Survival of pathogenic Escherichia coli on basil, lettuce, and spinach

A review of conditions influencing fate of Shiga toxin-producing Escherichia coli O157:H7 in leafy greens

The accuracy of predictive microbial models used in quantitative microbial risk assessment (QMRA) relies on the relevancy of conditions influencing growth or inactivation. The continued use of log-linear models in studies remains widespread, despite evidence that they fail to accurately account for biphasic kinetics or include parameters to account for the effect of environmental conditions within the model equation. Although many experimental studies detail conditions of interest, studies that do not do so lead to uncertainty in QMRA modeling because the applicability of the predictive microbial models to the conditions in the risk scenarios is questionable or must be extrapolated. The current study systematically reviewed 65 articles that provided quantitative data and documented the conditions influencing the inactivation or growth of Shiga toxin-producing Escherichia coli (STEC) O157:H7 in leafy greens. The conditions were identified and categorized as environmental, biological, chemical, and/or processing. Our study found that temperature (n = 37 studies) and sanitizing and washing procedures (n = 12 studies) were the most studied conditions in the farm-to-table continuum of leafy greens. In addition, relative humidity was also established to affect growth and inactivation in more than one stage in the continuum. This study proposes the evaluation of the interactive effects of multiple conditions in processing and storage stages from controlled experiments as they relate to the fate of STEC O157:H7 in leafy greens for future quantitative analysis.

A meta-analysis of factors influencing the inactivation of Shiga toxin-producing Escherichia coli O157:H7 in leafy greens

Recent advancements in modeling suggest that microbial inactivation in leafy greens follows a nonlinear pattern, rather than the simple first-order kinetics. In this study, we evaluated 17 inactivation models commonly used to describe microbial decline and established the conditions that govern microbial survival on leafy greens. Through a systematic review of 65 articles, we extracted 530 datasets to model the fate of Shiga toxin-producing Escherichia coli O157:H7 on leafy greens. Various factor analysis methods were employed to evaluate the impact of identified conditions on survival metrics. A two-parameter model (jm2) provided the best fit to most of both natural and antimicrobial-induced persistence datasets, whereas the one-parameter exponential model provided the best fit to less than 20% of the datasets. The jm2 model (adjusted R2 = .89) also outperformed the exponential model (adjusted R2 = .58) in fitting the pooled microbial survival data. In the context of survival metrics, the model averaging approach generated higher values than the exponential model for >4 log reduction times (LRTs), suggesting that the exponential model may be overpredicting inactivation at later time points. The random forest technique revealed that temperature and inoculum size were common factors determining inactivation in both natural and antimicrobial-induced die-offs.. The findings show the limitations of relying on the first-order survival metric of 1 LRT and considering nonlinear inactivation in produce safety decision-making.

Modeling Growth Kinetics of Escherichia coli and Background Microflora in Hydroponically Grown Lettuce

Hydroponic cultivation of lettuce is an increasingly popular sustainable agricultural technique. However, Escherichia coli, a prevalent bacterium, poses significant concerns for the quality and safety of hydroponically grown lettuce. This study aimed to develop a growth model for E. coli and background microflora in hydroponically grown lettuce. The experiment involved inoculating hydroponically grown lettuce with E. coli and incubated at 4, 10, 15, 25, 30, 36 °C. Growth models for E. coli and background microflora were then developed using Origin 2022 (9.9) and IPMP 2013 software and validated at 5 °C and 20 °C by calculating root mean square errors (RMSEs). The result showed that E. coli was unable to grow at 4 °C and the SGompertz model was determined as the most appropriate primary model. From this primary model, the Ratkowsky square root model and polynomial model were derived as secondary models for E. coli-R168 and background microflora, respectively. These secondary models determined that the minimum temperature (Tmin) required for the growth of E. coli and background microflora in hydroponically grown lettuce was 6.1 °C and 8.7 °C, respectively. Moreover, the RMSE values ranged from 0.11 to 0.24 CFU/g, indicating that the models and their associated kinetic parameters accurately represented the proliferation of E. coli and background microflora in hydroponically grown lettuce.

