"Recent nationwide outbreaks in 1998, particularly in California, and in 1999 in Chicago suggest that Shigella may be an emerging pathogen in the United States," said Meredith E. Agle, a doctoral student in food microbiology. "With the globalization of food and more people having more exotic tastes, we believe pathogens will be showing up more regularly from developing countries where poor sanitation and water quality make the shipment of bacteria-free produce very difficult."
Agle has been studying the bean salad recipe and Shigella's ability to survive in it. She shared preliminary data June 26 at the Institute of Food Technologists annual meeting in New Orleans. Among her findings: A commercial produce wash was no more effective than water for removing the Chicago outbreak strain, Shigella boydii, from parsley and cilantro.
In addition, she said, Shigella in the infected bean salad, re-created in the lab, did not grow but remained at infectious levels for up to six days of normal temperature storage in a refrigerator. At room temperature, Shigella grew rapidly, she said.
Agle said parsley and cilantro were suspected in the Chicago case because the infected plants in the 1998 outbreaks, which involved Shigella sonnei, had been traced to a Mexican farm. Many of the ingredients in the Chicago case were from Mexico and were not washed before being put in the bean salad. Shigella, which comes in four strains and is similar to E. coli, causes shigellosis, an infectious disease that leads to diarrhea, fever and stomach cramps, beginning about 24 hours after exposure and continuing for a week. Shigella sonnei is the most common strain associated with disease each year in the United States. Shigella boydii is associated with Mexico and South America.
Why shigella is so difficult to remove from parsley and cilantro is being studied. The pathogen may create a biofilm, similar to dental plaque, which clings firmly to the produce, Agle said. Irradiating parsley successfully removed pathogens but left it with a slightly cooked texture that many people may not find as palatable as raw parsley, she said.
"The message from this research is pretty clear," said Hans P. Blaschek, head of the UI department of food science and human nutrition, who supervises the lab where Agle works. "People need to properly store their fruits and vegetables in a refrigerator and, more importantly, wash them thoroughly. The actual physical manipulation of the produce during the washing process appears to be the most important factor in removal of the pathogen."