Identification of Genes Required for Growth ofEscherichia coliMG1655 at Moderately Low pH.
The survival of some pathotypes ofEscherichia coliin very low pH environments like highly acidic foods and the stomach has been well documented and contributes to their success as foodborne pathogens. In contrast, the ability ofE. colito grow at moderately low pH has received less attention, although this property can be anticipated to be also very important for the safety of mildly acidic foods. Therefore, the objective of this study was to identify cellular functions required for growth of the non-pathogenic strainE. coliMG1655 at low pH. First, the role of the fourE. coliamino acid decarboxylase systems, which are the major cellular mechanisms allowing extreme acid survival, was investigated using mutants defective in each of the systems. Only the lysine decarboxylase (CadA) was required for low pH growth. Secondly, a screening of 8544 random transposon insertion mutants resulted in the identification of six genes affecting growth in LB broth acidified to pH 4.50 with HCl. Two of the genes, encoding the transcriptional regulator LeuO and the elongation factorP-β-lysine ligase EpmA, can be linked to CadA production. Two other genes, encoding the diadenosine tetraphosphatase ApaH and the tRNA modification GTPase MnmE, have been previously implicated in the bacterial response to stresses other than low pH. A fifth gene encodes the LPS heptosyltransferase WaaC, and its mutant has a deep rough colony phenotype, which has been linked to reduced acid tolerance in earlier work. Finally,tatCencodes a secA-independent protein translocase that exports a few dozen proteins and thus is likely to have a pleiotropic phenotype. FormnmE, apaH, epmA, andwaaC, de novo in frame deletion and genetic complementation confirmed their role in low pH growth, and these deletion mutants were also affected in growth in apple juice and tomato juice. However, the mutants were not affected in survival in gastric simulation medium at pH 2.5, indicating that growth at moderately low pH and survival of extremely low pH depend mostly on different cellular functions.
Laboratory of Food Microbiology, Department of Microbial and Molecular Systems, and Leuven Food Science and Nutrition Research Centre (LFoRCe), KU Leuven Leuven, Belgium.
Bram
B
Vivijs
Laboratory of Food Microbiology, Department of Microbial and Molecular Systems, and Leuven Food Science and Nutrition Research Centre (LFoRCe), KU Leuven Leuven, Belgium.
Abram
A
Aertsen
Laboratory of Food Microbiology, Department of Microbial and Molecular Systems, and Leuven Food Science and Nutrition Research Centre (LFoRCe), KU Leuven Leuven, Belgium.
Chris W
CW
Michiels
Escherichia coli; acid stress; acidic foods; genetic analysis; stress tolerance genes