April 13, 2010

Salmonella-Spiced Food

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A year after the Peanut Corporation of America’s giant recall of peanut products, Salmonella strikes again. Early this year, investigators linked a foodborne outbreak caused by Salmonella Montevideo to another uncharacteristic culprit: black pepper.

In January, Daniele, Inc. of Rhode Island announced a recall of several ready-to-eat meat products, including some types of salami, because the Italian-style meats contained Salmonella-contaminated pepper. To date, 252 illnesses have been linked to these products. Salmonella infection causes the illness, “salmonellosis.”

In addition to Daniele, 14 other producers, who used pepper suspected as contaminated, have issued voluntary recalls on their own food items, according to the U.S. Food and Drug Administration’s website.

The Centers for Disease Control and Prevention (CDC) define foodbourne outbreaks as two or more cases of illness linked to a common food. Outbreak investigations strive for a very specific goal, that is, to trace the organism that made humans sick back to its source. While simple in theory, the reality of this outbreak detection requires that sick patients see doctors, that doctors identify samples (usually stool samples) as positive for a disease-causing organism, and that doctors send those samples to public health labs.

Doctors cannot identify a foodborne pathogen if sick humans choose not to seek help. For example, this may occur when symptoms of salmonellosis, including diarrhea and vomiting, are not so severe that victims seek medical attention.

If doctors do find positive samples, they send them to their state public health department for further identification. Public health departments monitor the number of cases of a certain foodborne illness all the time. Sporadic cases occur frequently, and they establish baseline levels. When officials see a spike from this baseline level, they suspect an outbreak may be responsible.

Connecting the dots between illnesses improved in the past two decades. DNA-based technologies became routine for identifying specific strains of bacteria. These methods offer a finer level of resolution for identifying strains than older methods. Currently, health officials consider pulsed field gel electrophoresis (PFGE) the “gold standard” method for strain identification.

PFGE involves taking DNA from a bacterium, and then, chopping it up at specific DNA sites with enzymes. Scientists then separate the pieces in an algae-derived gel using an alternating electric current. The movement of pieces – which are all different sizes – creates a unique pattern of bands that they may match to those generated from other samples. This pattern is commonly described a “DNA footprint.”

Prof. Martin Wiedmann, food science, studies PFGE and other molecular subtyping methods for identifying Salmonella. Thanks to these newer methods, Wiedmann said, “We’ve gotten much, much better at detecting outbreaks.”

The fingerprint analogy is not quite perfect, Wiedmann claimed. “Fingerprints are unique; PFGE patterns aren’t.”

Because bacteria reproduce asexually, not every ­­­bacterium is genetically unique. Bacteria may yield the same PFGE pattern even if they did not come from the same source. This biological fact makes linking a strain of bacteria to several cases and to a particular food product important for successfully identifying an outbreak.

Lorraine Rodriguez, food science, grad, performs PFGE typing of Salmonella in Wiedmann’s lab. She said, “You also need to link [matching PFGE patterns] epidemiologically. That means you need to find out if patients ate the same food, or were in the same places — the same market or the same restaurant, for example — to link them.”

In the recent black pepper outbreak, the CDC investigators used information from patients’ shopper cards to figure out whether they purchased Daniele brand meats. Investigators frequently use this technique to fill in the gaps left when patients try to recall their diets.

Illness from Salmonella occurs one to three days after eating the bacterium, and not everyone can remember the brand of deli meat they ate three lunches ago.

Salmonella exists in many niches because it is hardy and diverse.

Prof. Lorin Warnick, population medicine and diagnostic sciences, studies the prevalence and distribution of Salmonella on dairy farms. He stressed that the sources of Salmonella, in cases of human illness, can be as diverse as the strains themselves.

“It’s not always an animal source. It could be human contamination,” he said. “If it is an animal source, it’s not always an animal product. And if it is an animal, its not always domestic livestock.”

For instance, last year, investigators linked a salmonellosis outbreak to pet aquatic frogs that originated from one breeder in California.

No one knows the source of the Salmonella on the Daniele Inc.’s products. However, investigators linked another serovar (a classification of Salmonella strains, like Montevideo) of Salmonella to crushed red pepper used in Daniele products, which was supplied by another spice wholesaler.

According to a report by the U.S. Dept. of Agriculture, 60% of spices used in the U.S. are imported. However, while relatively few shipments are turned away from the U.S. before entry, up to 50% shipments of spices that were turned away from 1980 to 2000 were rejected because of Salmonella contamination.

Spices may become contaminated in fields or during processing, distribution and shipment.

To combat this risk, companies often treat their spices are with inactivation steps to kill pathogens. Irradiation is a common method used on high-volume ingredients like spices, especially those used in ready-to-eat foods, which are foods that are not cooked before eating after purchase, says Prof. Carmen Moraru, food science. Moraru investigates non-thermal methods to inactivate dangerous bacteria in foods.

Irradiation is commonly used for two reasons, she said, “Because of the effectiveness of the treatment against pathogens, and the fact that it doesn’t really impact the quality of the spices – the flavor profile, for example.” Moraru added, the spices consumers can buy on retail shelves are rarely irradiated since these products would have to be labeled as such.

According to Daniele’s hotline message, where consumers can hear a list of recalled products, “Daniele is now using only irradiated spices.” In addition to Daniele’s recall, customers of the two wholesale companies that supplied the black and red pepper have also recalled some of their products suspected of containing the same pepper.

The chain of events in this outbreak demonstrates that when basic food ingredients are contaminated with Salmonella, the impacts are far-reaching. Minimizing damage requires keeping track of products’ sources and destinations.

“People demand traceability, meaning if I eat something I want to know where does it come from and where do all the ingredients come from,” noted Wiedmann. “As we improve our system for that, we’re more likely to find out the sources.”

Original Author: Daina Ringus