Spoilage bacteria turn last week's roast chicken into a scene from Zombie Flesh Eaters. Like the undead, they're everywhere (air, soil, water, plants, and animals), so the invasion of your groceries is pretty much inevitable. Evolved to consume corpses and dead plants, which are customarily served cool, they're the dominant bacteria in your 35-40-degree fridge. (Conversely, temperatures above 85 degrees enervate them.) The stench is from the breakdown of amino acids into amines, which include the evocatively named cadaverine, putrescine, and spermidine. As repulsive as they are, only one, histamine, has been linked to negative health effects, and that's just for people who have allergies to it or who eat certain kinds of improperly stored fish. Spoilage bacteria are harmless.
Pathogenic bacteria make you wish you could exchange your Caribbean bungalow for a hospital room with an IV drip and a bedpan. The preferred habitat of salmonella, E. coli, and campylobacter—three moderately serious foodborne illnesses—is the nutrient-packed guts of warm-blooded animals, but these also do well at room temperature. The journey to your plate usually starts at the slaughterhouse or meat processing facility, where a minute amount of animal poop gets on your future burger. So long as the meat's under refrigeration, that's no big deal, since cold inhibits the proliferation of pathogens (most people can fight off low doses). But then—maybe at home, more likely at a food-service establishment—the ground beef sits out for several hours, allowing the bacteria to multiply. As a finishing touch, you order your patty medium rare, so the center doesn't get hot enough to kill its microscopic passengers. It smells good, looks good, tastes good. (Pathogenic bacteria provide no sensory clues as to their presence in food.) But hours to days later: stomach cramps, nausea, diarrhea, or worse.
Need more persuasion before you tuck into the green-streaked, two-week-old steak buried in your meat compartment? Consider this: We're lethal—at least to the decomposers. If the 98.6-degree heat doesn't get them ( an adaptation by warm-blooded animals to elude the fungal infections endemic to the rest of the animal and plant kingdoms), a dousing in stomach acid—pH 1-2, also good for tanning leather and sterilizing pools—will. And should a few spoilage bacteria survive to venture further downstream, they face almost certain death by a squad of intestinal immune cells. Pathogens, by contrast, are as well-prepared for this treacherous terrain as a USAF special op. They strike without warning. In the stomach, they may become temporarily acid-tolerant. In the small intestine, some can bind to and disrupt immunological command centers. And then there's their clever strategy for meeting new hosts. Excreted poisons or the infection itself make you feel very, very sick, triggering a release of bodily effluvia. Greetings, family and friends!
The morning after the dinner party, I called Hilary to see how she thought it had gone. Subtext: Were there any sudden deaths, ER visits, or irate phone calls from gastrointestinally afflicted guests? "It was just lovely," she said in her Irish lilt. "Amita even called to say what a good time she'd had—and to ask me to get her your gumbo recipe!"