irrigation water clustered together at high similarity values (>90%) using enterobacterial repetitive intergenic consensus-polymerase chan reaction analysis.

CONCLUSIONS

This study reported the presence of multidrug-resistant environmental E. coli throughout spinach production from farm, during processing and up to retail. Furthermore, the similarity of multi-drug resistant E. coli isolates suggests transfer from irrigation water to spinach in both scenarios, reiterating that irrigation water for vegetables consumed raw, should comply with standardized microbiological safety guidelines.

SIGNIFICANCE AND IMPACT OF STUDY

Multidrug-resistant E. coli presence throughout spinach production emphasizes the necessity of increased surveillance of AMR in fresh produce and the production environment within a One Health paradigm to develop AMR mitigation strategies.