Enterohemorrhagic E. coli O157:H7 gained great attention as a foodborne pathogen in 1982 after an outbreak from contaminated hamburgers in a fast-food chain. Although anyone can get ill from ingesting even low levels of this organism, the very young and the elderly are at greater risk for developing hemolytic uremic syndrome (HUS) as a result of an E. coli O157:H7 infection. HUS is a severe illness that can lead to permanent loss of kidney function and even death. According to estimates by the Centers for Disease Control (CDC), E. coli O157:H7 is the cause of 73,000 cases of shigatoxigenic E. coli (STEC) infections and 61 deaths each year in the United States.
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In 2002, the USDA’s Food Safety and Inspection Service (FSIS) mandated that beef plants reexamine HACCP plans and implement necessary measures to eliminate or significantly reduce the risk of E. coli O157:H7 in their products. FSIS also enhanced training programs for its inspectors and compliance officers, toughened its enforcement policies and held a series of workshops for small and very small plant operators.
It appears that these interventions have been effective. Data from the FSIS shows that the E. coli O157:H7 positive samples collected between 2000 and 2004 declined by more than 80 percent.1 Recalls have also shown a steady decline from 21 in 2002 to only six in 2004.1 And, most importantly, the CDC reported a 40 percent reduction in human illnesses associated with E. coli O157:H7 in 2005.2
Despite these encouraging results, we should not be lulled into a false sense of security. Improved prevention methods are a crucial part of the food safety solution, but they go hand-in-hand with reliable testing for the potentially deadly pathogen. Recent research indicates that E. coli O157:H7 is more complex and variable than previously understood, and its elusive nature challenges detection by the testing approaches in routine use today.
What Detection Methods Are Available?
Differences between E. coli O157:H7 and other types of E. coli can be detected directly by looking at the DNA or indirectly by looking at the products of the genes.
Polymerase chain reaction (PCR) is an example of a direct genetics-based approach that tests for actual fragments of target DNA. Lateral flow devices and other enzyme-linked methods rely on antigen-antibody responses that produce a chromogenic or fluorogenic signal. Culture methods look for color distinctions on plating media arising from E. coli O157:H7’s inactive b-glucaronidase, its inability to ferment sorbitol and/or its resistance to telluride.
The difficulties encountered in testing for E. coli O157:H7 are due to its highly dynamic nature, where the DNA readily undergoes change. This variability affects both the genetic structure and the genetic products.
Antigen-Antibody Reaction Problems
Sometimes this variability can affect the antigens that were used to name the pathogen: the O157 antigen on the cell surface and the H7 antigen on the flagella. Certain DNA changes within the organism can “turn off” expression of the H7 antigen and the production of flagella. The resulting variants are called O157:HNM (H-Non-Motile), and they have been explicitly included in the USDA food testing regulations.
A more recent discovery is that the same thing can happen with the O157 antigen on the cell surface. As first reported in 1998,3 the organism can undergo a genetic change that turns off expression of the O157 antigen. The resulting variants in this case are called “rough.” Rough isolates can be invisible not only to antigen-antibody tests, but they may also be missed by enrichment or confirmation methods that rely on antigen-mediated schemes such as antibody-coated bead capture for cell concentration prior to analysis.
These rough variants are not explicitly regulated by the USDA, but there is no reason to presume that they differ significantly in pathogenic potential from those that express O157. In fact, rough isolates have been isolated from clinical cases.4 In 2003, the USDA animal research center examined 1,697 E. coli O157:H7 isolates from cattle.5 They determined that approximately 10 percent were non-motile (did not express the H7 antigen) and approximately 1 percent were rough (did not express the O157 antigen).
Biochemical Reaction Problems
Biochemical or culture-based methods are also stymied by this organism. Because E. coli O157:H7 is believed to be telluride-resistant, many plating methods include telluride as a selective agent. (The T in CT-SMAC stands for telluride.) Although field-derived data is lacking, perhaps because telluride sensitivity precludes isolation with current plating procedures, data from laboratory strains indicate that telluride-sensitive isolates do exist.6