**7. Conclusions**

Drug-resistant TB is a significant challenge for the successful control of the disease worldwide. A comprehensive review of clinical, biological and microbiological issues favoring resistance development has been provided, helping in the development of new tools for the rapid diagnosis and treatment of drug-resistant TB. The review was based on the most recent updates on drug resistance mechanisms reported in the literature, and on the international recommendations of WHO to facilitate the clinical and microbiological management of MDR/XDR TB at global level.

**Author Contributions:** Conceptualization, writing, reviewing and editing: A.I., L.F., F.G. All authors have read and agree to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
