Irrigation regime, fruit water congestion and produce safety: parameter optimization to reduce susceptibility of tomatoes and peppers to post-harvest contamination, pathogen transfer and proliferation of Salmonella.

Recent multi-state outbreaks of vegetable-borne gastrointestinal illnesses demonstrate that human pathogens can contaminate produce at any stage of production. The fact that the outbreaks have been sporadic and the uncertainty regarding sources and routes of contamination argue for the possibility that some event(s) during the production cycle make vegetables more susceptible to contamination from environmental sources of pathogens. We hypothesize that irrigation-determined physical properties of tomatoes and peppers (fruit wetness, fruit water congestion, etc) may make them more vulnerable to contamination from various environmental sources, including transfer from rubber gloves or wiping cloths. Fruit wetness and fruit water congestion also increase susceptibility of vegetables to plant pathogens, and the association with plant pathogens has been well-documented to promote growth of Salmonella in vegetables. The overall goal of this project is to test how irrigation determined physical properties of tomatoes and peppers affect vulnerability of produce to infections by Salmonella. Upon completion of this project, we expect to have defined an optimal irrigation regime under which yields are maintained, yet the vulnerability of produce to contamination and proliferation of Salmonella are reduced. Following a successful completion of the first two production seasons, a full scale on-farm demonstration will be carried out.

As the number and severity of produce‐borne gastroenteritis outbreaks increase, we are coming to recognize that enteric pathogens can escape surface sanitation leading to contaminated fruits, vegetables and sprouts in the marketplace. The outbreaks associated with produce appear to be sporadic and do not generally result from an oversight by a producer or handler. We hypothesize that physical properties of produce, which are determined by the irrigation regime pre‐harvest, affect susceptibility of tomatoes and peppers to environmental sources of pathogens. We propose that by augmenting growing practices, safety of produce can be improved through the reduction in susceptibility of fruits to accidental sources of enteric pathogens. The overall goal of this proposal is to increase safety of vegetable produce by testing how irrigation regime pre‐harvest, fruit wetness and water congestion at harvest affect susceptibility of produce to post‐harvest contamination with Salmonella. The optimization of these parameters will promote produce safety either directly (by providing a less hospitable environment for Salmonella or by reducing transfer of pathogens from rubber gloves or wiping cloths onto fruits) or indirectly (by reducing injury from soft‐rot bacteria or sour rot pathogens, which increase growth of human pathogens on produce). 

The following specific objectives will help us reach this goal: 

Objective 1. To reduce post‐harvest susceptibility of tomatoes and peppers to Salmonella through the optimization of irrigation regime and fruit wetness at harvest With this objective, we will test how differences in physical properties of the fruit resulting from different irrigation regimes pre‐harvest and fruit wetness at harvest will affect susceptibility of fruit to contamination with Salmonella from rubber gloves and/or wiping cloths. 

Objective 2. To determine whether water congestion (with or without post‐harvest soft rot or sour rot decay) is a risk factor for Salmonella colonization or internalization. In the field, congestive water in fruits may result from root uptake, rainfall, saturated atmospheres or harvest related injuries. Experiments proposed here will determine whether fruit water congestion with or without postharvest decay is a risk factor for Salmonella contamination and/or internalization in tomato and pepper tissues under the simulated production line conditions. We will compare survival of Salmonella in water‐congested fruit at three stages of ripeness (green commercial harvest, breaker/pink (commercial vine‐ripe) or fully red). Because water congestion is known to predispose produce to infections with plant pathogens, and because soft‐rot pectobacteria and sour rot yeasts are known to promote growth of Salmonella in fruit tissues under laboratory conditions, we will test how these two factors affect proliferation of Salmonella in tomatoes and peppers post harvest. 

Objective 3. To define the role for spoilage bacteria in contamination of fruit with Salmonella. In soft rots caused by Pectobacteria, Salmonella grows to final populations that are 10‐100x higher than in mechanically damaged fruit. It is not yet known whether plant pathogens, when present at low numbers, will also promote contamination of produce with Salmonella. We will determine the mechanisms by which two common plant pathogens of tomato and pepper promote susceptibility of fruit to contamination and internalization of Salmonella.

Various aspects of Salmonella interaction with tomato fruit can be modeled by use of sections of pericarp. Pericarp sections are easy to handle, require little space, are easily stored under various environments and can be extracted for bacteria with 2 to 4 ml of sterile phosphate buffered saline. Growth of contaminating decay pathogens such as soft rot bacteria, soft-rotting fungi or fruit endophytes or epiphytes have not affected proliferation of Salmonella on fruit tissues. Inoculated sections have been held up to 72 h at 22o C. Salmonella populations on sections increased from ca. Log 3.0 after inoculation to Log. 7.5 by 24 h and up to Log 8.8 by 48 h. Pericarp sections can be water congested by floating them on water for 30 to 60 min. Movement of water would be through the cut edges as well as the bottom. The process did not seem to promote fruit decay or the growth of contaminating bacteria. Water moves into fruit rapidly, which emphasizes the hazards of harvesting fruit when the plants are wet or of allowing freshly harvested fruit to be exposed to uncontrolled water (such as rainfall, or any water that does not contain a sanitizer). Water movement into fruit appears to be reduced when salts are present suggesting that tissue water potential is a factor. Tomato fruit tissues respond differently to free water when pink or green as opposed to red. Salmonella proliferation is promoted (statistical increase in populations recovered) by water congestion in green or pink tomatoes but not red tomatoes. High storage temperatures (35o C) consistently promote proliferation. Tomato maturity appears to have the most significant effect on the proliferation of Salmonella in tomatoes once contamination takes place. Differences in irrigation regime (imposed within 2 weeks prior to harvest) did not significantly impact the ability of Salmonella to proliferate in the fruit. Because only three tomato varieties were tested in this field study, it is difficult to definitively establish and broadly conclude whether tomato genotype had a role in susceptibility of the crop to Salmonella. Consistent differences were observed in the proliferation of Salmonella in peppers vs tomatoes.