15.10.2026
25 years of TB genomics in Hamburg: Migration increases pathogen diversity, but local strains drive transmission
Migration has increased the genetic diversity of TB bacteria in Hamburg. Local transmission, however, has continued to be driven mainly by established strains. These findings are revealed in a long-term study recently published in Genome Medicine.
Researchers from the Research Center Borstel, Leibniz Lung Center (FZB), the Hamburg public health office and collaborating institutions analysed 3,131 Mtbc strains collected from patients in Hamburg between 1997 and 2021. They combined whole-genome sequencing (WGS) with epidemiological data to examine changes in the strain population and transmission patterns over time.
A more diverse TB strain population
The composition of Hamburg's Mycobacterium tuberculosis (Mtbc) population changed over the period. The share of Lineage 4 (L4), historically the most common lineage in Europe, fell from 83.5% in the first five years to 63.0% in the last five. The share of Lineage 3 (L3), found more often in parts of South and Central Asia and East Africa, rose from 4.8% to 18.1%. These changes coincided with an increase in the proportion of TB patients born outside Germany.
Yet the crucial finding was what did not happen: the greater diversity of strains was not accompanied by disproportionate transmission of the newly introduced lineages.
“Migration changes the genetic landscape of TB bacteria, but it does not automatically mean more local transmission”, says Professor Dr Stefan Niemann, Head of Molecular and Experimental Mycobacteriology Group at FZB. “Our data show that we need to distinguish carefully between the introduction of a strain and its successful onward transmission.”
Transmission differs between M. tubercolsis sublineages
Broad lineages categories can obscure important differences. L4 strains accounted for 81% of all identified genomic transmission clusters, but transmission patterns varied considerably between individual L4 sublineages. Some sublineages, including L4.1.2.1 and L4.8, were found in patients from many different geographical regions. That is consistant with the idea that some “generalist” strains can circulate across diverse human populations. Other strains appear to be more closely associated with particular populations or regions.
“The strain itself matters,” emphasizes Viola Dreyer from FZB. “Our results suggest that some sublineages are more successful than others at sustaining transmission chains. Detecting these differences requires genomic analysis at higher resolution than conventional strain typing can provide.”
Kontakt
Genomics distinguishes imported infections from local transmission
The findings are relevant for TB control in Germany and other low-incidence countries. A rise in TB cases alongside migration does not, by itself, show where infections occurred. Genomic data can help to distinguish infections acquired elsewhere from ongoing transmission within Germany. Combining pathogen genomes with demographic and epidemiological data therefore provides a much more precise picture of how TB spreads and can help public health teams to identify transmission chains and better target follow-up and control measures.
The 25-year analysis shows that migration, changes in pathogen population and local transmission are related, but they are not interchangeable measures of the same process.
The study “Migration and tuberculosis transmission in Hamburg, Germany: insights from 25 years of molecular epidemiology” was published in Genome Medicine on. It was supported by the German Center for Infection Research (DZIF).
Publication. Ullrich, N., Diel, R., Meywald-Walter, K. et al. Migration and tuberculosis transmission in Hamburg, Germany: insights from 25 years of molecular epidemiology. Genome Med 18, 137 (2026). https://doi.org/10.1186/s13073-026-01750-7