Project seeks better understanding of in-field pathogen die-off rates

Project seeks better understanding of in-field pathogen die-off rates

Under the FSMA Produce Rules, agricultural water that comes in direct contact with the edible portions of produce in the field must meet a microbial numerical criterion based on the Environmental Protection Agency's 2012 recreational water standards. If agricultural water does not meet these criteria, farmers are required to establish as soon as practical and no later than the following year corrective actions that will allow this water to meet them.

Farmers with agricultural water that does not initially meet the microbial criteria may also follow a set of alternative provisions, enabling them to use the water on their crops. One of these provisions allows growers to apply a time interval for in-field microbial die-off of a maximum of four consecutive days between the last irrigation and harvest where a predicted die-off rate of 0.5 log per day is expected during these four days. 

Dr. Eduardo Gutierrez-Rodriguez, assistant professor and Extension specialist in fresh produce safety at North Carolina State University, is studying these provisions and looking to optimize the existing knowledge in microbial die-off of avirulent and pathogenic enterohemorrhagic E. coli (EHEC) and Shiga toxin-producing E. coli (STEC). 

He is focusing on strawberries and cilantro to determine whether the proposed in-field microbial die-off rate can be adjusted to a linear model as listed by the FSMA Produce Rule. As part of the project, he and colleagues are examining whether the implementation of preharvest intervention strategies - including chlorine or PAA applications through the irrigation system - could enhance microbial die-offs and further reduce pathogen contamination when using agricultural water that does not meet the microbial standards. 

Gutierrez-Rodriguez said he hopes the research eventually helps to address many questions he continues to receive from growers about water-related contamination risks and in-field pathogen die-off. "The most tangible (benefit) is whether the application of those sanitizers prior to harvest could enhance in-field pathogen die-off without harming the crop and thus reduce the risk of contamination and pathogen persistence in cilantro or strawberry," he said.

Co-investigators-all from NCSU - are Dr. Chris Gunter, associate professor and vegetable production specialist; Dr. Sid Thakur, associate professor of molecular epidemiology; Dr. Victoria Albarracin, postdoctoral scholar at the Gutierrez-Rodriguez lab; and Dr. Carole Saravitz, director of the Phytotron. Their project will focus on strawberries and cilantro where few if any disinfection steps are commercially available after harvest and where large volumes of surface or well water are used for frost protection (strawberry) and overhead irrigation (cilantro), Gutierrez-Rodriguez said. Strawberries are grown on plastic covered beds, watered through drip irrigation and picked and packed in the field. In certain parts of the United States when temperatures dip below freezing, growers turn to sprinkler irrigation, row covers or a combination of both for frost protection. This puts the plants and fruit in direct contact with surface water. Cilantro cultural and harvest practices vary, depending on the region of the world. The low-growing herb is typically irrigated using drip, sprinkler or furrow irrigation.

As part of their proof-of-concept project, the researchers plan to compare die-off rates of potential surrogates and pathogenic strains of EHEC and STEC inside a biosafety level 2 or 3 greenhouse. In addition to the high level of biosecurity, the greenhouse also provides an equal playing field where die-off rates of surrogate and pathogens inoculated to the leaf or fruit of the plants can be compared side by side on plants grown following commercial practices. 

"We're talking to the strawberry industry, which is very important here in North Carolina and other parts of the South, to find out which treatments they would consider using," Gutierrez-Rodriguez said. "The products we are currently using are approved for treatment of agricultural irrigation systems and treatment of agricultural water used for fruit and vegetable crops, but we also need to evaluate the impact of these applications on the long-term sustainability of agriculture and whether these application cause any post-harvest losses during storage.

The data collected from these experiments will be used to develop and compare predictive die-off models between pathogens and surrogate organisms. If the die-off rates are similar between them, the next research effort will be to take the best surrogate out to farms and evaluate the die-off rates under field conditions - both in the absence and presence of chlorine and PAA run through the irrigation systems.

"This information, hopefully, will help us further complete our understanding of in-field microbial die-offs in cilantro and strawberry," Gutierrez-Rodriguez said. 

Project abstract may be found on the Center for Produce Safety website:

Establishing Die-off Rates of Surrogate and Virulent EHEC-STEC Strains from Strawberry and Cilantro Surfaces: Time, Inoculum Dose and Chemical Intervention - Proof of Concept