horizontal gene transfer

Bacteriophages are the most abundant biological entities on Earth. There are 10³¹ of them on the planet, and they infect 10²³ to 10²⁵ bacteria every second. That’s a lot of lysis, and it leads to the release of huge quantities of DNA that can be taken up by other organisms, leading to new traits. It seems that some bacteriophages are very, very good at releasing intact DNA, and they have been called superspreaders (link to paper).

In a very simple experiment, E. coli cells carrying a plasmid encoding ampicillin resistance were infected with the well studied phages T4 and T7 and also with a collection of 20 phages isolated from soil, water, and feces in Miami and Washington DC. After the cells lysed, DNA was extracted from the culture medium and introduced into antibiotic sensitive E. coli. Two phages, called SUSP1 and SUSP2, were thousands of times better at releasing plasmid DNA that readily conferred antibiotic resistance. These phages are superspreaders.

Superspreader phages can promote transformation by different plasmids, so their unique talent is not sequence specific. When these phages lyse cells, intact plasmid DNA is released. In contrast, phage T4 infection leads to degradation of plasmid DNA in the host cell. Superspreader phages lack genes encoding known  endonucleases – enzymes that degrade DNA, possibly explaining why plasmids are not degraded during infection. Other phages that lack such endonucleases, including mutants of lambda and T4, also promote plasmid mediated transformation.

Phages SUP1 and SUP2 don’t just spread plasmids to laboratory strains like E. coli. When crude mixtures of soil bacteria from Wyoming and Maryland were mixed with SUP1 and SUP2 lysates from E. coli, antibiotic resistance was readily transferred. One of the main recipients of plasmid DNA is a member of the Bacillus genus of soil bacteria, showing that superspreaders can move DNA into hosts of a species other than the one they can infect.

With so many bacteriophages on the planet, it is likely that there are many other superspreaders like SUP1 and SUP2 out there. The implication is that massive amounts of intact plasmid DNAs are being released every second. These DNAs can be readily taken up into other bacteria, leading to new phenotypes such as antibiotic resistance, altered host range, virulence, the ability to colonize new niches, and much more.

You might wonder if all that plasmid DNA, floating in the environment, can also enter eukaryotic cells – and the answer is yes. No wonder eukaryotes didn’t invent anything.