Transmission of antimicrobial resistant non-O157 Escherichia coli at the interface of animal-fresh produce in sustainable farming environments

The interaction of typical host adapted enteric bacterial pathogens with fresh produce grown in fields is complex. These interactions can be more pronounced in co-managed or sustainable farms where animal operations are, by design, close to fresh produce, and growers frequently move between the two production environments. The primary objectives of this study were to 1) determine the transmission of STEC or enteric pathogens from small and large animal herds or operations to fresh produce on sustainable farms in TN and NC, 2) identify the possible sources that impact transmission of AMR E. coli, specifically STEC on these systems, and 3) WGS to characterize recovered E. coli from these sources. Samples were collected from raw and composted manure, environment, and produce sources. The serotype, virulence, and genotypic resistance profile were determined using the assembled genome sequences sequenced by Illumina technology. Broth microdilution was used to determine the antimicrobial susceptibility of each isolate against a panel of fourteen antimicrobials. The prevalence of E. coli increased during the summer season for all sources tested. ParSNP trees generated demonstrated that the transmission of AMR E. coli is occurring between animal feeding operations and fresh produce. Ten isolates were identified as serotype O45, a serotype that is associated with the "Big Six" group that is frequently linked with foodborne outbreaks caused by non-O157 E. coli. However, these isolates did not possess the stx gene. The highest frequency of resistance was detected against streptomycin (n = 225), ampicillin (n = 190) and sulfisoxazole FIS (n = 140). A total of 35 (13.7%) isolates from two TN farms were positive for the blaCMY (n = 5) and blaTEM (n = 32) genes. The results of this study show the potential of AMR E. coli transmission between animal feeding operations and fresh produce, and more studies are recommended to study this interaction and prevent dissemination in sustainable farming systems.

Pre-Harvest Survival and Post-Harvest Chlorine Tolerance of Enterohemorrhagic Escherichia coli on Lettuce

In the field, foodborne pathogens such as enterohemorrhagic Escherichia coli (EHEC) are capable of surviving on produce over time, yet little is known about how these pathogens adapt to this environment. To assess the impact of pre-harvest environmental conditions on EHEC survival, we quantified survival on romaine lettuce under two relative humidity (75% and 45%) and seasonal conditions (March and June). Greenhouse-grown lettuce was spray-inoculated with EHEC and placed in a growth chamber, mimicking conditions typical for June and March in Salinas Valley, California. Bacteria were enumerated on days 0, 1, 3, and 5 post-inoculation. Overall, we found that the effect of relative humidity on EHEC survival depended on the seasonal conditions. Under June seasonal conditions, higher relative humidity led to lower survival, and lower relative humidity led to greater survival, five days post-inoculation. Under March seasonal conditions, the impact of relative humidity on EHEC survival was minimal over the five days. The bacteria were also tested for their ability to survive a chlorine decontamination wash. Inoculated lettuce was incubated under the June 75% relative humidity conditions and then washed with a 50 ppm sodium hypochlorite solution (40 ppm free chlorine). When incubated under June seasonal conditions for three to five days, EHEC strains showed increased tolerance to chlorine (adj. p < 0.05) compared to chlorine tolerance upon inoculation onto lettuce. This indicated that longer incubation on lettuce led to greater EHEC survival upon exposure to chlorine. Subsequent transcriptome analysis identified the upregulation of osmotic and oxidative stress response genes by EHEC after three and five days of incubation on pre-harvest lettuce. Assessing the physiological changes in EHEC that occur during association with pre-harvest lettuce is important for understanding how changing tolerance to post-harvest control measures may occur.

Roles of YcfR in Biofilm Formation in Salmonella Typhimurium ATCC 14028

An increasing number of foodborne diseases are currently attributable to farm produce contaminated with enteric pathogens such as Salmonella enterica. Recent studies have shown that a variety of enteric pathogens are able to colonize plant surfaces by forming biofilms and thereby persist for long periods, which can subsequently lead to human infections. Therefore, biofilm formation by enteric pathogens on plants poses a risk to human health. Here, we deciphered the roles of YcfR in biofilm formation by Salmonella enterica. YcfR is a putative outer membrane protein associated with bacterial stress responses. The lack of YcfR facilitated the formation of multicellular aggregates on cabbage leaves as well as glass surfaces while reducing bacterial motility. ycfR deletion caused extensive structural alterations in the outer membrane, primarily in lipopolysaccharides, outer membrane proteins, cellulose, and curli fimbria, thereby increasing cell surface hydrophobicity. However, the absence of YcfR rendered Salmonella susceptible to stressful treatments, despite the increased multicellular aggregation. These results suggest that YcfR is an essential constituent of Salmonella outer membrane architecture and its absence may cause multifaceted structural changes, thereby compromising bacterial envelope integrity. In this context, YcfR may be further exploited as a potential target for controlling Salmonella persistence on plants.

