Safe Food_ Bacteria, Biotechnology, and Bioterrorism - Marion Nestle [29]
Twenty years ago, three of today’s worst bacterial pathogens—Campylobacter, Listeria, and E. coli O157:H7 (described below)—were not recognized as hazards. Also new are bacteria capable of flourishing under refrigeration (Yersinia and Listeria) or acidic or dry conditions (E. coli O157:H7). The alarming survival features of such bacteria undoubtedly evolved in response to changes in methods of food production and distribution that select for the hardiest bacteria and encourage their wide dispersal. Whereas undercooked hamburger and ground beef products used to be the only known source of E. coli O157:H7, other foods cross-contaminated by exposure to infected cattle or meat are now involved: apple cider, sprouts, and any number of vegetables. Outbreaks of the especially virulent Salmonella enteritidis used to be restricted to eggs; now they have been traced to carriers as unlikely as tomatoes, melons, and orange juice. As we examine the societal and commercial forces that foster these unwelcome trends, we need to understand a bit more about one of the three newly emergent pathogens, E. coli O157:H7.
Introducing E. coli O157:H7
E. coli O157:H7 merits special attention not only because of its exceptional virulence but also because it illustrates so well how changes in the food system and in society provide new opportunities for spreading microbial disease through food. When I first encountered the more common form of E. coli in a college biology class, instructors presented it as a harmless inhabitant of the digestive tracts of animals and humans, spread by accidental transfer of excreted material. It was known best as an indicator of fecal contamination of water supplies; if water supplies contained E. coli, they were likely to contain more dangerous bacteria. We now know much more about the biology of this organism. Like many bacteria, E. coli is able to accept genes from related bacterial species to form “stable variants” that can pass the borrowed genes along to other bacteria as they divide and multiply (see appendix). The E. coli variant known as O157:H7 is especially dangerous; at some point, it picked up a Shigella gene for a toxin that destroys red blood cells and induces a syndrome of bloody diarrhea, kidney failure, and death. This toxin is particularly damaging to young children.16
Other features of the O157:H7 variant are also noteworthy. Unlike common E. coli, this type resists heat; it grows at temperatures up to 44°C (111°F). It also resists drying, can survive short exposures to strong acid (pH 2.5), and sometimes resists radiation and antibiotics. For these reasons, controlling it is not easy. Worse, E. coli O157:H7 is infectious at very low doses. The normal digestive tract contains hundreds of billions of bacteria that compete for space and nutrients. In this environment, it takes thousands of Salmonella to induce symptoms, but the lowest infectious dose of E. coli O157:H7 appears to be less than 50—a minuscule number in bacterial terms. Control measures, therefore, must do more than just prevent growth; they must eliminate the very presence of these bacteria. Foods containing E. coli O157:H7 must be cooked at temperatures high enough to kill all of them. Table 4 presents recommendations for food-handling techniques to prevent problems with this microbe.
E. coli O157:H7 infections originate with farm animals, and such animals increasingly harbor this variant. Although earlier studies suggested that perhaps 10% of adult ruminant (cud-chewing) animals—mainly cows and cattle—were infected with E. coli O157:H7, the proportion now is as high as 28%, and may exceed 40% in slaughtered animals not yet processed. Young infected animals exhibit mild diarrhea, but most do not appear sick and go untreated. Deer, sheep, goats, dogs, birds, and flies also harbor the variant, almost certainly because they have come