What is the difference between MDR, XDR, and PDR?

In the international interim standard definitions, multidrug-resistant (MDR) means non-susceptibility to at least one agent in three or more antimicrobial categories, extensively drug-resistant (XDR) means non-susceptibility to all but one or two categories, and pandrug-resistant (PDR) means non-susceptibility to all agents in all categories tested.

What does ESKAPE stand for?

ESKAPE is a mnemonic for a group of pathogens that frequently cause resistant infections: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species.

Clinically Significant Multidrug-Resistant Pathogens

Clinically significant multidrug-resistant (MDR) pathogens are bacteria that have acquired resistance to several classes of the antimicrobials normally used to treat them, narrowing therapeutic options and complicating the care of common infections. This area orients the reader to the small group of organisms that drive most of the clinical and public-health burden of antimicrobial resistance, the standardized definitions used to classify them, and the mechanisms by which they evade multiple drug classes.

Definition

A multidrug-resistant pathogen is an organism that is non-susceptible to at least one agent in three or more antimicrobial categories, per the international interim standard definitions; extensively drug-resistant and pandrug-resistant denote progressively broader non-susceptibility.

Scope

The area covers the definition and tiering of resistance (multidrug-resistant, extensively drug-resistant, and pandrug-resistant), the priority pathogens emphasized by international bodies, and the genetic and biochemical routes to multi-class resistance. It groups the detailed pathogen-level entries beneath it as a reference taxonomy and treats resistance descriptively; it is not a source of regimen selection or dosing guidance.

Sub-topics

Core questions

How are multidrug-, extensively drug-, and pandrug-resistant phenotypes defined and distinguished?
Which pathogens account for most of the clinical burden of antimicrobial resistance, and why?
By what genetic and biochemical mechanisms do bacteria become resistant to several drug classes at once?
How do surveillance and standardized definitions support comparison of resistance across settings?

Key concepts

Multidrug resistance (MDR)
Extensively drug-resistant (XDR) and pandrug-resistant (PDR) phenotypes
Antimicrobial category-based definitions
ESKAPE pathogens
WHO priority pathogen list
Horizontal gene transfer and mobile genetic elements
Intrinsic versus acquired resistance

Mechanisms

Multi-class resistance arises through enzymatic inactivation of drugs (for example beta-lactamases and carbapenemases), modification or protection of the drug target, reduced intracellular accumulation via porin loss and efflux pumps, and acquisition of alternative targets. Many of these determinants are carried on plasmids, transposons, and integrons that move between strains and species by horizontal gene transfer, allowing several resistance traits to accumulate on a single mobile element and spread rapidly. Standardized, category-based definitions let laboratories classify isolates consistently as multidrug-, extensively drug-, or pandrug-resistant.

Clinical relevance

Multidrug-resistant pathogens are central to infection prevention, diagnostic stewardship, and antimicrobial stewardship because they limit the agents likely to be effective and are associated with worse outcomes and higher costs. The grouping describes how resistance is classified and why certain organisms are prioritized; it is educational reference material and not a basis for individual diagnostic or treatment decisions.

Epidemiology

A short list of organisms, often summarized by the ESKAPE acronym and reflected in the WHO priority pathogen list, accounts for a disproportionate share of resistant healthcare-associated infections worldwide. International expert groups have published interim standard definitions and priority rankings to harmonize surveillance and direct research toward the pathogens posing the greatest threat.

History

As resistance accumulated across multiple antibiotic classes in the late twentieth and early twenty-first centuries, the field moved from describing single-drug resistance to needing shared definitions for multi-class phenotypes; an international expert group published interim standard definitions in 2012, and the World Health Organization released a global priority pathogen list in 2017 to focus antibiotic research and development.

Debates

How should resistance phenotypes be defined and ranked for surveillance?: Category-based definitions of multidrug-, extensively drug-, and pandrug-resistance and priority-pathogen rankings improve comparability, but agents tested and category lists differ across settings and evolve with new drugs, so classifications require periodic revision.

Seminal works

magiorakos-2012
tacconelli-2018

Frequently asked questions

What is the difference between MDR, XDR, and PDR?: In the international interim standard definitions, multidrug-resistant (MDR) means non-susceptibility to at least one agent in three or more antimicrobial categories, extensively drug-resistant (XDR) means non-susceptibility to all but one or two categories, and pandrug-resistant (PDR) means non-susceptibility to all agents in all categories tested.
What does ESKAPE stand for?: ESKAPE is a mnemonic for a group of pathogens that frequently cause resistant infections: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species.