Bacteria take a unique approach to sex. One builds a tiny bridge to another, then transfers DNA across. The frisky little guys will hook up with plant cells, and now, scientists have shown, donate DNA to mammalian cells. This result provides new fodder for a contentious debate over whether bacterial genes are present in the human genome.
Bacterial conjugation relies on a ring of DNA called a plasmid with the genes necessary to build the mating bridge and shoot DNA across it. Scientists used to think that bacteria mated only with bacteria. But in the past decade, they have learned that bumps on trees called galls are the result of conjugation between soil bacteria and plant cells, and they've seen Escherichia coli mate with yeast in the lab. But although researchers have long suspected that bacteria conjugate with mammalian cells, they've been unable to prove it.
Virginia Waters, a geneticist at the University of California, San Diego, set out to catch bacteria in the act. She added bacterial cells to a culture of a widely used mammalian cell line derived from the ovaries of Chinese hamsters. In the December issue of Nature Genetics, Waters reports that a few days later, one in every 10,000 mammalian cells tested positive for a bacterial plasmid. To demonstrate that the mammalian cells did not acquire bacterial genes by non-conjugational means--for example, by engulfing the bacteria, as eukaryotic cells sometimes do--Waters added an enzyme that digests DNA outside the cell membrane. That had no effect, lending weight to the theory that genes passed via the conjugation bridge. Furthermore, removing genes that facilitate conjugation halted gene transfer altogether, suggesting that they play a key role in the process.
It now seems that bacteria can mate with any organism with a cell membrane, says Stanley Maloy, a biologist at the University of Illinois, Urbana-Champaign. He says this idea has "very profound" implications for the debate over the origins of bacterial genes that are present in the human genome but absent in our closest relatives (Science, 8 June, p. 1903): The amount of conjugation Waters detected is "high enough to readily explain" the possible infiltration of bacterial genesinto our DNA, meaning that conjugation could have happened quickly enough to add genes only to humans, in the years since they split from the common ancestor they shared with chimpanzees.