Quantitative analysis and influences of contact dynamics on bacterial cross-contamination from contaminated fresh produce

Transfer of Salmonella enterica and Enterococcus faecium from food-contact surfaces to stone fruits

Contaminated food-contact surfaces are a potential route for spreading microorganisms to stone fruit during postharvest handling. The objective of this study was to investigate the factors that affect the transfer of bacteria from food-contact surfaces to stone fruits. Coupons (1 × 1 cm) of polyurethane (PU) or polyvinyl chloride (PVC) were inoculated with rifampin-resistant variants of Salmonella (five-strain cocktail) or Enterococcus faecium NRRL B-2354 at ~5 or ~7 log CFU/cm2. Inoculated coupons (n = 8-11) were attached to a texture analyzer, and uniform contact conditions (5 N, 5 s) were used to explore the impact of bacterial species, inoculation level, donor surface, the presence of dried peach juice or wax, recipient produce commodity, and the dryness of inoculum. Whole fruits were transferred to 20 mL of 0.1 % peptone, rubbed for 2 min, and then the diluent was plated onto tryptic soy agar supplemented with rifampin at 50 μg/mL. Whole fruits were enriched when populations were anticipated to fall below the limit of detection (1.6 log CFU/fruit). At an inoculum of ~5 log CFU/coupon, Salmonella and E. faecium were recovered from the fruit by enrichment but not by plating. At ~7 log CFU/coupon, transfer rates, i.e., ratio of populations on recipient fruit to donor surface, were not significantly (P > 0.05) influenced by either bacterial species (Salmonella [0.26 % ± 0.77 %] versus E. faecium [0.068 % ± 0.071 %]) or donor surface (PU [0.085 % ± 0.098 %] versus PVC [0.16 % ± 0.16 %]). The rates of transfer of E. faecium from contaminated PU to peaches (0.050 % ± 0.031 %), nectarines (0.066 % ± 0.076 %), and onion skins (0.048 % ± 0.059 %) were not significantly different. The mean transfer rates of E. faecium increased significantly (P < 0.05) in the presence of dried wax (18 % ± 16 %) or peach juice (1.3 % ± 2.6 %) on the PU surface compared with the control (0.080 % ± 0.086 %). The transfer rates of E. faecium from contaminated surfaces were also significantly influenced by the drying time post-inoculation; the drier the inoculum, the lower the transfer rates. The presence of residues or moisture on food-contact surfaces facilitated the transfer of microorganisms during dry handling of fresh stone fruits. The results underscore the importance of implementing adequate cleaning, sanitation and, where appropriate, drying of equipment surfaces to effectively remove organic residues and mitigate the risks of cross-contamination.

Transfer, survival and photoinactivation of Salmonella enterica on fresh produce and gloves

This study quantified the transfer of Salmonella between fruit (tomato and cucumber) and gloves (High-density polyethylene-HDPE, Vinyl-VY) after different contact time (2, 5, 10 and 30 s). The survival of Salmonella in discarded gloves and the photodynamic inactivation (PDI; curcumin 7.5 μM; 16.1, 20.2 and 24.2 J/cm2) of the pathogen were also evaluated. Scanning electronic micrographs showed the cell morphology of Salmonella on discarded HDPE and VY gloves. The highest transfer (p > 0.05) of Salmonella occurred after 30 s (4.0 %; -1.4 log %) from tomato to HDPE. Salmonella on contaminated gloves survived for up to 9 days (2.1-1.5 log CFU/sample). PDI reduced (p > 0.05) Salmonella counts ∼3.0 and 2.0 log CFU/sample after 12 min on tomato and cucumber, respectively. These results contribute important information about transfer and survival of Salmonella on gloves and fruit. The obtained data can be used to support the development of risk assessment models and measures to mitigate the risks of Salmonella contamination in fresh- produce.

