Part of a tripartite efflux system composed of MdtA, MdtB and MdtC. MdtB forms a heteromultimer with MdtC. Belongs to the resistance-nodulation-cell division (RND) (TC 2.A.6) family. MdtB subfamily. In strain 301 the corresponding sequence has an in-frame codon. The sequence has been checked and is believed to be correct.