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 he molytic uremic syndrome, can survive in a highly acidic environment. The ac id resistance of this organism, as measured by its ability to survive in lo w pH, depended on the density of the cells present during the assay. At low cell densities (less than or equal to 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. Exponentia l phase cultures did not exhibit the cell density effect, The increased aci d sensitivity at high cell densities was absent in the stationary phase cul tures of a rpoS mutant (rpoS::pRR10) of an E. coil O157:H7 strain. Cell den sity dependent acid sensitivity of the stationary phase cultures was also o bserved in other enterohemorrhagic E. coli and Shigella strains. The increa sed acid sensitivity at high cell densities was absent in Gram-positive org anisms. (C) 1999 Published by Elsevier Science B.V. All rights reserved.