As little is known about the genes involved in the induction of an acid tolerance response in Listeria monocytogenes, the role of the F0F1-ATPase was analyzed as a consequence of its role in the acid tolerance of a number of other bacteria and its conserved nature. It was found that acid adapted cells treated with N,N'-dicyclohexylcarbodiimide (DCCD) exhibited greatly enhanced sensitivity to low pH stress. Degenerate primers were designed to amplify and sequence a portion of the atpD gene. Subsequently, a PCR product from atpA to atpD was identified. While we were unable to create a deletion in the atpA gene, the plasmid pORI19 was inserted in a region between atpA and atpG to reduce, rather than eliminate, expression of the downstream genes. As expected this mutant displayed enhanced resistance to neomycin and exhibited slower growth than the wild type strain. This mutant could still induce an acid tolerance response and remained susceptible to DCCD treatment, but its relative acid sensitivity was difficult to assess as a consequence of its slow growth. (C) 2000 Elsevier Science B.V. All rights reserved.