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Applied and Environmental Microbiology, July 1999, p. 3048-3055, Vol. 65, No. 7
Microbiology Department, Unilever Research
Colworth, Sharnbrook, Bedfordshire MK44 1LQ,
England,1 and TEAGASC, National Dairy
Products Research Centre, Moorepark, Fermoy, County Cork,
Ireland2
Received 17 September 1998/Accepted 15 April 1999
Survival of a nontoxigenic isolate of Escherichia coli
O157:H7 at low pH (pH 3.0) was examined over prolonged time periods for
each of three population types: exponential-phase cells,
stationary-phase cells, and acid-adapted exponential-phase cells. In
each population, approximately 5 × 104 CFU
ml
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Survival of Low-pH Stress by Escherichia coli O157:H7:
Correlation between Alterations in the Cell Envelope and Increased
Acid Tolerance
1 were detected after a 24-h incubation at pH 3.0. Even
after 3 days at pH 3.0, significant numbers of survivors from each of the three populations could be detected. The high level of acid tolerance exhibited by these survivors was found to be quickly lost
once they were transferred to conditions which permitted growth to
resume, indicating that they were not mutants. Proton flux measurements
on the three populations of cells revealed that the initial rates of
viability loss at pH 3.0 correlated well with net proton accumulation.
Cells showing a high initial rate of viability loss (exponential-phase
cells) accumulated protons at the highest rate, whereas resistant
populations (adapted or stationary-phase cells) accumulated protons
only slowly. Differences in the protein composition of the cell
envelope between the three populations were studied by two-dimensional
polyacrylamide gel electrophoresis. Complex differences in the pattern
of proteins expressed by each population were uncovered. The
implications of these findings are discussed in the context of a
possible model accounting for acid tolerance in this important
food-borne pathogen.
*
Corresponding author. Present address: Department of
Molecular and Cell Biology, Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, Scotland. Phone: (44 1224) 273151. Fax: (44 1224) 273144. E-mail:
mbi106{at}abdn.ac.uk.
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