Escherichia coli (E. coli) is a gram-negative, facultative anaerobic, rod-shaped bacterium that exists as a normal flora in the intestinal tract of animals and humans. Most strains of E. coli are harmless, but some are pathogenic and can cause numerous intestinal or extraintestinal infections. These are referred to as avian pathogenic E. coli (APEC) in poultry. Colibacillosis is the collective term used to describe any localized or systemic infection caused partly or entirely by avian pathogenic Escherichia coli (APEC).
E. coli are commonly found in fecal matter and lower intestinal tract of warm-blooded organisms. Shedding of E. coli in feces makes it abundantly available in the environment. It can be found in water, soil, contaminated food material, and surfaces. Rodents are often carriers of APEC, and serve as a source of contamination to flocks.
APEC strains are often resistant to many antibiotics, including chloramphenicol, sulfonamides, aminoglycosides, tetracyclines, and beta-lactam antibiotics.