HOP-RESISTANT LACTOBACILLUS-BREVIS CONTAINS A NOVEL PLASMID HARBORINGA MULTIDRUG RESISTANCE-LIKE GENE

The bitter-tasting compounds derived from the flowers of the hop plant (Humulus lupulus L.) protect beer from bacterial spoilage. However, a few lactic acid bacteria, especially lactobacilli, are resistant to t hese compounds and sometimes cause serious spoilage in the beer indust ry. It is important to elucidate the mechanisms of hop-resistance in l actic acid bacteria. We selected mutants of Lactobacillus brevis resis tant to high concentrations of the hop compounds. The parental strain, L. brevis ABBC45, carries several plasmids. The copy number of one pl asmid, termed pRH45, was remarkably increased in one of the hop-resist ant mutants compared with that in the wild-type strain. pRH45 (15.0 kb ) contains an open reading frame of 1749 b, termed horA, the deduced p rotein of which includes six putative transmembrane domains and an ATP -binding domain. The amino acid sequence of this putative protein is s ignificantly homologous to half molecules of a mammalian multidrug res istance gene product, P-glycoprotein, and to several bacterial ABC tra nsporters. Furthermore, the hop-resistant mutant was found to be weakl y resistant to novobiocin and ethidium bromide, which are structurally and functionally unrelated to the hop compounds. A possible role of t he potential drug efflux pump gene in the hop-resistance of L. brevis is discussed.