Abstract
The present study has attempted to determine the colonization resistance (CR) of the digestive tract by biotyping Enterobacteriaceae in four faecal samples per subject of five different animal species as well as man. The results indicate that the degree of bacterial contamination with Enterobacteriaceae from the environment may strongly influence the outcome. Both conventionally living chicken and man, showed a much wider range of the 'confidence limits of the mean' of the mean number of biotypes per faecal sample between individual subjects, than was found between subjects maintained under laboratory circumstances. Yet there appeared a statistically significant difference in CR between some of the animal species as a group. Man did not differ from monkeys, however, both differed from the rodents species studied. Monkeys differed also from dogs and the latter from rodents. It is concluded that the CR measured by determining the mean number of biotypes of Enterobacteriaceae can only be used for accurate comparison of the CR between subjects, if the 'bacteriological environment' is known; i.e. the sources of contamination.