The expression of gadA and gadB, which encode two glutamate decarboxylases (GADs) of Escherichia coli, is induced by an acidic environment and participate in acid resistance. In this study, we constructed a polyamine-deficient mutant and investigated the role of polyamines in acid resistance. The expression of gadA and gadB was shown to be dependent on polyamines. For that reason, the polyamine-deficient mutant was completely devoid of GAD activity and was very susceptible to low pH if large amount of polyamines were not provided. We also showed that the polyamine-deficient mutant contained higher cAMP than the isogenic polyamine-proficient wild type and cAMP negatively regulated the expression of gadA and gadB. Therefore, introduction of the cya (encoding adenylate cyclase) mutation allele into the polyamine-deficient mutant resulted in the increment of GAD activity and thus restored the reduced acid resistance of the mutant. GAD activity. The positive regulators, H-NS (histone-like protein, encoded by hns gene) and RpoS (alternative RNA polymerase s subunit, encoded by rpoS gene), also significantly governed the expression of gadA and gadB respectively, but polyamines did not regulate either the intracellular H-NS level or rpoS expression in this culture condition. These results strongly suggest that there are at least two different regulatory systems in acid resistance, one is positive regulation via a H-NS/RpoS system and the other is negative regulation via a polyamine/cAMP system.