When Theodor Escherich (1885a, b) first describedEscherichia colihe looked on it as a saprophytic organism. Soon several investigators found that colibacteria could be isolated from intestinal infections and from many infections outside the intestine, like urinary tract infections (UTI), cholecystitis, wound infections, meningitis, septicaemia, pulmonary infections, and many more. Uhlenhuth (1897) showed that coli strains from pathological processes were more pathogenic in animal experiments than strains isolated from the normal intestine. Smith (1927), who examined strains from white scours in calves, showed that spontaneous acapsular mutants could be obtained from certain colibacteria, and that such mutants were less virulent when injected intra-peritoneally into guinea-pigs.