A species of gram-negative, facultatively anaerobic, rod-shaped bacteria (gram-negative facultatively anaerobic rods) commonly found in the lower part of the intestine of warm-blooded animals. It is usually nonpathogenic, but some strains are known to produce diarrhea and pyogenic infections. Pathogenic strains (virotypes) are classified by their specific pathogenic mechanisms such as toxins (enterotoxigenic escherichia coli), etc.