MDR-TB Outbreak among HIV-Negative Tunisian Patients followed during 11 Years.

BACKGROUND: Multidrug-resistant tuberculosis (MDR-TB) outbreaks that evolve, from the outset, in a context strictly negative for HIV infection deserve special consideration since they reflect the true intrinsic epidemic potential of the causative strain. To our knowledge, the long-term evolution of such exceptional outbreaks and the treatment outcomes for the involved patients has never been reported hitherto. Here we provide a thorough description, over an 11-year period, of an MDR-TB outbreak that emerged and expanded in an HIV-negative context, Northern Tunisia. METHODOLOGY/PRINCIPAL FINDINGS: From October 2001 to June 2011, the MDR-TB outbreak involved 48 HIV-negative individuals that are mainly young (mean age 31.09 yrs; 89.6% male) and noninstitutionalized. Drug susceptibility testing coupled to mutational analysis revealed that initial transmission involved an isolate that was simultaneously resistant to isoniazid, rifampicin, ethambutol, and streptomycin. The causative Haarlem3-ST50 outbreak strain expanded mainly as an 11-banded IS6110 RFLP profile (77.1%), from which a 12-banded subclone evolved. After undergoing a 2-year treatment with second-line drugs, 22 (45.8%) patients were cured and 3 (6.2%) completed treatment, thus yielding an overall treatment success rate of 52.1%. Among the patients that experienced unfavorable treatment outcomes, 10 (20.8%) failed treatment, 3 (6.2%) were lost to follow-up, 5 (10.4%) died, and 5 (10.4%) could not be evaluated. Poor adherence to treatment was found to be the main independent predictor of unfavorable outcomes (HR: 9.15; 95% CI 1.72-48.73; P = 0.014). Intriguingly, the evolved 12-banded subclone proved significantly associated with unfavorable outcomes (HR: 4.90; 95% CI 1.04-23.04, P = 0.044). High rate of fatality and relapse was further demonstrated at the long-term, since 70% of those whose treatment failed have died, and 24% among those deemed successfully treated have relapsed. CONCLUSIONS/SIGNIFICANCE: Taken together, the data obtained in this study indicate that MDR-TB clinical isolates could become fit enough to cause large and severe outbreaks in an HIV-negative context. Such MDR-TB outbreaks are characterized by low treatment success rates and could evolve towards increased severity, thus calling for early detection of cases and the necessity to raise the bar of surveillance throughout and beyond the treatment period.

Evidence for the critical role of a secondary site rpoB mutation in the compensatory evolution and successful transmission of an MDR tuberculosis outbreak strain.

BACKGROUND: MDR Mycobacterium tuberculosis clinical strains that cause large outbreaks, particularly among HIV-negative patients, are likely to have undergone the most successful compensatory evolution. Hence, mutations secondary to the acquisition of drug resistance are worthy of consideration in these highly transmissible strains. Here, we assessed the role of a mutation within rpoB, rpoB V615M, secondary to the rifampicin resistance-conferring mutation rpoB S531L, which is associated with a major MDR tuberculosis outbreak strain that evolved in an HIV-negative context in northern Tunisia. METHODS: Using BCG as a model organism, we engineered strains harbouring either the rpoB S531L mutation alone or the double mutation rpoB S531L, V615M. Individual and competitive in vitro growth assays were performed in order to assess the relative fitness of each BCG mutant. RESULTS: The rpoB V615M mutation was found to be invariably associated with rpoB S531L. Structural analysis mapped rpoB V615M to the same bridge helix region as rpoB compensatory mutations previously described in Salmonella. Compared with the rpoB single-mutant BCG, the double mutant displayed improved growth characteristics and fitness rates equivalent to WT BCG. Strikingly, the rpoB double mutation conferred high-level resistance to rifampicin. CONCLUSIONS: Here, we demonstrated the fitness compensatory role of a mutation within rpoB, secondary to the rifampicin resistance mutation rpoB S531L, which is characteristic of an MDR M. tuberculosis major outbreak strain. The finding that this secondary mutation concomitantly increased the resistance level to rifampicin argues for its significant contribution to the successful transmission of the MDR-TB strain.