Purpose: The objectives of the present study were to characterize the clinical isolates prevailing in the northeast of Jiangsu and to investigate the mode of transmission. The study also aimed to explore the extent to which Mycobacterium tuberculosis  strains contributed to drug resistance and the possible factors related to the recent transmission.
Patients and methods: We consecutively enrolled 912 culture-confirmed pulmonary tuberculosis (TB) cases from 1 January 2013 to 31 December 2014 in Lianyungang City, which is located in the center of China’s vast ocean area and the northeast of Jiangsu province. Isolates were genotyped using 15-locus mycobacterial interspersed repetitive unit-variable number tandem repeat (MIRU-VNTR) typing. The Hunter–Gaston discrimination index (HGDI) was used to estimate the discriminatory power and diversity of molecular markers.
Results: Among 741 successfully genotyped isolates, 144 (19.43%) strains formed 46 clusters, while 597 (80.57%) isolates had the unique MIRU pattern. The total HGDI for all 15 loci was 0.999. The average cluster size was 3 (2–13) patients. The estimated proportion of recent transmission was 13.34%. Patients with unfavorable treatment outcomes were infected with clustered strains at a higher proportion than were those with favorable treatment outcomes (adjusted OR: 1.78, 95% CI: 1.14–2.85, P =0.012). 
Conclusion: The probability of recent TB transmission was relatively low in the study site, while the cases mainly arose from the activation of previous infection. Spatial analysis showed that strains forming larger clusters had the characteristics of regional aggregation.
Keywords: tuberculosis, molecular epidemiology, genotype, transmission, drug resistance