Identification of Genes Required for Growth of Escherichia coli MG1655 at Moderately Low pH

Abstract

The survival of some pathotypes of Escherichia coli in 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 of E. coli to 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 strain E. coli MG1655 at low pH. First, the role of the four E. coli amino 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 factor P-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, tatC encodes a secA-independent protein translocase that exports a few dozen proteins and thus is likely to have a pleiotropic phenotype. For mnmE, apaH, epmA, and waaC, 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.

Non-canonical activation of OmpR drives acid and osmotic stress responses in single bacterial cells

“…1c, d ), suggesting that cadBA was the major system that eliminated protons upon acid stress in E. coli over the approximate pH i range of 6.1–6.5. This was not surprising, since the glutamate and arginine decarboxylation systems exhibit much lower pH optima (pH 4 and 5, respectively) 20 , compared to lysine (pH 6.1–6.5) 21 , S-adenosylmethionine decarboxylase (pH 7.4) 22 and ornithine decarboxylase systems (pH 7.0) 23 . Furthermore, the pH i response of the cadBA null strain was similar to the wild type (Fig.…”

A New Role of OmpR in Acid and Osmotic Stress in Salmonella and E. coli

“…In E. coli , the intracellular pH was less acidic and OmpR regulated different pathways. In E. coli , OmpR represses speF , the ornithine decarboxylation system, which has a higher pH optimum of 7 ( Vivijs et al, 2016 ) compared to the glutamate and arginine decarboxylation systems (pH optima 4 and 5, respectively) ( Bearson et al, 2009 ). Normally, ornithine decarboxylase decarboxylates arginine, consumes protons and produces putrescine, which allows for recovery from acidification.…”

Sublethal concentrations of undissociated acetic acid may not always stimulate acid resistance in Salmonella enterica sub. enterica serovar Enteritidis Phage Type 4: Implications of challenge substrate associated factors

“…Typhimurium was protected when inoculated into ground beef and boiled egg white but not when rice was used. In general, discrepancies between phenotypes in culture media and food matrices has been found by other authors as well [ 56 ].…”