bacteria don't have species

Astrobiology | AM: You have argued that bacteria don't have species. I wonder if you could explain that idea.

Lynn Margulis: Bacteria are much more of a continuum. They drop their genes all the time. Like we say in What is Life?, it's like going swimming in a swimming pool, going in blue-eyed and coming out brown-eyed, just because you've gulped the water. Obviously, animals don't do that. But that's what bacteria do, all the time. They just pick up genes, they throw away genes, and they are very flexible about that.

Say you have a bacterium like Azotobacter. This is a nitrogen-fixing bacterium. It takes nitrogen out of the air and puts it into useable food. Nitrogen fixing is a big deal. It takes a lot of genes. If you put a little something like arsenium bromide in a test tube with these organisms, and put it in a refrigerator overnight, lo and behold, the next day the cells can't do this anymore, they can't fix nitrogen. So by definition you have to change them from one genus to another.

I'll give you another example: E. coli. It's a normal inhabitant of the human gut. If you put a particular plasmid into E. coli, all of a sudden you have Klebsiella and not E. coli. You've changed not only the species, but the genus. It's like changing a person to a chimpanzee. Can you imagine doing that, putting a chimpanzee in the refrigerator, and getting him out the next morning, and now he's a person?

Sorin Sonea, who was the chair of the microbiology department at the Université de Montreal, in Canada, has been saying for 25 or 30 years that you either have to consider all the bacteria on Earth as one species, or you have to consider them as no species at all. The criteria we use for species, which are good ones for animals and plants and fungi, do not apply, because bacteria can change overnight. You have all sorts of gradations, where adding or removing a few genes will change an organism's name, because those genes are what define the organism.