Enterobacteriaceae associated with animals in health and disease

Members of at least eight of the 14 general of the Enterobacteriaceae are found associated with animals. A large number of these are present as members of the microbiota of the gut of many animals without causing harmful effects. Indeed, their presence as part of the stable flora contributes to the defences of the animal by excluding enteric pathogens by competitive exclusion which contributes to the phenomenon of [colonization resistance' (van der Waaij 1983). some of these strains can cause disease if they contaminate a normally 'sterile' site such as the body cavities. In contrast, particular strains within each bacterial species are overt pathogens and are regularly associated with disease. Nevertheless, even these may colonize the animal without producing clinical illness. In this paper E. coli and Salmonella spp. have been used to illustrate the various aspect of host-parasite relationships.