According to microbiologist Russell, acid-resistant strains of bacteria have evolved to overcome the protective barrier of the gastric stomach. The ongoing process of natural selection allows organisms with the appropriate genes to survive and multiply where others cannot. This strategy has the potential to control the production of other acid-resistant bacteria, including virulent strains of E. coli that have not yet evolved. He holds the rank of adjunct professor of microbiology at Cornell, and the other authors of the Science report were his students when the feeding studies were conducted. The gastrointestinal tract of animals and man is an ideal habitat for the growth of bacteria, and cell densities can be as high as a trillion cells per gram of digesta.
Most gut bacteria are harmless types, and they can even provide essential nutrients to the host.
When animals consume contaminated food, the native bacteria compete with the invaders and provide at least some protection against food-borne illness. Escherichia coli is a common bacterium in the GI tract, but it is usually outnumbered by other types. E. coli is never a beneficial bacterium, but under normal circumstances the animal and E. coli tolerate each other. Some strains of E. coli, however, are not people-friendly, and these highly virulent forms can cause acute illness or even death. Victims may experience severe cramping and abdominal pain, watery or bloody diarrhea, vomiting or low-grade fever for an average of eight days.