15 key produce safety lessons

Bob Whitaker, Produce Marketing Association’s chief science officer and chairman of the Center for Produce Safety’s (CPS) Technical Committee, has provided a list of 15 take-away points from reports at the recent Produce Research Symposium:

Key Points

• Leafy Green Marketing Agreement “buffers” seem to work.

• Improper composting can result in pathogen survival.

• Initial data suggest sheep can be carriers of Salmonella.

• Pathogens do not survive well in the production environment. Attenuated E. coli O157:H7, applied by a spray to simulate what might happen if an overhead irrigation water source were contaminated with the pathogen, does not survive well on the leaf surfaces of either spinach or lettuce. The pathogen dies off and is hard to detect after two days.

• Pathogens are not taken up through the roots of a plant. Attenuated E. coli O157:H7 delivered to the roots of growing spinach plants via drip irrigation does not traverse the root and get taken up by the plant.

• Pathogens do not seem to move through the soil. Attenuated E. coli O157:H7 inoculated into the soil or sprayed on top of the soil did not survive past seven days.

• Pathogens may survive for longer periods when associated with organic matter. Spinach inoculated with E. coli O157:H7 and turned under the ground was recoverable from the soil for 100 days. This work will be repeated in 2010 using practices for preparing fields for replanting.

• A technology to permit storage of pathogen DNA can aid investigations. FTE filter papers can be used to store DNA from bacteria for up to 10 months at room temperature. By storing sample extracts on filter papers, hundreds of samples can be taken and then assayed at a future date.

• A “perfect storm” can result in pathogen growth. Moisture, temperature and perhaps other factors may create conditions where pathogens can survive. A rainfall event followed by warm temperatures created a situation where multiple genetic variants of E. coli O157:H7 were recovered in both raw lettuce and finished products.

Reports showed that buffer zones work:

• Leafy Green Marketing Agreement, or LGMA, “buffers” appear to work. Rapid-response experiments show that following a field intrusion by feral pigs, elevated levels of generic E. coli were found where the pigs obviously contacted the crop, but not beyond the 10-foot buffer zone prescribed by LGMA metrics.

• Larger sample sizes increase the chance of finding pathogens.

• Nonpathogenic bacteria may be used to identify conditions that permit pathogen survival. Bacterial populations that exist on the surface of leaf vegetables change by location and season. In a risk-based testing system, these nonpathogen bacteria could be used in conjunction with other measurements as “indicators” to identify when there may be an elevated risk of pathogen contamination or survival.

• Filth flies may be a potential vector for E. coli O157:H7. Flies have the capacity to transmit E. coli O157:H7 to the surface of vegetables under laboratory conditions. A very low prevalence of flies captured near vegetable production fields test positive for E. coli O157:H7.

• Modification of lettuce-coring knives can reduce the risk of pathogen contamination. By extending the common coring tool away from the cutting blade, the chance of cross-contamination is reduced. Further, by polishing joint welds, the tools are much easier to sanitize.

• LAMP technology (a software package) has been shown to have the potential to provide a quantitative assay of Salmonella that is 10 times more sensitive than PCR (this polymerase chain reaction is a scientific technique to amplify a single or few copies of a piece of DNA). The use of an intercalating agent permits the assay to distinguish between live and dead cells.

The reports covered whether sheep can carry Salmonella.

• Preliminary data suggest that sheep can be carriers of Salmonella. Bands of sheep sampled for Salmonella were shown to be carriers. Further experimentation is required to determine if grazing sheep represent a contamination risk.

• Improper composting can result in pathogen survival. Moisture, “heat-up times,” temperature, turns and other factors significantly affect the ability of pathogens in compost to survive the process.

• Bacteriophages (viruses that infect bacteria) may be a useful tool to improve the sensitivity of assaying finished compost for pathogens. It has been difficult to develop reliable tests for pathogens in complex organic backgrounds like compost, where many nonpathogenic species are also present. Using bacteriophages to kill competing species can improve pathogen recovery.

10102440  A.tif

PRODUCE IRRIGATION-SAFE: E. coli O157:H7, when applied by a spray that simulates what might happen if an overhead irrigation water source were contaminated with the pathogen, does not survive well on leaf surfaces of either spinach or lettuce.

ARS photo

This article published in the October, 2010 edition of CALIFORNIA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.