Clustering of Tuberculosis Cases Based on Variable-Number Tandem- Repeat Typing in Relation to the Population Structure of

The population structure of 3,776Mycobacterium tuberculosis isolates was determined using variable-number tandem-repeat (VNTR) typing. The degree of clonality was so high that a more relaxed definition of clustering cannot be applied. Among recent immigrants with non-Euro-American isolates, transmission is overestimated if based on identical VNTR patterns. DNA typing is a powerful tool to trace tuberculosis (TB) trans-mission and outbreaks. Clustering of Mycobacterium tuber-culosis isolates based on identical DNA fingerprints is commonly used as a proxy for recent transmission (1). However, this as-sumption is not always correct and depends onmany factors, such as circulation of genetically similar strains, evolution ofM. tuber-culosis over time, transmission rate, DNA typingmethods applied, duration of the study period, sampling, and effectiveness of TB control (2, 3). Various studies have shown that not all cases in DNA fingerprint clusters have epidemiological links with other cases in the cluster (4, 5). Moreover, epidemiological links have been found between cases caused by bacteria with slightly differ