The Escherichia coli asr gene and its orthologues in the genomes of other members of the family Enterobacteriaceae represent stress response elements regulated by a low environmental acidity (pH < 5.0) [1]. They code for small basic 82–139 amino acid proteins of unknown function. While important for exponential phase E. coli growth in liquid culture at pH 4.5, asr is also required for bacterial colonization of the lower intestine of the animal host under competitive pressure and for competition in long-term culture [2]. Here we demonstrate that the Enterobacter cloacae asr gene is able to restore the colonization deficiency of the E. coli asr mutant under competition in the mouse model. E. cloacae Asr protein shows a 76% amino acid identity to E. coli Asr and undergoes proteolytic processing induced by acid stress. These observations imply that the asr gene function is conserved through Enterobacteriaceae and is important for animal host colonization.

Keywords: Enterobacter cloacae, asr gene, mouse gastrointestinal tract colonisation