Quantification of low and high levels of Salmonella enterica serovar Typhimurium on leaves

Cultivar was more influential than bacterial strain and other experimental factors in recovery of Escherichia coli O157:H7 populations from inoculated live Romaine lettuce plants

The varied choice of bacterial strain, plant cultivar, and method used to inoculate, retrieve, and enumerate Escherichia coli O157:H7 from live plants could affect comparability among studies evaluating lettuce-enterobacterial interactions. Cultivar, bacterial strain, incubation time, leaf side inoculated, and sample processing method were assessed for their influence in recovering and quantifying E. coli O157:H7 from live Romaine lettuce. Cultivar exerted the strongest effect on E. coli O157:H7 counts, which held up even when cultivar was considered in interactions with other factors. Recovery from the popularly grown green Romaine "Rio Bravo" was higher than from the red variety "Outredgeous." Other modulating variables were incubation time, strain, and leaf side inoculated. Sample processing method was not significant. Incubation for 24 hours post-lettuce inoculation yielded greater counts than 48 hours, but was affected by lettuce cultivar, bacterial strain, and leaf side inoculated. Higher counts obtained for strain EDL933 compared to a lettuce outbreak strain 2705C emphasized the importance of selecting relevant strains for the system being studied. Inoculating the abaxial side of leaves gave higher counts than adaxial surface inoculation, although this factor interacted with strain and incubation period. Our findings highlight the importance of studying interactions between appropriate bacterial strains and plant cultivars for more relevant research results, and of standardizing inoculation and incubation procedures. The strong effect of cultivar exerted on the E. coli O157:H7-lettuce association supports the need to start reporting cultivar information for illness outbreaks to facilitate the identification and study of plant traits that impact food safety risk.IMPORTANCEThe contamination of Romaine lettuce with Escherichia coli O157:H7 has been linked to multiple foodborne disease outbreaks, but variability in the methods used to evaluate E. coli O157:H7 association with live lettuce plants complicates the comparability of different studies. In this study, various experimental variables and sample processing methods for recovering and quantifying E. coli O157:H7 from live Romaine lettuce were assessed. Cultivar was found to exert the strongest influence on E. coli O157:H7 retrieval from lettuce. Other modulating factors were bacterial incubation time on plants, strain, and leaf side inoculated, while sample processing method had no impact. Our findings highlight the importance of selecting relevant cultivars and strains, and of standardizing inoculation and incubation procedures, in these types of assessments. Moreover, results support the need to start reporting cultivars implicated in foodborne illness outbreaks to facilitate the identification and study of plant traits that impact food safety risk.

In-House Validation of a Rinse-Membrane Filtration Method for Processing Fresh Produce Samples for Downstream Cultural Detection of Salmonella, Escherichia coli O157:H7, and Listeria

More efficient sampling and detection methods of pathogens on fresh produce are needed. The purpose of this study was to compare a novel rinse-membrane filtration method (RMFM) to a more traditional sponge rubbing or stomaching method in processing jalapeño peppers and cantaloupe samples for detection of Escherichia coli, Salmonella enterica, and Listeria monocytogenes. For jalapeño peppers inoculated with 106, 104, and 102 CFU of each pathogen and cantaloupes inoculated at 106 and 104 CFU, all pathogens were detected in all (100%) samples by RMFM at a 10-mL filtration volume, as well as by the stomacher and sponge rubbing methods. However, for cantaloupe inoculated at 102 CFU, detection differed by pathogen: S. enterica (20% RMFM, 60% stomacher, and 20% sponge), L. monocytogenes (40% RMFM, 60% stomacher, and 20% sponge), and E. coli O157:H7 (100% RMFM, 75% stomacher, and 75% sponge). When RMFM was compared with the other methods, in accordance with guidelines in the International Organization for Standardization 16140:2003 protocol, it produced values >95% in relative accuracy, relative specificity, and relative sensitivity. Overall, the RMFM performed similar to or better than the homogenization and sponge surface rubbing methods and is a good alternative for processing large numbers of produce samples for bacterial pathogen detection.

Identifying and modeling meteorological risk factors associated with pre-harvest contamination of Listeria species in a mixed produce and dairy farm

This study sought to investigate the prevalence of Listeria species (including L. monocytogenes) in a mixed produce and dairy farm and to identify specific meteorological factors affecting Listeria spp. presence. Environmental samples were collected monthly from locations within the mixed farm over 14months and were analyzed for Listeria spp. Meteorological factors were evaluated for their association with the presence of Listeria spp. by using logistic regression (LR) and random forest (RF). The developed LR model identified wind speed and precipitation as significant risk factors (P<0.05), indicating higher wind speed at day 2 prior to sampling and higher average precipitation over the previous 25days before sampling increased the probability of isolation of Listeria spp. from the mixed farm. Results from RF revealed that average wind speed at day 2 prior to sampling and average precipitation in the previous 25days before sampling were the most important factors influencing the presence of Listeria spp., which supported the findings from LR. These findings indicate that the occurrence of Listeria spp. was influenced by wind speed and precipitation, suggesting run-off and wind-driven dust might be possible routes of pathogen transmission in mixed farms. The developed LR and RF models, with robust predictive performances as measured by the area under the receiver operating characteristic curves, can be used to predict Listeria spp. contamination risk in a mixed farm under different weather conditions and can help with the evaluation of farm management practices and the development of control strategies aimed at reducing pre-harvest microbial contamination in a mixed farming system.

