Inhibition of Escherichia coli K12 by short-chain organic acids: lack of evidence for induction of the SOS response.

Sublethal concentrations of formic acid (10 mmol/l) and propionic acid (5 mmol/l) at pH 5.0 preferentially inhibit DNA synthesis and stop cell multiplication in the absence of a corresponding cessation in the increase of culture turbidity. The possibility that the acids induce the SOS response by starving cells of thymine or by causing physical damage to the DNA molecule has now been investigated. Accumulation of thymine into the cytoplasm of whole cells was not inhibited by either acid. Mutants defective in excision repair (uvr A6), recombination repair (rec A56) and polymerase activity (pol A1) were not more sensitive to the acids than their isogenic parent. No significant increase in cell length was observed from measurements of transmission electron microscope images of acid-treated cells. It is concluded, therefore, that sublethal concentrations of formic and propionic acid inhibit DNA synthesis without physically damaging DNA molecule, or starving the cell of essential thymine or otherwise inducing an SOS response.