A research group has discovered that bacteria in remote, isolated regions share some stretches of DNA and are suggesting that this is due to bacteria travelling through the air. Human understanding of how bacteria travels across the globe has changed a lot in recent decades. It was originally assumed that bacteria are transported on humans and animals, and in that manner they could be ‘carried’ around and brought to new places.
Bacteria found in rainwater showed us that they were present into clouds, and it was soon found that both viruses and bacteria are carried into the atmosphere in small particles from dust and sea spray.
However, new research, published in Philosophical Transactions of Royal Society B now shows, that bacteria from incredibly remote regions share stretches of DNA, and they hypothesise that this is a result of bacteria travelling through the air, and have named this idea the ‘air bridge hypothesis’.
The group came to this hypothesis by collecting Thermus thermophilus, a heat-loving type of bacteria, from hot gravel and springs from locations in Italy, Chile, and Russia. Analysis of the DNA from these sample bacteria identified similarities in regions of the genome which contain “molecular memory” of virus interaction.
These so-called memories are made when a bacteria is infected by a bacteriophage, a kind of virus, and the bacteria incorporates some of the viral DNA into its own so that it can recognise future infection. The viral DNA is stored in a region of the genome to itself, and the order that DNA from different viruses is stored in corresponds to the order in which they infected the bacterium.
Bacteria can swap and share pieces of DNA with bacteria close by, in a process called horizontal gene transfer. This allows them to spread beneficial genes to more than just their daughter cells. These genes usually give the recipient some advantage it did not have before; for example it may encode a protein pumps to remove a specific toxin.
There are three mechanisms by which they can achieve this, but all involve making a copy of the DNA called a plasmid, a small circle of DNA, which is then passed onto another bacterium. The recipient can then incorporate this new DNA into its own and pass it onto the next generation.
The scientists expected to find that bacteria within the same area would have shared DNA due to horizontal gene transfer, but that they would not share DNA between areas due to isolation and having independent histories of viral infection.
Analysis of the bacteria from the five regions tested showed plenty of shared DNA “memories”. The most striking finding was that not only did the bacteria from the different areas share the viral DNA, but that they had it in the same order. This could have been achieved if all of the colonies of bacteria had been infected by the bacteriophages in the same order, but it is more likely that the set of genes was distributed when it was already in that configuration. This suggests that bacteria are travelling thousands of miles and sharing DNA across the globe.
Bacteria share more than just DNA for viral infection. It is well known that bacteria can share genes for antibiotic resistance by horizontal gene transfer. If the ‘air bridge hypothesis’ which these findings have been used to suggest, is true, it means that bacteria which have developed genes for resistance to an antibiotic in one area may be able to share these genes with other bacteria around the world. Antibiotic resistance in bacteria is a global health issue, and any information about how it spreads could be useful in future research.
The group intends to explore this hypothesis by testing the bacteria in air at different altitudes and locations to see how high into the sky the bacteria can reach and the wider effects of this hypothesis if it is true.
This post was written by Molly Eastol and edited by Ella Mercer