Cell density dependent acid sensitivity in stationary phase cultures of enterohemorrhagic Escherichia coli O157:H7.

Escherichia coli O157:H7, the causative agent of hemorrhagic colitis and hemolytic uremic syndrome, can survive in a highly acidic environment. The acid resistance of this organism, as measured by its ability to survive in low pH, depended on the density of the cells present during the assay. At low cell densities (</=2 x 10(7) ml(-1)), about 100% of the stationary phase cells survived in Luria broth pH 2.5 at 37 degrees C for at least 7 h. The same cultures at high cell densities (2-5 x 10(9) ml(-1)) were about 1000-fold more sensitive under identical conditions. Exponential phase cultures did not exhibit the cell density effect. The increased acid sensitivity at high cell densities was absent in the stationary phase cultures of a rpoS mutant (rpoS::pRR10) of an E. coli O157:H7 strain. Cell density dependent acid sensitivity of the stationary phase cultures was also observed in other enterohemorrhagic E. coli and Shigella strains. The increased acid sensitivity at high cell densities was absent in Gram-positive organisms.