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c e Summary. Tuberculosis (TB) remains a worldwide emergency (8 million new cases per year, 2 million deaths annually) mostly affecting poor countries. To this old, persistent threat, a new emergency is adding further challenge, i.e., the multidrug-resistant TB, which in its extreme and highly lethal form is called XDR TB (extensively drug-resistant TB). How to fight XDR TB is a high research and public health priority. Key words: Mycobacterium tuberculosis, drug susceptibility testing, first-line drugs, second-line drugs, drug resistance. Riassunto (Tubercolosi estrema: una vecchia e nuova minaccia). La tubercolosi (TB) continua ad esser