Sweet Basil (Ocimum basilicum L.) Productivity and Raw Material Quality from Organic Cultivation

Sweet basil is one of the most important culinary herbs. Currently, its production is carried out mainly in accordance with conventional agriculture. However, its cultivation in organic systems seems to be better adjusted to consumer demands connected with the lack of pesticide residues in foods and their safety. In the present study, two methods of basil cultivation in organic farming system were applied, i.e., in the open air and under foil tunnels. During the experiment, in central European climatic conditions, it was possible to obtain four successive cuts of herb. The herb was subjected to chemical analysis, including determination of the content of essential oil, phenolic compounds, and chlorophylls. Gas chromatography coupled with mass spectrometry (GC–MS) and flame ionization detector (GC-FID) analysis of the essential oil was performed, whereas the fresh herb was subjected to sensory analysis. The cumulative mass of fresh herb was distinctly higher in the cultivation under foil tunnels (44.7 kg∙10 m−2) in comparison to the open field (24.7 kg∙10 m−2). The content of essential oil, flavonoids, and phenolic acids was also higher in the raw material collected from plants grown under foil tunnels (0.81, 0.36, and 0.78 g·100g−1 DW, respectively) than from the open field (0.48, 0.29, and 0.59g·100g−1 DW, respectively). In turn, the dominant compound of the essential oil, i.e., linalool, was present in higher amounts in the essential oil obtained from plants cultivated in the open field. The sensory and microbiological quality of herb was comparable for both methods of cultivation. The obtained results indicate that, in central European climatic conditions, it is possible to obtain good-quality yield of basil herb. However, for its better productivity, it seems that cultivation under foil tunnels is preferable.

Salmonella enterica in Soils Amended with Heat-Treated Poultry Pellets Survived Longer than Bacteria in Unamended Soils and More Readily Transferred to and Persisted on Spinach

Untreated biological soil amendments of animal origin (BSAAO) are commonly used as biological fertilizers but can harbor foodborne pathogens like Salmonella enterica, leading to potential transfer from soils to fruits and vegetables intended for human consumption. Heat-treated poultry pellets (HTPP) can provide produce growers with a slow-release fertilizer with a minimized risk of pathogen contamination. Little is known about the impact of HTPP-amended soil on the survival of Salmonella enterica The contributions of RpoS and formation of viable but nonculturable cells to Salmonella survival in soils are also inadequately understood. We quantified the survival of Salmonella enterica subsp. enterica serovar Newport wild-type (WT) and rpoS-deficient (ΔrpoS mutant) strains in HTPP-amended and unamended soil with or without spinach plants over 91 days using culture and quantitative PCR methods with propidium monoazide (PMA-qPCR). Simulated "splash" transfer of S. Newport from soil to spinach was evaluated at 35 and 63 days postinoculation (dpi). The S. Newport WT and ΔrpoS mutant reached the limit of detection, 1.0 log CFU/g (dry weight), in unamended soil after 35 days, whereas 2 to 4 log CFU/g (dry weight) was observed for both WT and ΔrpoS mutant strains at 91 dpi in HTPP-amended soil. S. Newport levels in soils determined by PMA-qPCR and plate count methods were similar (P > 0.05). HTPP-amended soils supported higher levels of S. Newport transfer to and survival on spinach leaves for longer periods of time than did unamended soils (P < 0.05). Salmonella Newport introduced to HTPP-amended soils survived for longer periods and was more likely to transfer to and persist on spinach plants than was S. Newport introduced to unamended soils.IMPORTANCE Heat-treated poultry pellets (HTPP) often are used by fruit and vegetable growers as a slow-release fertilizer. However, contamination of soil on farms may occur through contaminated irrigation water or scat from wild animals. Here, we show that the presence of HTPP in soil led to increased S. Newport survival in soil and to greater likelihood of its transfer to and survival on spinach plants. There were no significant differences in survival durations of WT and ΔrpoS mutant isolates of S. Newport. The statistically similar populations recovered by plate count and estimated by PMA-qPCR for both strains in the amended and unamended soils in this study indicate that all viable populations of S. Newport in soils were culturable.

