Thesis (M. Sc. (Medical Sciences)) -- University of Limpopo, 2015 / Tuberculosis (TB) patients may have mixed infections with both drug-susceptible and drug-resistant Mycobacterium tuberculosis (MTB) strains. This phenomenon termed heteroresistance presents a challenge TB management and is considered a preliminary stage to full resistance. Heteroresistance is more likely to occur in high TB incidence areas and in chronic patients as they have more opportunity to become infected with various strains of TB and has been proven to occur in new cases, treatment failure and relapse. Methods: Sputum samples were collected from new consulting and hospitalised patients who were on treatment for MDR TB. A total of 231 samples were run on MTBDRplus to determine heteroresistance of Mycobacterium tuberculosis to isoniazid and rifampicin. To determine heteroresistance to second-line drugs, 91 samples were run on MTBDRsl. Nineteen (19) samples that were heteroresistant to 2nd line drugs were subjected to spoligotyping to determine the families/lineages they belonged to. Results: A total of 66 were confirmed as Mycobacterium tuberculosis complex by the line probe assays. Out of the 66 MTBC, rifampicin resistance was found in 22 (10%) and 44 (19%) were reported susceptible. Isoniazid resistance was found in 39 (17%) and 27 (12%) were reported susceptible. Of the 66 MTBC positive samples, moxifloxacin resistance was found in 33 (16%) and 14 (7%) were reported susceptible. Kanamycin resistance was found in 17 (8%) and 30 (14%) were reported susceptible. Ethambutol resistance was found in 25 (12%) and 22 (10%) were reported susceptible. Heteroresistance was evident in 22 (10%) samples for the first-line and in 23 (11%) for the second-line drugs. Results of a total of 19 heteroresistant samples subjected to spoligotyping when compared to those in the international spolDB4 database indicated that 4 of them matched existing shared spoligotype international types, 15 were unknown (orphans). Eighteen (18) of 19 heteroresistant samples subjected to spoligotyping were also MDR. Fourteen of the samples that were resistant to both RIF and INH were orphans. Of the 14 MDR, 3 samples belonged to clades T1, T-H37RvV817 and LAM 3 with SITs: 879, 568 and 2301, respectively. One sample with SIT 1196 had an unknown clade was resistant to RIF but susceptible to INH. Conclusion: This study has shown that heteroresistance remains an important phenomenon in clinical tuberculosis, especially in highly endemic areas. According to the current study, heteroresistance was associated more with recurrent cases who are on initiation or continuation phase than new cases and a larger percentage of heteroresistance was reported in second-line drugs than there is in first-line drugs. The T1 genotype was found to be predominant amongst recurrent cases. The LAM3 and T-H37RvV817 lineages were found amongst the new cases. In the present study there was no significant association between heteroresistance and the patient’s treatment history as indicated by a P-value of 0.473 and between heteroresistance and spoligotype families (P-value, 0.991). The predominance of orphan SITs and unknown clades followed by non-Beijing strains in the study may be due to the migration of carriers from the neighboring countries as the Limpopo Province is flanked by Botswana, Zimbabwe and Mozambique. Further studies with larger numbers of patients should focus on the prevalence to associate heteroresistance with patients‟ treatment history and establish the contributing MTBC strain lineages.