Perhaps our most intimate acquaintance with the flow of genetic information is our family--parents to be exact. If we expand this idea further, not just through a couple hundred years of genealogy but through eons, we get an overview, an evolutionary picture if you will, that will look the most familiar to us as a phylogenetic tree. The passing of genetic information from parent to offspring is vertical gene transfer.
Another method of moving genes from one place to another is called horizontal gene transfer--from one organism to another of a completely unrelated species. If you picture this on a phylogenetic tree, the arrows would be going left and right instead of up and down. Molecular biologists know that this can be achieved artificially in the lab. A simple example of this involves a technique called transformation and the molecular biologists' favorite work horse, the E. coli bacterium. Disregarding the technicalities for the moment, the principle is this: the scientist takes the gene of interest (which could be from any organism*), mixes the DNA with some competent E. coli, and gives the cells a brief shock to make them take up the DNA.
Horizontal gene transfer can also occur naturally. Up until now, pretty much all known instances of this were believed to occur with bacteria. In a way, this is not surprising--microorganisms seem to lead a much more adventurous sex life than your neighborhood pervert. This can be deadly--especially if you're talking about antibiotic resistance. A recent example of bacteria gaining antibiotic resistance by transfer of antibiotic resistance genes from another bacteria is that of Staphylococcus aureus or as the news likes to scare people with--the "superbug". What's so super about it? S. aureus is resistant to a whole host of antibiotics, but until recently, it could still be treated with one of the biggest weapons in the antibiotic arsenal, vancomycin. But due to a tryst with vancomycin resistant Enterococcus facaelis, certain strains of S. aureus (VISA/VRSA) can live just as happily with or without the antibiotic.
A recent Genome Biology paper by Budd et al. presents evidence horizontal gene transfer may not be just bacteria-bacteria conjugation. In analyzing genomic sequences of many organisms, the group noticed that a gene encoding a protease inhibitor, a2-macroglobulin (a2M), used by eukaryotes to fight off infections was popping up in all sorts of bacterial sequences. The inheritance of the gene among bacteria did not correlate with vertical transfer, but did imply that perhaps horizontal gene transfer occurred between bacteria and eukaryotes.
Did this gene transfer happen recently? The appearance of a2M is scattered throughout the bacterial kingdom--from the harmless aquatic species to the pathogenic--which suggest, as Budd et al. have, that this gene transfer happened a long time ago between a bacterial ancestor and one of our single-celled metazoan ancestors. Exactly in which ancestor the gene originated is still debatable.
But all this discussion about seemingly indiscriminate gene swapping doesn't tell us one thing--what is it good for? The answer to that is rather self-evident for pathogenic bacteria. If a disease causing bacteria can obtain a host gene, perhaps it can use it to its advantage to evade the immune system and live in its preferred niche, be it me or you or the poor houseplant on the back porch. And what about apparently free-living bacteria? The paper proposes that they might not be free-living at all and that a2M might act as a colonizing factor, helping the bacteria cling to their particular environmental niche. Whatever the case, it gives us something new to think about. We may think we're clever in examining bacterial genomes for targets we can exploit, but the evidence points to the fact that these microbes have been mining our genomes to their advantage before we even showed up on earth.
*There are, of course, some caveats to this. You can pretty much zap any DNA into E. coli, but it gets more complicated when you want to express your gene of interest in another organism. If you want to express a gene from Bacillus subtilis (another bacterium) in E. coli, you probably won't have any problems, but mammalian genes are another matter. Our genes are structured differently than that of bacteria. Bacterial genes are pared down and efficient whereas ours contain numerous introns that must be spliced out before the gene is translated. E. coli doesn't have the proper machinery to do all that splicing.
