Overexpression of vmeTUV encoding a multidrug efflux transporter of Vibrio parahaemolyticus causes bile acid resistance

We isolated deoxycholate-resistant mutants from Vibrio parahaemolyticus RTM34, which lacks four multidrug efflux transporters belonging to the resistance nodulation cell division (RND) family. RTM34 showed sensitivity to many antimicrobial agents such as cholate and deoxycholate. Deoxycholate-resistant mutants from RTM34 have elevated resistance to not only deoxycholate, but also antibiotics, disinfectants, and dyes. RT-PCR analysis revealed that the expression of vmeV, which encodes an RND-type multidrug efflux transporter, was higher in deoxycholate-resistant mutants than in parental strain RTM34. VPA0806, designated as vdeR, was located upstream of the vmeTUV operon, was oriented in the opposite direction of this operon, and encoded a putative TetR family transcriptional regulator. We determined the nucleotide sequences of vdeR and the vmeT promoter region in the genomes of deoxycholate-resistant mutants. A point mutation was identified in vdeR of seven deoxycholate-resistant mutants and a deletion mutation was identified in vdeR of one deoxycholate-resistant mutant. Since most mutations cause a frame shift mutation and premature stop codon, the function of VdeR is thought to be lost in these mutants. Taken together, the results of the present study indicate that deoxycholate resistance in these mutants was due to the overexpression of vmeTUV caused by a loss in the repression by VdeR.