Extensively drug-resistant mycobacterium tuberculosis

**Definition:** Extensively drug-resistant tuberculous mycobacteria are resistant to at least four of the core anti-TB drugs. XDR-TB mycobacteria are resistant to the two most powerful anti-TB drugs, isoniazid and rifampicin, also known as multidrug-resistance, in addition to resistance to any of the fluoroquinolones (such as levofloxacin or moxifloxacin) and to at least one of the three injectable second-line drugs (amikacin, capreomycin or kanamycin).