This work describes a novel phenomenon in which exposure of Escherichia coli to low concentrations of citric acid induces the uptake of exogenous plasmid DNA and transformation to antibiotic resistance. Transformation to ampicillin resistance using pUC18 was observed at an efficiency of 10⁴ cells per microgram DNA. This acid stimulus-triggered acquisition of competency in E. coli was caused not only by citric acid but also by lactic acid, succinic acid, and malic acid. Transformation efficiency was not adversely affected by the washing of E. coli cells with 200 mM NaCl immediately after exposure to citric acid. Thus, DNA uptake can occur concurrently with stimulus by citric acid. The acid stimulus-triggered transformation was investigated using transposon insertion-disrupted E. coli strains. The transformation efficiency of an ompA disruptant decreased to 5% of that of the wild-type parent, suggesting that OmpA is involved in acid stimulus-triggered acquisition of competency in E. coli.