Survival of Salmonella enterica and Enterococcus faecium on Abiotic Surfaces During Storage at Low Relative Humidity

Currently, there is limited knowledge on the survival of bacteria on surfaces during postharvest handling of dry products such as onions. Extended survival of microorganisms, coupled with a lack of established and regular, validated cleaning or sanitation methods could enable cross-contamination of these products. The aim of the study was to evaluate the survival of a potential surrogate, Enterococcus faecium, and Salmonella enterica on typical onion handling surfaces, polyurethane (PU), and stainless steel (SS), under low relative humidity. The influence of onion extract on the survival of E. faecium and Salmonella on PU and SS was also investigated. Rifampin-resistant E. faecium NRRL B-2354 and a five-strain cocktail of Salmonella suspended in 0.1% peptone or onion extract were separately inoculated onto PU and SS coupons (2 × 2 cm), at high, moderate, or low (7, 5, or 3 log CFU/cm2) levels. The inoculated surfaces were stored at ∼34% relative humidity and 21°C for up to 84 days. Triplicate samples were enumerated at regular intervals in replicate trials. Samples were enriched when populations fell below the limit of detection by plating (0.48 log CFU/cm2). Scanning electron microscopy was used to observe the cell distribution on the coupons. Reductions of E. faecium of less than ∼2 log were observed on PU and SS over 12 weeks at all inoculum levels and with both inoculum carriers. In 0.1% peptone, Salmonella populations declined by 2 to 3 log over 12 weeks at the high and moderate inoculum levels; at the low inoculum level, Salmonella could not be recovered by enrichment at 84 days. Survival of E. faecium and Salmonella was significantly (P < 0.05) enhanced over 84 days of storage when suspended in onion extract, where cells were covered by a layer of onion extract. E. faecium might have utility as a conservative surrogate for Salmonella when evaluating microbial survival on dry food-contact surfaces.

Surface inoculation method impacts microbial reduction and transfer of Salmonella Enteritidis PT 30 and potential surrogates during dry sanitation

Dry sanitation methods are often limited to physical removal strategies such as brushing or wiping with sanitary cleaning tools. However, the relative efficacy of these approaches to remove microbiota on surfaces, and the risk of transferring cells to other surfaces via the cleaning tool, is unclear. The effect of dry wiping with a single-use towel on the removal of four different bacteria (Salmonella Enteritidis, Enterococcus faecium, Listeria innocua, Escherichia coli) was investigated. We also quantified the number of cells transferred to the towel itself during dry cleaning. Three different surface inoculation methods (spot, glass bead, contaminated milk powder) were assessed and significantly impacted initial surface microbial load. Higher initial counts corresponded to lower transfer coefficients (e.g., proportion of transferred cells). The effect of bacterial identity was significant on reduction after dry wiping for all three inoculation methods. Moreover, both bacterial identity and inoculation method had significant effects on the number of cells transferred to the towel. In most scenarios, dry wiping resulted in a reduction <1.0 log CFU/coupon. Although, on surfaces inoculated via contaminated milk powder, reductions of up to 1.6 ± 0.3 log CFU/coupon were obtained. Overall, E. faecium transferred more readily to the towel. These results may help guide experimental design for future research on dry sanitation.

Transfer of Enterococcus faecium and Salmonella enterica during simulated postharvest handling of yellow onions (Allium cepa)

Bacterial transfer during postharvest handling of fresh produce provides a mechanism for spreading pathogens, but risk factors in dry environments are poorly understood. The aim of the study was to investigate factors influencing bacterial transfer between yellow onions (Allium cepa) and polyurethane (PU) or stainless steel (SS) under dry conditions. Rifampin-resistant Enterococcus faecium NRRL B-2354 or a five-strain cocktail of Salmonella was inoculated onto onion skin or PU surfaces at high or moderate levels using peptone, onion extract, or soil water as inoculum carriers. Transfer from inoculated to uninoculated surfaces was conducted using a texture analyzer to control force, time, and number of contacts. Transfer rates (ratio of recipient surface to donor surface populations) of E. faecium (4-5%) were significantly higher than those of Salmonella (0.5-0.6%) at the high (7 log CFU/cm2) but not moderate (5 log CFU/cm2) inoculum levels. Significantly higher populations of E. faecium transferred from onion to PU than from PU to onion. The transfer rates of E. faecium were impacted by inoculum carrier (61% [onion extract], 1.6% [peptone], and 0.31% [soil]) but not by inoculation level or recipient surface (PU versus SS). Bacterial transfer during dry onion handling is significantly dependent on bacterial species, inoculation levels, inoculum carrier, and transfer direction.

