The effect of intestinal colonisation by a strain on the establishment in the gut of an isogenic mutant administered orally 24 h after the first strain was studied in gnotobiotic pigs. Irrespective of the clinical outcome of the infection, the extensive colonisation of one strain prevented a similar degree of colonisation by an otherwise isogenic antibiotic resistant strain; in some cases the second strain was hardly detectable. The poor colonisation of the challenge strains was generally reflected in very low counts of organisms in the tissues. Colonisation by a strain of reduced the rate of establishment of an isogenic , strain but did not prevent colonisation by an Typhimurium strain. Typhimurium with mutations in the (serine chemotaxis receptor protein) or (transcriptional regulator of anaerobic metabolism) genes did not inhibit colonisation. Mutations in (adenylate cyclase), and (chemotaxis receptor proteins for aspartate and ribose respectively) genes were less inhibitory, while (non-motile) and (impaired motility) mutants were fully inhibitory.


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