The uropathogenic Escherichia coli strain 536 carries two pathogenicity islands, each of which is associated with either of the tRNA genes selC or leuX, respectively. Growth competition in CD-1 mouse mucus between the wild-type strain E. coli 536, its leuX mutant 536v102 and its mutant 536R3, lacking both pathogenicity islands but expressing a functional tRNA S veu , revealed a major impact of leuX on E. coli survival in bladder mucus. The impaired survival in CD-1 mouse mucus observed upon deletion of the leuX gene was abolished after complementation with the leuX gene. The survival of bacteria in bladder mucus was not influenced by the presence of pathogenicity islands I and II. z 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V.