Prevalence of Shiga-toxigenic and atypical enteropathogenic Escherichia coli in untreated surface water and reclaimed water in the Mid-Atlantic U.S

The microbial quality of irrigation water has increasingly become a concern as a source of contamination for fruits and vegetables. Non-traditional sources of water are being used by more and more growers in smaller, highly diversified farms in the Mid-Atlantic region of the U.S. Shiga-toxigenic E. coli (STEC) have been responsible for several outbreaks of infections associated with the consumption of leafy greens. Our study evaluated the prevalence of the "big seven" STEC serogroups and the associated enterohemorrhagic E. coli (EHEC) virulence factors (VF) genes in conventional and nontraditional irrigation waters in the Mid-Atlantic region of the U.S. Water samples (n = 510) from 170 sampling events were collected from eight untreated surface water sites, two wastewater reclamation facilities, and one vegetable processing plant, over a 12-month period. Ten liters of water were filtered through Modified Moore swabs (MMS); swabs were then enriched into Universal Pre-enrichment Broth (UPB), followed by enrichment into non-O157 STEC R&F broth and isolation on R & F non-O157 STEC chromogenic plating medium. Isolates (n = 2489) from enriched MMS from water samples were screened for frequently reported STEC serogroups that cause foodborne illness: O26, O45, O103, O111, O121, O145, and O157, along with VF genes stx1, stx2, eae, and ehxA. Through this screening process, STEC isolates were found in 2.35% (12/510) of water samples, while 9.0% (46/510) contained an atypical enteropathogenic E. coli (aEPEC) isolate. The eae gene (n = 88 isolates) was the most frequently detected EHEC VF of the isolates screened. The majority of STEC isolates (stx1 or stx2) genes mainly came from either a pond or reclamation pond water site on two specific dates, potentially indicating that these isolates were not spatially or temporally distributed among the sampling sites. STEC isolates at reclaimed water sites may have been introduced after wastewater treatment. None of the isolates containing eae were determined to be Escherichia albertii. Our work showed that STEC prevalence in Mid-Atlantic untreated surface waters over a 12-month period was lower than the prevalence of atypical EPEC.

Microbial pollution and food safety

Microbial pollution is a serious food safety issue because it can lead to a wide range of foodborne diseases. A great number of foodborne diseases and outbreaks are reported in which contamination of fresh produce and animal products occurs from polluted sources with pathogenic bacteria, viruses and protozoa and such outbreaks are reviewed and the sources are revealed. Investigations of foodborne outbreaks involved meat production and fresh produce, namely, that occurred at the early stages of the food chain have shown certain sources of contamination. Domesticated food animals, as well as wild animals, flies and rodents can serve as a source of contamination of nearby produce-growing fields and can lead to human infection through direct contact at farms and, mostly, mail order hatcheries. The most of the fresh produce associated outbreaks have followed wildlife intrusion into growing fields or fecal contamination from nearly animal production facilities that likely led to produce contamination, polluted water used for irrigation and improper manure. Preventive measures, as part of implemented good agricultural practice systems are described. Controlling and minimizing pre-harvest contamination may be one of the key aspects of food safety.

Survival and interaction of Escherichia coli O104:H4 on Arabidopsis thaliana and lettuce (Lactuca sativa) in comparison to E. coli O157:H7: Influence of plant defense response and bacterial capsular polysaccharide

Shiga toxin-producing Escherichia coli (STEC) has been associated with illnesses and outbreaks linked to fresh vegetables, prompting a growing public health concern. Most studies regarding interactions of STEC on fresh produce focused on E. coli O157:H7. Limited information is available about survival or fitness of E. coli O104:H4, non-O157 pathogen that was linked to one of the largest outbreaks of hemolytic uremic syndrome in 2011. In this study, survival of E. coli O104:H4 was evaluated on Arabidopsis thaliana plant and lettuce for 5 days compared with E. coli O157:H7, and expression of pathogenesis-realted gene (PR1; induction of plant defense response) was examined by reverse transcription quantitative PCR, and potential influence of capsular polysaccharide (CPS) on the bacterial fitness on plant was investigated. Populations of E. coli O104:H4 strains (RG1, C3493, and LpfA) on Arabidopsis and lettuce were significantly (P < 0.05) greater than those of E. coli O157:H7 strains (7386 and sakai) at day 5 post-inoculation, indicating E. coli O104:H4 may have better survival ability on the plants. In addition, the E. coli O104:H4 strains produced significantly (P < 0.05) higher amounts of CPS compared with the E. coli O157:H7 strains. RG1 strain (1.5-fold) initiated significantly (P < 0.05) lower expression of PR1 gene indicating induction of plant defense response compared with E. coli O157:H7 strains 7386 (2.9-fold) and sakai (2.7-fold). Collectively, the results in this study suggests that different level of CPS production and plant defense response initiated by each STEC strain might influence the bacterial survival or persistence on plants. The present study provides better understanding of survival behavior of STEC, particularly E. coli O104:H4, using a model plant and vegetable under pre-harvest conditions with plant defense response.

