Attachment of Salmonella strains to a plant cell wall model is modulated by surface characteristics and not by specific carbohydrate interactions

Tomato Cultivar Nyagous Fruit Surface Metabolite Changes during Ripening Affect Salmonella Newport

ABSTRACT

Tomatoes are a valuable crop consumed year-round. Ripe fruit is picked for local sale, whereas tomatoes intended for transit may be harvested at late mature green or breaker stages when fruit firmness preserves quality. In this study, we evaluated Solanum lycopersicum cv. BHN602 association with three Salmonella serotypes and S. lycopersicum cv. Nyagous with Salmonella Newport using fruit at two ripeness stages. Counts of Salmonella Javiana and Typhimurium were higher from red ripe fruit surfaces of BHN602, and counts of Salmonella Newport were higher from ripe Nyagous fruit than from mature green fruit (P < 0.05). Aqueous fruit washes containing fruit surface compounds collected from ripe Nyagous fruit supported more Salmonella Newport growth than green fruit washes (P < 0.05). Growth curve analysis showed that between 2 and 6 h, Salmonella Newport grew at a rate of 0.25 log CFU/h in red fruit wash compared with 0.17 log CFU/h in green fruit wash (P < 0.05). The parallel trend in Salmonella interaction between fruit and wash suggested that surface metabolite differences between unripe and ripe fruit affect Salmonella dynamics. Untargeted phytochemical profiling of tomato fruit surface washes with gas chromatography time-of-flight mass spectrometry showed that ripe fruit had threefold-lower amino acid and fourfold-higher sugar (fructose, glucose, and xylose) levels than green fruit. Green fruit had higher levels of lauric, palmitic, margaric, and arachidic acids, whereas red fruit had more capric acid. The phenolics ferulic, chlorogenic, and vanillic acid, as well as tyrosol, also decreased with ripening. Although limitations of this study preclude conclusions on how specific compounds affect Salmonella, our study highlights the complexity of the plant niche for foodborne pathogens and the importance of understanding the metabolite landscape Salmonella encounters on fresh produce. Fruit surface phytochemical profiling generated testable hypotheses for future studies exploring the differential Salmonella interactions with tomato varieties and fruit at various ripeness stages.

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The activity of BcsZ of Salmonella Typhimurium and its role in Salmonella-plants interactions

Salmonella enterica is one of the most common human pathogens associated with fresh produce outbreaks. The present study suggests that expression of BcsZ, one of the proteins in the bcs complex, enhances the survival of Salmonella Typhimurium on parsley. BcsZ demonstrated glucanase activity with the substrates carboxymethylcellulose and crystalline cellulose, and was responsible for a major part of the S. Typhimurium CMCase activity. Moreover, there was constitutive expression of BcsZ, which was also manifested after exposure to plant polysaccharides and parsley-leaf extract. In an in-planta model, overexpression of BcsZ significantly improved the epiphytic and endophytic survival of S. Typhimurium on/in parsley leaves compared with the wild-type strain and bcsZ null mutant. Interestingly, necrotic lesions appeared on the parsley leaf after infiltration of Salmonella overexpressing BcsZ, while infiltration of the wild-type S. Typhimurium did not cause any visible symptoms. Infiltration of purified BcsZ enzyme, or its degradation products also caused symptoms on parsley leaves. We suggest that the BcsZ degradation products trigger the plant’s defense response, causing local necrotic symptoms. These results indicate that BcsZ plays an important role in the Salmonella-plant interactions, and imply that injured bacteria may take part in these interactions.

Inoculum Preparation Conditions Influence Adherence of Salmonella enterica Serovars to Red Leaf Lettuce (Lactuca sativa)

