Drug-resistant strains of M. tuberculosis are becoming more common, particularly in developing countries. Multidrug-resistant TB (MDR-TB), defined as resistance to at least isoniazid and rifampin, and extensively drug-resistant TB (XDR-TB), defined as MDR-TB strains that are also resistant to fluoroquinolones and second-line injectable agents, are a growing concern. Researchers at Vanderbilt, with collaborators in Tennessee as well as Lima, Peru and Cape Town and Durban, South Africa, study resistance to fluoroquinolones, widely used for the treatment of bacterial infections, and how it may affect the diagnosis and treatment of TB.