Microbiological quality of spinach irrigated with reclaimed wastewater and roof-harvest water

AIMS

The effect of reclaimed wastewater (RCW) and roof-harvest rainwater (RHW) on the microbiological quality of irrigated spinach was investigated.

METHODS AND RESULTS

Spinach grown in the controlled environment chamber was irrigated by RCW, RHW or creek water (CW; control water) for 4 weeks, and then six replicate spinach samples from each treatment were collected weekly at 0 h and 24 h postirrigation. Spinach samples were analysed for populations of faecal bacterial indicators and pathogens. Bacterial populations in alternative irrigation water samples were determined by the membrane filtration technique. The RCW samples contained the highest faecal bacterial indicator populations, followed by the CW and RHW throughout the entire study. Irrigation waters containing higher populations of total and faecal coliforms did not necessarily result in higher populations of these bacteria on the irrigated spinach. Higher numbers of E. coli-positive spinach samples were reported from RCW-irrigated spinach, especially with repeated irrigation. Pathogens were not detected from any water or spinach samples.

CONCLUSIONS

Spinach irrigated with RHW did not significantly affect the populations of faecal indicator bacteria when compared with CW-irrigated spinach. Repeat irrigation with RCW is not recommended due to the increased contamination of E. coli on spinach leaves.

SIGNIFICANCE AND IMPACT OF THE STUDY

RHW may potentially be used as alternative irrigation water without deleteriously affecting the microbiological safety of the spinach.

Survival of Escherichia coli on Lettuce under Field Conditions Encountered in the Northeastern United States

Although wildlife intrusion and untreated manure have been associated with microbial contamination of produce, relatively few studies have examined the survival of Escherichia coli on produce under field conditions following contamination (e.g., via splash from wildlife feces). This experimental study was performed to estimate the die-off rate of E. coli on preharvest lettuce following contamination with a fecal slurry. During August 2015, field-grown lettuce was inoculated via pipette with a fecal slurry that was spiked with a three-strain cocktail of rifampin-resistant nonpathogenic E. coli. Ten lettuce heads were harvested at each of 13 time points following inoculation (0, 2.5, 5, and 24 h after inoculation and every 24 h thereafter until day 10). The most probable number (MPN) of E. coli on each lettuce head was determined, and die-off rates were estimated. The relationship between sample time and the log MPN of E. coli per head was modeled using a segmented linear model. This model had a breakpoint at 106 h (95% confidence interval = 69, 142 h) after inoculation, with a daily decrease of 0.70 and 0.19 log MPN for 0 to 106 h and 106 to 240 h following inoculation, respectively. These findings are consistent with die-off rates obtained in similar studies that assessed E. coli survival on produce following irrigation. Overall, these findings provide die-off rates for E. coli on lettuce that can be used in future quantitative risk assessments.

Fitness of Enterohemorrhagic Escherichia coli (EHEC)/Enteroaggregative E. coli O104:H4 in Comparison to That of EHEC O157: Survival Studies in Food and In Vitro

In 2011, one of the world's largest outbreaks of hemolytic-uremic syndrome (HUS) occurred, caused by a rare Escherichia coli serotype, O104:H4, that shared the virulence profiles of Shiga toxin-producing E. coli (STEC)/enterohemorrhagic E. coli (EHEC) and enteroaggregative E. coli (EAEC). The persistence and fitness factors of the highly virulent EHEC/EAEC O104:H4 strain, grown either in food or in vitro, were compared with those of E. coli O157 outbreak-associated strains. The log reduction rates of the different EHEC strains during the maturation of fermented sausages were not significantly different. Both the O157:NM and O104:H4 serotypes could be shown by qualitative enrichment to be present after 60 days of sausage storage. Moreover, the EHEC/EAEC O104:H4 strain appeared to be more viable than E. coli O157:H7 under conditions of decreased pH and in the presence of sodium nitrite. Analysis of specific EHEC strains in experiments with an EHEC inoculation cocktail showed a dominance of EHEC/EAEC O104:H4, which could be isolated from fermented sausages for 60 days. Inhibitory activities of EHEC/EAEC O104:H4 toward several E. coli strains, including serotype O157 strains, could be determined. Our study suggests that EHEC/EAEC O104:H4 is well adapted to the multiple adverse conditions occurring in fermented raw sausages. Therefore, it is strongly recommended that STEC strain cocktails composed of several serotypes, instead of E. coli O157:H7 alone, be used in food risk assessments. The enhanced persistence of EHEC/EAEC O104:H4 as a result of its robustness, as well as the production of bacteriocins, may account for its extraordinary virulence potential.

