Lung infection could be less transmissible than thought

Recent research found that various strains of the bacterium, Mycobacterium abscessus, were genetically similar, stoking fears that it was spreading from person to person.

But a new study by Harvard Medical School researchers published May 22 in PNAS, calls those findings into question, offering an alternative explanation behind the genetic similarity of clinical clusters. This suggests that the pathogen may not be that prone to person-to-person transmission after all. The team set out to investigate a hypothesis that the samples appeared genetically similar because the pathogen was evolving at a very slow rate.

The scientists first used a large dataset of M. abscessus genomes to create a “tree of life,” a kind of genetic family tree for the bacterium. They looked at branches of the tree with clusters of genetically similar strains, then tried to calculate their evolutionary rate. They found that these genetically similar clusters were evolving around 10 times more slowly than typical M. abscessus strains.

Next, they used computer modeling to determine whether the genetic similarities could be explained by the relatively small population size of these bacteria. But even when they simulated extreme population sizes, the result didn’t change. This was an indicator that the high genetic similarity is best explained by a slower evolutionary rate.

Finally, researchers conducted experiments to see how fast different strains of M. abscessus evolved to develop resistance when exposed to antibiotics in the lab. They found that the genetically similar strains evolved much more slowly than other strains.

Reference:

Mutation rates and adaptive variation among the clinically dominant clusters of Mycobacterium abscessus,Proceedings of the National Academy of Sciences,DOI 10.1073/pnas.2302033120