Multidrug efflux pump that probably functions as a Na(+)/drug antiporter. Confers resistance to many drugs such as fluoroquinolones (norfloxacin, ciprofloxacin, enoxacin) and tetraphenylphosphonium ion (TPP) (PubMed:9661020, PubMed:11566977). Also to deoxycholate, doxorubicin, trimethoprim, chloramphenicol, fosfomycin, ethidium bromide and benzalkonium (PubMed:11566977). Also able to export peptides; when overexpressed, allows cells deleted for multiple peptidases (pepA, pepB, pepD and pepN) to grow in the presence of dipeptides Ala-Gln or Gly-Tyr which otherwise inhibit growth (PubMed:20067529). Cells overexpressing this protein have decreased intracellular levels of Ala-Gln dipeptide, and in a system that produces the Ala-Gln dipeptide overproduction of this protein increases export of the dipeptide (PubMed:20067529). Belongs to the multi antimicrobial extrusion (MATE) (TC 2.A.66.1) family. MdtK subfamily.