The isolation and description of Bacillus coli commune by Escherich a hundred years ago marked the start of a series of scientific investigations which have led to some of the most important discoveries in microbial pathogenicity and genetics that have been made since that time. It is not difficult to find the reasons why so much effort has been concentrated on this organism. Escherichia coli is present in the gut of all warm-blooded animals generally forming the predominant aerobic flora; it is of medical and veterinary importance being responsible for a variety of infections in the human and animal populations and it has provided a useful tool for geneticists.