Borrelia: travelers from Asia

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Lyme disease is an infection caused by bacteria from the genus Borrelia. It is spread by ticks, the vectors that transmit the Borrelia bacteria from host to host. As part of a German Research Foundation (DFG) project, evolutionary biologist Noémie Becker has employed genetic analyses to investigate how adaptation to various hosts and vectors has influenced the evolution of the pathogens. In our interview, she talks about her results and what conclusions can be drawn from them for human infections.

Which Borrelia species did you study?

Noémie Becker: We investigated three species: Borrelia bavariensis – not called so, incidentally, because it originates in Bavaria, but because the species was named by the National Reference Center for Borrelia in Oberschleißheim, Munich – and Borrelia afzelii, both of which are mainly found in rodents, and Borrelia garinii, which is adapted to birds.

All species are prevalent throughout Eurasia, and they particularly interested us because they can all cause Lyme disease in humans. They have various hosts, but the same vectors: In Europe this is chiefly the tick species Ixodes ricinus; in Asia the tick Ixodes persulcatus. We isolated Borrelia from over 2,000 ticks that we collected in Germany, Russia, and Japan. Borrelia is a difficult genus for microbiologists, as the bacteria do not grow very well in culture media. But we managed to grow cultures from which we were able to extract the DNA and sequence the genome.

What have you discovered about the origin of the Borrelia bacteria?

Becker: It is very likely that all three species originated in Asia. We make this inference on the grounds that the genetic diversity of Borrelia is greater in Asia, and so it would seem that their evolution started there. This hypothesis of an Asian origin was also confirmed by the phylogenetic tree of our bacterial strains, which we reconstructed. In the course of spreading to Europe, Borrelia switched vector and adapted to our tick I. ricinus. In particular, the bird-adapted species B. garinii is apparently able to switch vectors with relative ease and regularity, which means it can migrate faster and adapt better. The two rodent-adapted species, by contrast, wandered much more rarely between the continents and presumably made the leap to another vector one time only. This meant they could not adapt as well or as quickly as B. garinii.

Do infections differ according to the Borrelia species?

Becker: All three species we investigated can infect humans, but patients sometimes develop different symptoms. We tend to find B. bavariensis and B. garinii more frequently in neuroborreliosis, so these species seem to be more effective at infecting the brain. Meanwhile, we tend to find B. afzelii in the joints or occasionally in the skin. But it depends very much on the patient and is difficult to predict.

How can you prevent an infection?

Becker: The best way of preventing Lyme disease is to remove the ticks as quickly as possible. If you recognize and remove them on the same day or the next day, it is very unlikely that the tick will have transmitted Borrelia. The bacteria live in the gastrointestinal tract of the ticks and when the tick begins to suck blood, this milieu changes. This causes the Borrelia bacteria to migrate to the tick’s salivary glands, and only then can they be transmitted. This takes 1-2 days.
Identifying Lyme disease is not always easy.

Can any conclusions be drawn from your results for the diagnosis or treatment of Lyme disease?

Becker: It was already known of a certain group of genes that they help Borrelia circumvent human immune defenses. To our surprise, we found two Borrelia strains in samples from infected patients that do not possess these genes. But the strains were infectious nonetheless, which shows that Borrelia probably possess several mechanisms for getting round the immune system. This result is interesting because these strains could now be used to find other circumvention mechanisms. Humans are not actually the normal hosts of Borrelia, but other vertebrates such as rodents and birds. The actual hosts do not develop any diseases from the bacteria. If we understand better how Borrelia adapt to these hosts, then we could also understand better why things are different in humans and possibly identify new targets for a vaccine.