May 5, 2004
Recent findings from a genetic analysis of 100 tuberculosis samples from the almost 3,000 cases occurring during the last 13 years in San Francisco suggest ways the bacteria can escape a host's immune system or develop antibiotic resistance. And in follow-up research, the researchers found that people from different regions of the world carried different TB strains. This raises the possibility that the pathogen evolves within a geographic population group and does not spread to other groups.
Peter Small, M.D., of Stanford University, and colleagues examined the genomes of the TB strains to identify specific, irreversible genetic changes that served as "fingerprints." His team used genetic analysis to discriminate between TB cases contracted from another San Franciscan and cases that arose from much earlier infections.
Using those fingerprints, the researchers could track transmission through a community, said Aaron Hirsh, Ph.D., lead author of the second paper. That information could be graphically represented to illustrate commonalities between strains. Researchers assigned each strain its own color, based on the national origin of the infected person.
"Immigrants from Asia don't really commingle with immigrants from other parts of the world or with native-born Americans," said Small, who is currently on leave from Stanford serving as a senior program officer in the Global Health Program of the Bill and Melinda Gates Foundation. "But the puzzling part is that Asian tuberculosis must have been introduced around the time of the Gold Rush and you'd think in the intervening 150 years, there'd be ample opportunity for those strains to spread around. We simply didn't see that."
"Co-evolution is highly speculative, but it's an intriguing possibility," said Small. "This has profound implications for vaccine development. We may ultimately need to develop different vaccines for different parts of the world. It's a completely open question, but we can start to answer it now."
The papers "Functional and Evolutionary Genomics of Mycobacterium tuberculosis: Insights from Genomic Deletions in 100 Strains," and "Stable Association Between Strains of Mycobacterium tuberculosis and Their Human Host Populations," appeared in the same edition of Proceedings of the National Academy of Sciences (2004;101(14):4865-4870 and 4871-4876, respectively).
TB & Outbreaks Week