FACTORS AFFECTING COLONIZATION of the gut by bacteria have been considered in broad outline in a review by Savage (1977). Greater insight into the interactions between bacterial species and their environment may be gained by a detailed appraisal of what is known about a single species. Escherichia coli is an obvious candidate. Its population, of between lo5 and lo8 cells/g faeces is but a small proportion of a total faecal bacterial population in excess of 1O"/g (Moore et al. 1975) and yet it is usually considered to be a typical and constant member of the gut flora. The commensal Esch. coli are residents of the large intestine although there is argument as to whether their normal habitat is the lumen (Maier et al. 1972) or whether they are more typically adsorbed to the intestinal mucosa (Gorbach 1971). In contrast, the pathogenic strains are distinguished by their ability to attach to the wall of the small intestine (Smith & Linggood 1971), to produce enterotoxin or to invade epithelial cells (Dupont et al. 1971).

From an ecological rather than an anthropocentric standpoint the commensal Esch. coli strains are considerably more important than the pathogenic strains. Certainly in terms of numbers they are more successful and there appears to be only a single report in the literature of an individual found to be frequently lacking coliforms (Gerhardt & Iglewski 1976). Despite its relatively small proportion Esch. coli has a considerable effect upon man both directly and indirectly. As one of the major group of facultative