AIMS Oenococcus oeni is a lactic acid bacterium used in wine fermentation. Two open reading frames (orfB and orfC) were identified in the upstream region of the hsp18 gene, encoding the small heat-shock protein Lo18. Expression of these genes in conditions of acid stress was studied in Escherichia coli. METHODS AND RESULTS Sequence analysis showed that orfB encodes a putative transcriptional regulator of the LysR family. The protein encoded by orfC shares homologies with multi-drug resistance systems. Heterologous expression of orfB, orfC and hsp18 genes in Escherichia coli significantly enhanced the viability of the host strain under acidic conditions. CONCLUSION It was demonstrated that the three genes were needed for acquisition of this acid tolerance phenotype. SIGNIFICANCE AND IMPACT OF THE STUDY Heterologous expression of Oenococcus genes could be used to confer acidophilic behaviour on strains of biotechnological interest.