My recent article Bacteria vs. genetic predisposition: the spread of Th1 disease in families discusses how the bacteria responsible for causing chronic disease can be passed from generation to generation. At the same time, the genetic mutations created by these pathogens are also passed from mother to child.

Just this month, researchers led by John H. Werren at the University of Rochester in New York elucidated yet another way that bacterial DNA is likely passed from person to person.[1] This demonstrates just how easy it is for bacterial DNA to become incorporated into human DNA – a reality that is central to biomedical researcher Trevor Marshall’s model of chronic disease in which pathogens are constantly swapping genetic material with each other and their host.

The study describes how due to horizontal gene transfer – or the reality that once inside the body, organisms swap genetic material with each other, and also with the host – bacterial DNA often ends up integrated into human DNA. This integrated genetic material is then passed from generation to generation, and it is very likely that many of these acquired segments of DNA may help bacteria survive more easily in the body. “Our data are indicating that [DNA transfer] is going on all the time,” says Werren.

“The mechanism therefore provides an alternative to mutation of existing DNA as a way for the species to acquire new genetic traits,” states Patrick Barry ofScience News. “The transfer of DNA from bacteria means that an individual could acquire and pass on genes that it had not inherited.”

Warren’s team looked at several species of insects and roundworms infected by a parasitic bacterium called Wolbachia pipientis. The bacterium lives inside the animals’ cells, including their egg cells, giving it ready access to the chromosomes that are passed on to the animals’ offspring.

When the researchers compared the genetic code of the bacterium with the code of 11 other species: four roundworms, four fruit flies, and three wasps, they found that all but three of the fruit fly species had segments of the bacterium’s genetic code embedded in their DNA.

The team also scanned an archive of published genomes for 21 other invertebrate species and found bacterial genes in nine of them – proving that bacterial DNA can indeed be passed from mother to child. Whether this occurs in humans has not yet been demonstrated, but in principle, seems quite possible.

But this process has been taking place for centuries. Why hasn’t it been analyzed sooner?

“Such bacterial genetic code is routinely ignored during the sequencing of animals’ genomes because most scientists have assumed that the foreign DNA is a sign of contamination, Werren says. However, the new research rules out the possibility of contamination, says the scientist.


  1. Hotopp, J. C. D., Clark, M. E., Oliveira, D. C. S. G., Foster, J. M., Fischer, P., Torres, M. C. M., et al. (2007) Widespread lateral gene transfer from intracellular bacteria to multicellular eukaryotes. Science (New York, N.Y.), 317(5845), 1753-6. []