IMPORTANCE

In 2011, a severe outbreak caused by an EHEC/EAEC serovar O104:H4 strain led to many HUS sequelae. In this study, the persistence of the O104:H4 strain was compared with those of other outbreak-relevant STEC strains under conditions of fermented raw sausage production. Both O157:NM and O104:H4 strains could survive longer during the production of fermented sausages than E. coli O157:H7 strains. E. coli O104:H4 was also shown to be well adapted to the multiple adverse conditions encountered in fermented sausages, and the secretion of a bacteriocin may explain the competitive advantage of this strain in an EHEC strain cocktail. Consequently, this study strongly suggests that enhanced survival and persistence, and the presumptive production of a bacteriocin, may explain the increased virulence of the O104:H4 outbreak strain. Furthermore, this strain appears to be capable of surviving in a meat product, suggesting that meat should not be excluded as a source of potential E. coli O104:H4 infection.

Efficacy of post-harvest rinsing and bleach disinfection of E. coli O157:H7 on spinach leaf surfaces

Attachment and detachment kinetics of Escherichia coli O157:H7 from baby spinach leaf epicuticle layers were investigated using a parallel plate flow chamber. Mass transfer rate coefficients were used to determine the impact of water chemistry and common bleach disinfection rinses on the removal and inactivation of the pathogen. Attachment mass transfer rate coefficients generally increased with ionic strength. Detachment mass transfer rate coefficients were nearly the same in KCl and AGW rinses; however, the detachment phase lasted longer in KCl than AGW (18 ± 4 min and 4 ± 2 min, respectively), indicating that the ions present during attachment play a significant role in the cells' ability to remain attached. Specifically, increasing bleach rinse concentration by two orders of magnitude was found to increase the detachment mass transfer rate coefficient by 20 times (from 5.7 ± 0.7 × 10^-11 m/s to 112.1 ± 26.8 × 10^-11 m/s for 10 ppb and 1000 ppb, respectively), and up to 88 ± 4% of attached cells remained alive. The spinach leaf texture was incorporated within a COMSOL model of disinfectant concentration gradients, which revealed nearly 15% of the leaf surface is exposed to almost 1000 times lower concentration than the bulk rinse solution.

Transfer of Pathogens from Cantaloupe Rind to Preparation Surfaces and Edible Tissue as a Function of Cutting Method

Whole and cut cantaloupes have been implicated as vehicles in foodborne illness outbreaks of norovirus, salmonellosis, and listeriosis. Preparation methods that minimize pathogen transfer from external surfaces to the edible tissue are needed. Two preparation methods were compared for the transfer of Listeria monocytogenes, Salmonella enterica serovar Typhimurium LT2, murine norovirus, and Tulane virus from inoculated cantaloupe rinds to edible tissue and preparation surfaces. For the first method, cantaloupes were cut into eighths, and edible tissue was separated from the rind and cubed with the same knife used to open the cantaloupes. For the second method, cantaloupes were scored with a knife around the circumference sufficient to allow manual separation of the cantaloupes into halves. Edible tissue was scooped with a spoon and did not contact the preparation surface touched by the rind. Bacteria and virus were recovered from the rinds, preparation surfaces, and edible tissue and enumerated by culture methods and reverse transcription, quantitative PCR, respectively. Standard plate counts were determined throughout refrigerated storage of cantaloupe tissue. Cut method 2 yielded approximately 1 log lower recovery of L. monocytogenes and Salmonella Typhimurium from edible tissue, depending on the medium in which the bacteria were inoculated. A slight reduction was observed in murine norovirus recovered from edible tissue by cut method 2. The Tulane virus was detected in approximately half of the sampled cantaloupe tissue and only at very low levels. Aerobic mesophilic colony counts were lower through day 6 of storage for buffered peptone water-inoculated cantaloupes prepared by cut method 2. No differences were observed in environmental contamination as a function of cutting method. Although small reductions in contamination of edible tissue were observed for cut method 2, the extent of microbial transfer underscores the importance of preventing contamination of whole cantaloupes.