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.

Effect of the surfactant tween 80 on the detachment and dispersal of Salmonella enterica serovar Thompson single cells and aggregates from cilantro leaves as revealed by image analysis

Salmonella enterica has the ability to form biofilms and large aggregates on produce surfaces, including on cilantro leaves. Aggregates of S. enterica serovar Thompson that remained attached to cilantro leaves after rigorous washing and that were present free or bound to dislodged leaf tissue in the wash suspension were observed by confocal microscopy. Measurement of S. Thompson population sizes in the leaf washes by plate counts failed to show an effect of 0.05% Tween 80 on the removal of the pathogen from cilantro leaves 2 and 6 days after inoculation. On the contrary, digital image analysis of micrographs of single cells and aggregates of green fluorescent protein (GFP)-S. Thompson present in cilantro leaf washes revealed that single cells represented 13.7% of the cell assemblages in leaf washes containing Tween 80, versus 9.3% in those without the surfactant. Moreover, Tween 80 decreased the percentage of the total S. Thompson cell population located in aggregates equal to or larger than 64 cells from 9.8% to 4.4% (P < 0.05). Regression analysis of the frequency distribution of aggregate size in leaf washes with and without Tween 80 showed that the surfactant promoted the dispersal of cells from large aggregates into smaller ones and into single cells (P < 0.05). Our study underlines the importance of investigating bacterial behavior at the scale of single cells in order to uncover trends undetectable at the population level by bacterial plate counts. Such an approach may provide valuable information to devise strategies aimed at enhancing the efficacy of produce sanitization treatments.

Resistance to essential oils affects survival of Salmonella enterica serovars in growing and harvested basil

The number of outbreaks of food-borne illness associated with consumption of fresh products has increased. A recent and noteworthy outbreak occurred in 2007. Basil contaminated with Salmonella enterica serovar Senftenberg was the source of this outbreak. Since basil produces high levels of antibacterial compounds the aim of this study was to investigate if the emerging outbreak reflects ecological changes that occurred as a result of development of resistance to ingredients of the basil oil. We irrigated basil plants with contaminated water containing two Salmonella serovars, Typhimurium and Senftenberg, and showed that Salmonella can survive on the basil plants for at least 100 days. S. Senftenberg counts in the phyllosphere were significantly higher than S. Typhimurium, moreover, S. Senftenberg was able to grow on stored harvested basil leaves. Susceptibility experiments demonstrated that S. Senftenberg is more resistant to basil oil and to its antimicrobial constituents: linalool, estragole and eugenol. This may indicate that S. Senftenberg had adapted to the basil environment by developing resistance to the basil oil. The emergence of resistant pathogens has a significant potential to change the ecology, and opens the way for pathogens to survive in new niches in the environment such as basil and other plants.

The endophytic lifestyle of Escherichia coli O157:H7: quantification and internal localization in roots

The foodborne pathogen Escherichia coli O157:H7 is increasingly associated with fresh produce (fruit and vegetables). Bacterial colonization of fresh produce plants can occur to high levels on the external tissue but bacteria have also been detected within plant tissue. However, questions remain about the extent of internalization, its molecular basis, and internal location of the bacteria. We have determined the extent of internalization of E. coli O157:H7 in live spinach and lettuce plants and used high-resolution microscopy to examine colony formation in roots and pathways to internalization. E. coli O157:H7 was found within internal tissue of both produce species. Colonization occurred within the apoplast between plant cells. Furthermore, colonies were detected inside the cell wall of epidermal and cortical cells of spinach and Nicotiana benthamiana roots. Internal colonization of epidermal cells resembled that of the phytopathogen Pectobacterium atrosepticum on potato. In contrast, only sporadic cells of the laboratory strain of E. coli K-12 were found on spinach, with no internal bacteria evident. The data extend previous findings that internal colonization of plants appears to be limited to a specific group of plant-interacting bacteria, including E. coli O157:H7, and demonstrates its ability to invade the cells of living plants.

Presence and persistence of Salmonella enterica serotype typhimurium in the phyllosphere and rhizosphere of spray-irrigated parsley

Salmonella enterica is one of the major food-borne pathogens associated with ready-to-eat fresh foods. Although polluted water might be a significant source of contamination in the field, factors that influence the transfer of Salmonella from water to the crops are not well understood, especially under conditions of low pathogen levels in water. The aim of this study was to investigate the short- and long-term (1 h to 28 days) persistence of Salmonella enterica serotype Typhimurium in the phyllosphere and the rhizosphere of parsley following spray irrigation with contaminated water. Plate counting and quantitative real-time PCR (qRT-PCR)-based methods were implemented for the quantification. By applying qRT-PCR with enrichment, we were able to show that even irrigation with water containing as little as ∼300 CFU/ml resulted in the persistence of S. Typhimurium on the plants for 48 h. Irrigation with water containing 8.5 log CFU/ml resulted in persistence of the bacteria in the phyllosphere and the rhizosphere for at least 4 weeks, but the population steadily declined with a major reduction in bacterial counts, of ∼2 log CFU/g, during the first 2 days. Higher levels of Salmonella were detected in the phyllosphere when plants were irrigated during the night compared to irrigation during the morning and during winter compared to the other seasons. Further elucidation of the mechanisms underlying the transfer of Salmonella from contaminated water to crops, as well as its persistence over time, will enable the implementation of effective irrigation and control strategies.