Resistance to antimicrobial agents is a growing and worrying phenomenon. Extensively drug-resistant bacteria, or XDR bacteria, are a type of multidrug-resistant organisms that are resistant to almost all or all approved antimicrobial agents.
The origin of XDR bacteria
The term was first coined in response to Mycobacterium tuberculosis that was resistant to first-line agents (isoniazid and rifampicin), a fluoroquinolone, and to one or more second-line parenteral drugs (amikacin, kanamycin, and capreomycin). This was later expanded to include other bacteria that were resistant to most standard antimicrobial treatments.
Defining XDR bacteria
XDR bacteria are different from multidrug-resistant bacteria and pan-drug resistant bacteria. The difference is mainly interpreted as to what extent the bacteria are drug-resistant, with multidrug-resistant bacteria being unsusceptible to one or more antimicrobial agents and pan-drug resistant bacteria being unsusceptible to all antimicrobial agents.
Therefore, XDR bacteria rest somewhere in between these two categories. Occasionally, definitions are based on broader categories of antimicrobials rather than agents, which therefore narrows the bacteria fitting the description. However, this system is often seen as more informative by healthcare professionals treating afflicted patients with various antimicrobial classes.
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