Fecal Contamination in the Wastewater Irrigation System and its Health Threat to Wastewater-Based Farming Households in Addis Ababa, Ethiopia

Due to rapidly growing demand, the production of vegetables is increasing along the Akaki Rivers. The objective of this study was to examine the degree of fecal contamination and levels of fecal contamination and dissemination throughout the wastewater irrigation system. Irrigation water, irrigated soil, and leafy vegetables were collected twice during 2 vegetable growing seasons, at the maturity period of the growing season, from 19 sampling points along the 2 Akaki Rivers. Composite samples were taken from all sampling points and E.coli was enumerated. The mean E.coli load in wastewater and non-wastewater sources were 1.1⁶±5.5³ CFU/100 ml and 2.23²±1.29² CFU/100 ml respectively. All counts of E. coli in the wastewater exceeded the WHO's standards indicating that the irrigation water quality was unacceptable. In the wastewater-irrigated and non-wastewater-irrigated soil, the mean E.coli were 3.6² ±1.58² CFU/g and 1.32²±87.1 CFU/g respectively. Meanwhile, the mean E.coli counts on the lettuce and Swiss chard were 78 ± 2 CFU/g and 44 ±3CFU/g respectively. The E.coli count on the leafy vegetables was found to be associated with the E.coli in the wastewater and soil. The production of leafy vegetables using wastewater with unacceptably high levels of E.coli and high occupational exposure introduces high levels of risk to the farming communities and to the consumers. Leafy, low-growing raw edible vegetables need careful treatment during food production and harvesting procedures or activities.

Monitoring the effect of environmental conditions on safety of fresh produce sold in Qatar's wholesale market

Fresh produce imported by Qatar are mostly sold at the wholesale produce market (WPM) located in open-air and near major animal markets and slaughterhouses. This study was the first in Qatar to monitor the effect of environmental conditions on the microbial quality and safety of fresh produce sold at the WPM over 1 year. The monitoring involved the collection of 540 produce samples along with samples of air, soil, and surface swabs. Samples were analyzed for total aerobic bacteria (TAB); generic Listeria spp., Staphylococcus spp., Salmonella spp.; total coliforms and total fungi. Bacterial and fungal isolates were identified using 16S rRNA/ITS rRNA markers. Environmental/sanitary factors significantly impacted the prevalence of microorganisms in all samples tested. Produce quality was rated 'poor' during the months of November-February or May-August, with TAB and coliform counts exceeding 6 and 4 log₁₀ CFU/g, respectively. Bacillus subtilus, Enterobacter cloacae, E. faecium, P. expansium, P. aurantiocandidum, and A. niger were the most abundant species with prevalence rate of 11-30%. The high microbial load of environmental samples indicates that the location of the WPM near livestock markets is likely impacting the microbial quality of fresh produce. Therefore, effective control measures need to be implemented at WPM to improve produce safety yearlong.

Biobased Chicken Eggshell Powder for Efficient Delivery of Low-Dose Silver Nanoparticles (AgNPs) to Enhance Their Antimicrobial Activities against Foodborne Pathogens and Biofilms

