E. coli and other food-borne bacteria (like salmonella and campylobacter) are tasteless. Contaminated food gives no obvious sign of the single-celled organisms that lurk there, waiting to colonize your body. This raises an excellent question: If dangerous bacteria don’t leave obvious signs, what’s making that week-old package of ground beef smell so bad?
The answer is spoilage bacteria, a family of bacteria that thrives on just about any food. As these bacteria replicate, they coat your food with slime. The waste products they leave behind cause the objectionable changes in smell and taste. However, for all their obvious repulsiveness, eating them probably won’t make you sick. That’s because spoilage bacteria is ideally suited to the world of decaying grocery produce, not the high acid environment of your stomach.
So does this mean that you can add rotten food to your dinner table without harm? Well, not quite. As spoilage bacteria break down food, other organisms hitch a ride. Molds quickly join the party (for example, the fuzzy green fur on forgotten salami). Some are dangerous and create poisonous substances that permeate food, and they remain even after cooking.
Furthermore, if conditions are ideal for spoilage bacteria, it’s a safe bet that they’re good for pathogenic bacteria, too. In other words, if your food is spoiled, it’s also more likely to hold a teeming population of pathogenic bacteria, and therefore to pose a greater health risk.
Source of Information : Oreilly - Your Body Missing Manual