A mathematical model of human micturition dynamics and bacterial growth predicts the population growth rate required for a bladder infection to become established in the absence of adhesin-mediated surface growth. Escherichia coli strains isolated from the urinary tract have significantly higher in vitro growth rates in urine than strains isolated from the intestinal flora. The results suggest that, for E. coli isolated from the urinary tract, adhesin-mediated surface growth may not be required for infections to become established and persist. The growth-rate differences observed between urinary tract and intestinal isolates suggests that the ability to survive and efficiently utilize the resources available in urine is an important adaptation for E. coli inhabiting the urinary tract.