Aims: To determine whether horC confers beer-spoilage ability and to evaluate the validity of horC as a trans-species genetic marker for differentiating the beer-spoilage ability of lactic acid bacteria (LAB). Methods and Results: Hop-sensitive Lactobacillus brevis ABBC45(cc) was transformed with an expression plasmid, pHYchorBC, containing putative multidrug resistance gene horC and its putative regulator horB, and the transformant was designated as ABBC45(cc)/pHYchorBC. As a control, ABBC45(cc) was transformed with pHYchorB that contains horB, and the transformed strain was designated as ABBC45(cc)/pHYchorB. As a result of beer-spoilage assay of these transformants, ABBC45(cc)/pHYchorBC exhibited beer-spoilage ability, whereas ABBC45(cc)/pHYchorB did not. Furthermore ABBC45(cc)/pHYchorBC showed higher hop resistance than ABBC45(cc)/pHYchorB, accounting for the differences in beer-spoilage ability observed between the two transformants. ABBC45(cc)/pHYchorBC also exhibited higher resistance to various structurally unrelated drugs, compared with ABBC45(cc)/pHYchorB. Conclusions: horC was shown to confer hop resistance and beer-spoilage ability on ABBC45(cc) by presumably encoding a multidrug transporter. Significance and Impact of the Study: The finding that horC plays an important role in hop resistance and beer-spoilage ability supports the validity of horC as a trans-species genetic marker for differentiating the beer-spoilage ability of LAB.