Non-susceptibility to at least one agent in all but one or two antimicrobial categories (i.e. bacterial isolates remain susceptible to drugs from at most two classes of antibiotic).
XDR organisms are resistant to more drugs than multidrug-resistant (MDR) microbes and often require use of relatively toxic drugs, high doses of drugs or drug combinations. Acquisition of additional resistance mechanisms has the potential to turn them into pandrug-resistant (PDR) organisms, resistant to all available antibiotics.
Video definition by Marc Mendelson, University of Cape Town (South Africa).