Microbiological Status and Food Safety Compliance of Commercial Basil Production Systems

Basil has been implicated in a number of microbe-associated foodborne illnesses across the world, and the source of contamination has often been traced back to the production and/or processing stages of the supply chain. The aim of this study was to evaluate the microbiological quality of fresh basil from the point of production to the retail outlet in the Gauteng and Northwest Provinces of South Africa. A total of 463 samples were collected over a 3-month period from two large-scale commercial herb producing and processing companies and three retail outlets. The microbiological quality of the samples was assessed based on the presence or absence of Escherichia coli O157:H7 and Salmonella Typhimurium and the levels of the indicator bacteria E. coli and total coliforms. Salmonella Typhimurium was detected on four basil samples (0.9%) arriving at the processing facility and at dispatch, but no E. coli O157:H7 was detected throughout the study. Total coliform counts were 0.4 to 4.1 CFU/g for basil, 1.9 to 3.4 log CFU/ml for water, and 0.2 to 1.7 log CFU/cm(2) for contact surfaces, whereas E. coli was detected in the water samples and only once on basil. The Colilert-18 and membrane filter methods were used to analyze water samples, and a comparison of results revealed that the Colilert-18 method was more sensitive. Strong evidence suggests that high numbers of coliforms do not necessarily indicate the presence of Salmonella Typhimurium. The study results highlight the importance of effective implementation of food safety management systems in the fresh produce industry.

Survival of Salmonella on basil plants and in pesto

Leafy greens, including fresh herbs, have repeatedly been involved in outbreaks of foodborne disease. Although much effort has been put into studying leafy greens and products such as head lettuce and baby leaves, less is known about fresh leafy herbs, such as basil. The goal of this study was to investigate the survival of Salmonella on basil plants and in pesto. A mix of three Salmonella strains (Reading, Newport, and Typhimurium) was inoculated onto basil leaves and pesto and survived during the experimental period. Whereas the mix of Salmonella survived in pesto stored at 4°C for 4 days, Salmonella was recovered from inoculated leaves for up to 18 days at 20 to 22°C. Although the steady decline of Salmonella on leaves and in pesto suggests a lack of growth, it appears that pesto is a hostile environment for Salmonella because the rate of decline in pesto was faster (0.29 log CFU/g/day) than on leaves (0.11 log CFU/g/day). These findings suggest that the dilution of contaminated ingredients and the bactericidal effect of the pesto environment helped to further reduce the level of enteric organisms during storage, which may have applications for food safety.

Biofilm formation by enteric pathogens and its role in plant colonization and persistence

The significant increase in foodborne outbreaks caused by contaminated fresh produce, such as alfalfa sprouts, lettuce, melons, tomatoes and spinach, during the last 30 years stimulated investigation of the mechanisms of persistence of human pathogens on plants. Emerging evidence suggests that Salmonella enterica and Escherichia coli, which cause the vast majority of fresh produce outbreaks, are able to adhere to and to form biofilms on plants leading to persistence and resistance to disinfection treatments, which subsequently can cause human infections and major outbreaks. In this review, we present the current knowledge about host, bacterial and environmental factors that affect the attachment to plant tissue and the process of biofilm formation by S. enterica and E. coli, and discuss how biofilm formation assists in persistence of pathogens on the plants. Mechanisms used by S. enterica and E. coli to adhere and persist on abiotic surfaces and mammalian cells are partially similar and also used by plant pathogens and symbionts. For example, amyloid curli fimbriae, part of the extracellular matrix of biofilms, frequently contribute to adherence and are upregulated upon adherence and colonization of plant material. Also the major exopolysaccharide of the biofilm matrix, cellulose, is an adherence factor not only of S. enterica and E. coli, but also of plant symbionts and pathogens. Plants, on the other hand, respond to colonization by enteric pathogens with a variety of defence mechanisms, some of which can effectively inhibit biofilm formation. Consequently, plant compounds might be investigated for promising novel antibiofilm strategies.