ABSTRACT

Salmonella enterica has been increasingly linked to outbreaks involving consumption of fresh produce. Although researchers have identified genes whose products are involved in mediating S. enterica-plant interactions, the use of various experimental approaches, serovars, and plant types has generated variable and conflicting data. The purpose of this study was to determine whether conditions under which inocula are prepared for in vitro plant interaction studies influence the outcome of these studies. Seven S. enterica serovars were grown in media that differed in salinity and physical state with incubation at 25 or 37°C. These cultures were then used to inoculate red leaf lettuce, and adherent microbes were subsequently recovered. Although all Salmonella serovars were influenced by inoculum preparation conditions, the amount of variation differed. Analysis of pooled serovar data revealed that inocula prepared from either agar plates or biphasic cultures had higher levels of interaction with red leaf lettuce than those prepared from broth cultures. Incubation at 37°C enhanced adherence after 30 s or 5 days of contact time, and adherence after 1 h of contact time was increased in low-salt medium. Broth inoculum cultures were highly influenced by medium salinity and incubation temperature, whereas plate and biphasic inoculum cultures were only minimally affected. Therefore, inocula prepared from bacteria grown on plates or in biphasic culture would be most suitable for evaluation of strategies used to interfere with plant-Salmonella interactions. However, pooled data mask serovar-specific responses, and care should be taken when extrapolating these findings to individual serovars. The previous association of a serovar with outbreaks involving leafy greens was not correlated with levels of interaction with red leaf lettuce, suggesting that the occurrence of these serovars in or on these commodities does not reflect their fitness in the plant environment.

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Attachment of Salmonella enterica on Mangoes and Survival Under Conditions Simulating Commercial Mango Packing House and Importer Facility

Consumption of raw mangoes has led to multiple Salmonella-associated foodborne outbreaks in the United States. Although several studies have investigated the epiphytic fitness of Salmonella on fresh produce, there is sparse information available on the survival of Salmonella on mangoes under commercial handling and storage conditions. Hence, the objective of the study was to evaluate the survival of Salmonella on mangoes under ambient conditions simulating the mango packing house and importer facility. Further, the ability of the pathogen to adhere and attach on to the mango fructoplane was also investigated. For the attachment assays, mango skin sections were inoculated with fifty microliters of S. Newport suspension (6.5 log CFU/skin section) and minimum time required for adhesion and attachment were recorded. With the survival assays, unwaxed mangoes were spot inoculated with the Salmonella cocktail to establish approximately 4 and 6.5 log CFU/mango. The fruits were then subjected to different storage regimens simulating fruit unloading, waxing, and storage at the packing house and ripening and storage at the importer facility. Results of our study reveal that Salmonella was able to adhere on to the fructoplane immediately after contact. Further, formation of attachment structures was seen as early as 2 min following inoculation. With the survival assays, irrespective of the inoculum levels, no significant increase or decrease in pathogen population was observed when fruit were stored either at ambient (29-32°C and RH 85-95%, for 48 h), ripening (20-22°C and RH 90-95% for 9 days) or refrigerated storage (10-15°C and 85-95% for 24-48 h) conditions. Therefore, once contaminated, mangoes could serve as potential vehicles in the transmission of Salmonella along the post-harvest environment. Hence development and adoption of effective food safety measures are warranted to promote the microbiological safety of mangoes.

Attachment and Survival of Escherichia coli O157:H7 on In-Shell Hazelnuts

The multistate Escherichia coli (E. coli) O157:H7 outbreak associated with in-shell hazelnuts highlights the pathogen's ability to involve non-traditional vehicles in foodborne infections. Furthermore, it underscores significant gaps in our knowledge of pathogen survivability and persistence on nuts. Therefore, this study investigated the ability of E. coli O157:H7 to attach and survive on in-shell hazelnuts. In-shell hazelnuts were inoculated with a four-strain mixture of E. coli O157:H7 at 7.6 log colony forming units (CFU)/nut by wet or dry inoculation, stored at ambient conditions (24 ± 1 °C; 40% ± 3% relative humidity (RH) and sampled for twelve months. For the attachment assay, in-shell hazelnuts were inoculated and the adherent population was enumerated at 30 s^-1 h following inoculation. Irrespective of the inoculation method, ~5 log CFU of adherent E. coli O157:H7 was recovered from the hazelnuts as early as 30 s after inoculation. Conversely, pathogen survival was significantly reduced under dry inoculation with samples being enrichment negative after five months of storage (p < 0.05). On the other hand, wet inoculation led to a significantly longer persistence of the pathogen with ~3 log CFU being recovered from the in-shell nuts at 12 months of storage (p < 0.05). These results indicate that E. coli O157:H7 can survive in significant numbers on in-shell hazelnuts when stored under ambient conditions.