Ecology of foodborne pathogens

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Dec 3, 2008 10:51 AM, Mark A. Trent
UC Davis
Farm Advisor, Imperial County

Health conscious consumers love the convenience of purchasing fresh, bagged salads. Increasing demand for fresh-cut produce has reached an estimated $12 billion in annual sales with $5 billion attributed to cut, packed salad and vegetables.

This increased demand also increases the chance for foodborne illnesses to occur. Even though millions of people eat tons of fresh-cut greens and other vegetables each year, a single foodborne outbreak can seriously affect the public’s confidence in our food supply and change people’s eating habits. The cost of the 2006 E. coli outbreak in spinach has exceeded $100 million, with sales declining more than 30 percent.

Just as understanding the ecology of insect pests and the organisms that cause plant diseases is crucial to crop management, knowledge of the life systems of foodborne pathogenic bacteria can help us to produce and provide a safe and healthy product for consumers. In general, bacteria are classified according to their genus and species.

For example, Escherichia coli or E. coli and Salmonella enterica or S. enterica. However, because of the diversity among certain species of bacteria it is often necessary to divide the species further into groups known as subspecies, strains, or serovars.

For example the genus Salmonella consists of two species, S. enterica and S. bongori. Diversity lies in the over 2,000 serovars that comprise S. enterica. An example of a serovar is S. typhi, which causes systemic infections and typhoid fever. The complete classification for S. typhi is: Salmonella enterica subsp. enterica serovar typhi.

One reason for subspecies, strains and serovars is that a subspecies of a certain bacterial species may or may not cause disease. Also, many subspecies, serovars or strains of a certain pathogenic bacteria species may cause disease in one host, but not in another.

Typically, E. coli colonizes the gastrointestinal tract of newborns within hours after birth and is the predominant facultative anaerobe in the humans and other warm-blooded animals’ colonic flora. Most strains of E .coli are beneficial; humans and other species cannot survive without them.

However, some strains have evolved to be pathogenic. Pathogenic E. coli have short, hair-like projections or appendages called “fimbriae” on their outer surface that act as a virulence factor by promoting adherence. In their natural environment, pathogenic E. coli use these fimbriae to attach themselves to the gut of an animal. When the animal sheds these bacteria through defecation the life of the bacteria outside its host is limited. However, they do have a survival mechanism. If the bacteria come into contact with alternative host such as a living plant it can use its fimbriae to attach, derive nutrients and multiply.

Salmonella bacteria are very common on raw egg shells, in poultry, and red meat; it is also a natural part of the bacterial flora of reptiles and amphibians. In addition to these common habitats for Salmonella, contaminated water is one of the major sources for contraction of Salmonella illnesses worldwide. Similar to E. coli, Salmonella has the ability to attach itself to plants as a survival mechanism.

Anything that comes into contact with fresh produce has the potential to contaminate it. A major source of microbial contamination of fresh produce is indirect or direct contact with feces. Potential sources of fecal contamination include animals, untreated manure used as a soil amendment, water, infected workers, or conditions in the field or packing facility, such as unclean containers and tools used in harvesting and packing.



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