Despite the current sanitation practices to decontaminate food-contact surfaces, persistent biofilms still pose significant threats to human health by inducing cross-contamination. This study aims to enhance the antimicrobial activity of low-dose silver nanoparticles (AgNPs) against foodborne pathogens and their biofilms through the development of a biobased delivery carrier for metallic nanoparticles. In this study, chicken eggshell powder (EP) was used as a biocompatible delivery carrier, and it possesses a strong ability to encapsulate green-synthesized AgNPs with an encapsulation efficiency of 80.18%. The EP carriers stabilized AgNPs in an organic-rich environment and prevented the aggregation of nanoparticles. The results of antimicrobial test demonstrate that EP significantly enhanced the antimicrobial efficacy of low-dose AgNPs (2 μg/mL), enabling 5-log reductions of planktonic Escherichia coli and Listeria innocua within 25 min and 60 min treatments, respectively, even in the presence of high organic content (chemical oxygen demand, COD = 1000 mg/L). Due to the high affinity of EP to bind biofilms, the encapsulated low-dose AgNPs can inactivate approximately 2-log CFU/cm2 of biofilms within a 2-h treatment. The proposed AgNPs@EP composite with a low silver concentration (2 μg/mL) can effectively inactivate and remove biofilms from food-contact surfaces in which such a low concentration of AgNPs is unlikely to induce negative impacts on human health and environment. Therefore, this antimicrobial AgNPs@EP composite can potentially be used as a biobased sanitizer for food-contact surfaces in a food manufacturing plant.

Determining the Potential Food Safety Risks Associated with Dropped Produce on Floor Surfaces in Grocery Stores

ABSTRACT

Grocery stores handle fresh produce in large quantities daily. According to the Food and Drug Administration Food Code, food is to be stored at least 15 cm above the floor, and all foods shall be protected from any source of contamination or otherwise discarded. It is reported in the literature that dropped produce could be a potential source of microbial contamination. Both consumers and employees often drop produce on the floor and then place it back into a display case or bin, which could potentially serve as a source of contamination. This study aims to determine the bacterial transfer rate on different produce types when dropped for various contact times onto floor surfaces contaminated with Listeria monocytogenes. Apples, peaches, and romaine lettuce were dropped separately onto carpet and tile surfaces from a distance of 1 m and held for 5 s, 1 min, 10 min, 1 h, and 4 h. Results showed that transfer from all produce types occurred from both the carpet (10.56%) and tile (3.65%) surfaces. Still, percent transfer was not statistically significant among different times used in this study (P > 0.05). Dropped romaine lettuce had the most transfer (28.97%) from both the surfaces combined, followed by apples (8.80%) and peaches (7.32%) with minimal transfer. Even with a low transfer level, grocery stores should include signage to alert consumers not to pick up dropped produce and should train their employees accordingly.

HIGHLIGHTS

Hydrophobic and adhesive patterns of lactic acid bacteria and their antagonism against foodborne pathogens on tomato surface (Solanum lycopersicum L.)

AIMS

To evaluate tomato epiphyte lactic acid bacteria (LAB) hydrophobicity and auto-aggregation as an indicator of bacteria adhesion to tomato. Likewise, use LAB adhesion and co-aggregation as mechanisms to antagonize pathogen attachment.

METHODS AND RESULTS

Fifty-four LAB were screened to evaluate their hydrophobic, auto- and co-aggregative properties against Salmonella Typhimurium, Saintpaul, Montevideo and Escherichia coli O157:H7. Subsequently, tomato adhesion of Enterococcus faecium Col1-1C, Weisella cibaria 11-E-2 and W. confusa Col 1-13 with high, medium and low hydrophobicity and high co-aggregation was investigated as well as their pathogen antagonism. Results indicate that bacteria hydrophobicity and auto-aggregation correspond to LAB adhesion to tomato. Enterococcus faecium Col1-1C interfered in most of the pathogen adhesion and micrographs revealed that such effect could be related to the inhibition of structures-type biofilm on E. coli O157:H7 and the aggregate formation on Salmonella.

CONCLUSIONS

Lactic acid bacteria hydrophobicity and auto-aggregation can estimate bacteria adhesion to tomato and adhesive and co-aggregative properties could serve as a tool to antagonize foodborne pathogens under specific conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study evidence the interference of Ent. faecium Col1-1C in E. coli O157:H7 biofilm formation and Salmonella colonization.